summaryrefslogtreecommitdiffstats
path: root/Documentation/security/keys/core.rst
blob: 811b905b56bf7fc9455f351b90af2434f72db9a3 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
============================
Kernel Key Retention Service
============================

This service allows cryptographic keys, authentication tokens, cross-domain
user mappings, and similar to be cached in the kernel for the use of
filesystems and other kernel services.

Keyrings are permitted; these are a special type of key that can hold links to
other keys. Processes each have three standard keyring subscriptions that a
kernel service can search for relevant keys.

The key service can be configured on by enabling:

	"Security options"/"Enable access key retention support" (CONFIG_KEYS)

This document has the following sections:

.. contents:: :local:


Key Overview
============

In this context, keys represent units of cryptographic data, authentication
tokens, keyrings, etc.. These are represented in the kernel by struct key.

Each key has a number of attributes:

	- A serial number.
	- A type.
	- A description (for matching a key in a search).
	- Access control information.
	- An expiry time.
	- A payload.
	- State.


  *  Each key is issued a serial number of type key_serial_t that is unique for
     the lifetime of that key. All serial numbers are positive non-zero 32-bit
     integers.

     Userspace programs can use a key's serial numbers as a way to gain access
     to it, subject to permission checking.

  *  Each key is of a defined "type". Types must be registered inside the
     kernel by a kernel service (such as a filesystem) before keys of that type
     can be added or used. Userspace programs cannot define new types directly.

     Key types are represented in the kernel by struct key_type. This defines a
     number of operations that can be performed on a key of that type.

     Should a type be removed from the system, all the keys of that type will
     be invalidated.

  *  Each key has a description. This should be a printable string. The key
     type provides an operation to perform a match between the description on a
     key and a criterion string.

  *  Each key has an owner user ID, a group ID and a permissions mask. These
     are used to control what a process may do to a key from userspace, and
     whether a kernel service will be able to find the key.

  *  Each key can be set to expire at a specific time by the key type's
     instantiation function. Keys can also be immortal.

  *  Each key can have a payload. This is a quantity of data that represent the
     actual "key". In the case of a keyring, this is a list of keys to which
     the keyring links; in the case of a user-defined key, it's an arbitrary
     blob of data.

     Having a payload is not required; and the payload can, in fact, just be a
     value stored in the struct key itself.

     When a key is instantiated, the key type's instantiation function is
     called with a blob of data, and that then creates the key's payload in
     some way.

     Similarly, when userspace wants to read back the contents of the key, if
     permitted, another key type operation will be called to convert the key's
     attached payload back into a blob of data.

  *  Each key can be in one of a number of basic states:

      *  Uninstantiated. The key exists, but does not have any data attached.
     	 Keys being requested from userspace will be in this state.

      *  Instantiated. This is the normal state. The key is fully formed, and
	 has data attached.

      *  Negative. This is a relatively short-lived state. The key acts as a
	 note saying that a previous call out to userspace failed, and acts as
	 a throttle on key lookups. A negative key can be updated to a normal
	 state.

      *  Expired. Keys can have lifetimes set. If their lifetime is exceeded,
	 they traverse to this state. An expired key can be updated back to a
	 normal state.

      *  Revoked. A key is put in this state by userspace action. It can't be
	 found or operated upon (apart from by unlinking it).

      *  Dead. The key's type was unregistered, and so the key is now useless.

Keys in the last three states are subject to garbage collection.  See the
section on "Garbage collection".


Key Service Overview
====================

The key service provides a number of features besides keys:

  *  The key service defines three special key types:

     (+) "keyring"

	 Keyrings are special keys that contain a list of other keys. Keyring
	 lists can be modified using various system calls. Keyrings should not
	 be given a payload when created.

     (+) "user"

	 A key of this type has a description and a payload that are arbitrary
	 blobs of data. These can be created, updated and read by userspace,
	 and aren't intended for use by kernel services.

     (+) "logon"

	 Like a "user" key, a "logon" key has a payload that is an arbitrary
	 blob of data. It is intended as a place to store secrets which are
	 accessible to the kernel but not to userspace programs.

	 The description can be arbitrary, but must be prefixed with a non-zero
	 length string that describes the key "subclass". The subclass is
	 separated from the rest of the description by a ':'. "logon" keys can
	 be created and updated from userspace, but the payload is only
	 readable from kernel space.

  *  Each process subscribes to three keyrings: a thread-specific keyring, a
     process-specific keyring, and a session-specific keyring.

     The thread-specific keyring is discarded from the child when any sort of
     clone, fork, vfork or execve occurs. A new keyring is created only when
     required.

     The process-specific keyring is replaced with an empty one in the child on
     clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is
     shared. execve also discards the process's process keyring and creates a
     new one.

     The session-specific keyring is persistent across clone, fork, vfork and
     execve, even when the latter executes a set-UID or set-GID binary. A
     process can, however, replace its current session keyring with a new one
     by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous
     new one, or to attempt to create or join one of a specific name.

     The ownership of the thread keyring changes when the real UID and GID of
     the thread changes.

  *  Each user ID resident in the system holds two special keyrings: a user
     specific keyring and a default user session keyring. The default session
     keyring is initialised with a link to the user-specific keyring.

     When a process changes its real UID, if it used to have no session key, it
     will be subscribed to the default session key for the new UID.

     If a process attempts to access its session key when it doesn't have one,
     it will be subscribed to the default for its current UID.

  *  Each user has two quotas against which the keys they own are tracked. One
     limits the total number of keys and keyrings, the other limits the total
     amount of description and payload space that can be consumed.

     The user can view information on this and other statistics through procfs
     files.  The root user may also alter the quota limits through sysctl files
     (see the section "New procfs files").

     Process-specific and thread-specific keyrings are not counted towards a
     user's quota.

     If a system call that modifies a key or keyring in some way would put the
     user over quota, the operation is refused and error EDQUOT is returned.

  *  There's a system call interface by which userspace programs can create and
     manipulate keys and keyrings.

  *  There's a kernel interface by which services can register types and search
     for keys.

  *  There's a way for the a search done from the kernel to call back to
     userspace to request a key that can't be found in a process's keyrings.

  *  An optional filesystem is available through which the key database can be
     viewed and manipulated.


Key Access Permissions
======================

Keys have an owner user ID, a group access ID, and a permissions mask. The mask
has up to eight bits each for possessor, user, group and other access. Only
six of each set of eight bits are defined. These permissions granted are:

  *  View

     This permits a key or keyring's attributes to be viewed - including key
     type and description.

  *  Read

     This permits a key's payload to be viewed or a keyring's list of linked
     keys.

  *  Write

     This permits a key's payload to be instantiated or updated, or it allows a
     link to be added to or removed from a keyring.

  *  Search

     This permits keyrings to be searched and keys to be found. Searches can
     only recurse into nested keyrings that have search permission set.

  *  Link

     This permits a key or keyring to be linked to. To create a link from a
     keyring to a key, a process must have Write permission on the keyring and
     Link permission on the key.

  *  Set Attribute

     This permits a key's UID, GID and permissions mask to be changed.

For changing the ownership, group ID or permissions mask, being the owner of
the key or having the sysadmin capability is sufficient.


SELinux Support
===============

The security class "key" has been added to SELinux so that mandatory access
controls can be applied to keys created within various contexts.  This support
is preliminary, and is likely to change quite significantly in the near future.
Currently, all of the basic permissions explained above are provided in SELinux
as well; SELinux is simply invoked after all basic permission checks have been
performed.

The value of the file /proc/self/attr/keycreate influences the labeling of
newly-created keys.  If the contents of that file correspond to an SELinux
security context, then the key will be assigned that context.  Otherwise, the
key will be assigned the current context of the task that invoked the key
creation request.  Tasks must be granted explicit permission to assign a
particular context to newly-created keys, using the "create" permission in the
key security class.

The default keyrings associated with users will be labeled with the default
context of the user if and only if the login programs have been instrumented to
properly initialize keycreate during the login process.  Otherwise, they will
be labeled with the context of the login program itself.

Note, however, that the default keyrings associated with the root user are
labeled with the default kernel context, since they are created early in the
boot process, before root has a chance to log in.

The keyrings associated with new threads are each labeled with the context of
their associated thread, and both session and process keyrings are handled
similarly.


New ProcFS Files
================

Two files have been added to procfs by which an administrator can find out
about the status of the key service:

  *  /proc/keys

     This lists the keys that are currently viewable by the task reading the
     file, giving information about their type, description and permissions.
     It is not possible to view the payload of the key this way, though some
     information about it may be given.

     The only keys included in the list are those that grant View permission to
     the reading process whether or not it possesses them.  Note that LSM
     security checks are still performed, and may further filter out keys that
     the current process is not authorised to view.

     The contents of the file look like this::

	SERIAL   FLAGS  USAGE EXPY PERM     UID   GID   TYPE      DESCRIPTION: SUMMARY
	00000001 I-----    39 perm 1f3f0000     0     0 keyring   _uid_ses.0: 1/4
	00000002 I-----     2 perm 1f3f0000     0     0 keyring   _uid.0: empty
	00000007 I-----     1 perm 1f3f0000     0     0 keyring   _pid.1: empty
	0000018d I-----     1 perm 1f3f0000     0     0 keyring   _pid.412: empty
	000004d2 I--Q--     1 perm 1f3f0000    32    -1 keyring   _uid.32: 1/4
	000004d3 I--Q--     3 perm 1f3f0000    32    -1 keyring   _uid_ses.32: empty
	00000892 I--QU-     1 perm 1f000000     0     0 user      metal:copper: 0
	00000893 I--Q-N     1  35s 1f3f0000     0     0 user      metal:silver: 0
	00000894 I--Q--     1  10h 003f0000     0     0 user      metal:gold: 0

     The flags are::

	I	Instantiated
	R	Revoked
	D	Dead
	Q	Contributes to user's quota
	U	Under construction by callback to userspace
	N	Negative key


  *  /proc/key-users

     This file lists the tracking data for each user that has at least one key
     on the system.  Such data includes quota information and statistics::

	[root@andromeda root]# cat /proc/key-users
	0:     46 45/45 1/100 13/10000
	29:     2 2/2 2/100 40/10000
	32:     2 2/2 2/100 40/10000
	38:     2 2/2 2/100 40/10000

     The format of each line is::

	<UID>:			User ID to which this applies
	<usage>			Structure refcount
	<inst>/<keys>		Total number of keys and number instantiated
	<keys>/<max>		Key count quota
	<bytes>/<max>		Key size quota


Four new sysctl files have been added also for the purpose of controlling the
quota limits on keys:

  *  /proc/sys/kernel/keys/root_maxkeys
     /proc/sys/kernel/keys/root_maxbytes

     These files hold the maximum number of keys that root may have and the
     maximum total number of bytes of data that root may have stored in those
     keys.

  *  /proc/sys/kernel/keys/maxkeys
     /proc/sys/kernel/keys/maxbytes

     These files hold the maximum number of keys that each non-root user may
     have and the maximum total number of bytes of data that each of those
     users may have stored in their keys.

Root may alter these by writing each new limit as a decimal number string to
the appropriate file.


Userspace System Call Interface
===============================

Userspace can manipulate keys directly through three new syscalls: add_key,
request_key and keyctl. The latter provides a number of functions for
manipulating keys.

When referring to a key directly, userspace programs should use the key's
serial number (a positive 32-bit integer). However, there are some special
values available for referring to special keys and keyrings that relate to the
process making the call::

	CONSTANT			VALUE	KEY REFERENCED
	==============================	======	===========================
	KEY_SPEC_THREAD_KEYRING		-1	thread-specific keyring
	KEY_SPEC_PROCESS_KEYRING	-2	process-specific keyring
	KEY_SPEC_SESSION_KEYRING	-3	session-specific keyring
	KEY_SPEC_USER_KEYRING		-4	UID-specific keyring
	KEY_SPEC_USER_SESSION_KEYRING	-5	UID-session keyring
	KEY_SPEC_GROUP_KEYRING		-6	GID-specific keyring
	KEY_SPEC_REQKEY_AUTH_KEY	-7	assumed request_key()
						  authorisation key


The main syscalls are:

  *  Create a new key of given type, description and payload and add it to the
     nominated keyring::

	key_serial_t add_key(const char *type, const char *desc,
			     const void *payload, size_t plen,
			     key_serial_t keyring);

     If a key of the same type and description as that proposed already exists
     in the keyring, this will try to update it with the given payload, or it
     will return error EEXIST if that function is not supported by the key
     type. The process must also have permission to write to the key to be able
     to update it. The new key will have all user permissions granted and no
     group or third party permissions.

     Otherwise, this will attempt to create a new key of the specified type and
     description, and to instantiate it with the supplied payload and attach it
     to the keyring. In this case, an error will be generated if the process
     does not have permission to write to the keyring.

     If the key type supports it, if the description is NULL or an empty
     string, the key type will try and generate a description from the content
     of the payload.

     The payload is optional, and the pointer can be NULL if not required by
     the type. The payload is plen in size, and plen can be zero for an empty
     payload.

     A new keyring can be generated by setting type "keyring", the keyring name
     as the description (or NULL) and setting the payload to NULL.

     User defined keys can be created by specifying type "user". It is
     recommended that a user defined key's description by prefixed with a type
     ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting
     ticket.

     Any other type must have been registered with the kernel in advance by a
     kernel service such as a filesystem.

     The ID of the new or updated key is returned if successful.


  *  Search the process's keyrings for a key, potentially calling out to
     userspace to create it::

	key_serial_t request_key(const char *type, const char *description,
				 const char *callout_info,
				 key_serial_t dest_keyring);

     This function searches all the process's keyrings in the order thread,
     process, session for a matching key. This works very much like
     KEYCTL_SEARCH, including the optional attachment of the discovered key to
     a keyring.

     If a key cannot be found, and if callout_info is not NULL, then
     /sbin/request-key will be invoked in an attempt to obtain a key. The
     callout_info string will be passed as an argument to the program.

     To link a key into the destination keyring the key must grant link
     permission on the key to the caller and the keyring must grant write
     permission.

     See also Documentation/security/keys/request-key.rst.


The keyctl syscall functions are:

  *  Map a special key ID to a real key ID for this process::

	key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id,
			    int create);

     The special key specified by "id" is looked up (with the key being created
     if necessary) and the ID of the key or keyring thus found is returned if
     it exists.

     If the key does not yet exist, the key will be created if "create" is
     non-zero; and the error ENOKEY will be returned if "create" is zero.


  *  Replace the session keyring this process subscribes to with a new one::

	key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name);

     If name is NULL, an anonymous keyring is created attached to the process
     as its session keyring, displacing the old session keyring.

     If name is not NULL, if a keyring of that name exists, the process
     attempts to attach it as the session keyring, returning an error if that
     is not permitted; otherwise a new keyring of that name is created and
     attached as the session keyring.

     To attach to a named keyring, the keyring must have search permission for
     the process's ownership.

     The ID of the new session keyring is returned if successful.


  *  Update the specified key::

	long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload,
		    size_t plen);

     This will try to update the specified key with the given payload, or it
     will return error EOPNOTSUPP if that function is not supported by the key
     type. The process must also have permission to write to the key to be able
     to update it.

     The payload is of length plen, and may be absent or empty as for
     add_key().


  *  Revoke a key::

	long keyctl(KEYCTL_REVOKE, key_serial_t key);

     This makes a key unavailable for further operations. Further attempts to
     use the key will be met with error EKEYREVOKED, and the key will no longer
     be findable.


  *  Change the ownership of a key::

	long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid);

     This function permits a key's owner and group ID to be changed. Either one
     of uid or gid can be set to -1 to suppress that change.

     Only the superuser can change a key's owner to something other than the
     key's current owner. Similarly, only the superuser can change a key's
     group ID to something other than the calling process's group ID or one of
     its group list members.


  *  Change the permissions mask on a key::

	long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm);

     This function permits the owner of a key or the superuser to change the
     permissions mask on a key.

     Only bits the available bits are permitted; if any other bits are set,
     error EINVAL will be returned.


  *  Describe a key::

	long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer,
		    size_t buflen);

     This function returns a summary of the key's attributes (but not its
     payload data) as a string in the buffer provided.

     Unless there's an error, it always returns the amount of data it could
     produce, even if that's too big for the buffer, but it won't copy more
     than requested to userspace. If the buffer pointer is NULL then no copy
     will take place.

     A process must have view permission on the key for this function to be
     successful.

     If successful, a string is placed in the buffer in the following format::

	<type>;<uid>;<gid>;<perm>;<description>

     Where type and description are strings, uid and gid are decimal, and perm
     is hexadecimal. A NUL character is included at the end of the string if
     the buffer is sufficiently big.

     This can be parsed with::

	sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc);


  *  Clear out a keyring::

	long keyctl(KEYCTL_CLEAR, key_serial_t keyring);

     This function clears the list of keys attached to a keyring. The calling
     process must have write permission on the keyring, and it must be a
     keyring (or else error ENOTDIR will result).

     This function can also be used to clear special kernel keyrings if they
     are appropriately marked if the user has CAP_SYS_ADMIN capability.  The
     DNS resolver cache keyring is an example of this.


  *  Link a key into a keyring::

	long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key);

     This function creates a link from the keyring to the key. The process must
     have write permission on the keyring and must have link permission on the
     key.

     Should the keyring not be a keyring, error ENOTDIR will result; and if the
     keyring is full, error ENFILE will result.

     The link procedure checks the nesting of the keyrings, returning ELOOP if
     it appears too deep or EDEADLK if the link would introduce a cycle.

     Any links within the keyring to keys that match the new key in terms of
     type and description will be discarded from the keyring as the new one is
     added.


  *  Move a key from one keyring to another::

	long keyctl(KEYCTL_MOVE,
		    key_serial_t id,
		    key_serial_t from_ring_id,
		    key_serial_t to_ring_id,
		    unsigned int flags);

     Move the key specified by "id" from the keyring specified by
     "from_ring_id" to the keyring specified by "to_ring_id".  If the two
     keyrings are the same, nothing is done.

     "flags" can have KEYCTL_MOVE_EXCL set in it to cause the operation to fail
     with EEXIST if a matching key exists in the destination keyring, otherwise
     such a key will be replaced.

     A process must have link permission on the key for this function to be
     successful and write permission on both keyrings.  Any errors that can
     occur from KEYCTL_LINK also apply on the destination keyring here.


  *  Unlink a key or keyring from another keyring::

	long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key);

     This function looks through the keyring for the first link to the
     specified key, and removes it if found. Subsequent links to that key are
     ignored. The process must have write permission on the keyring.

     If the keyring is not a keyring, error ENOTDIR will result; and if the key
     is not present, error ENOENT will be the result.


  *  Search a keyring tree for a key::

	key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring,
			    const char *type, const char *description,
			    key_serial_t dest_keyring);

     This searches the keyring tree headed by the specified keyring until a key
     is found that matches the type and description criteria. Each keyring is
     checked for keys before recursion into its children occurs.

     The process must have search permission on the top level keyring, or else
     error EACCES will result. Only keyrings that the process has search
     permission on will be recursed into, and only keys and keyrings for which
     a process has search permission can be matched. If the specified keyring
     is not a keyring, ENOTDIR will result.

     If the search succeeds, the function will attempt to link the found key
     into the destination keyring if one is supplied (non-zero ID). All the
     constraints applicable to KEYCTL_LINK apply in this case too.

     Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search
     fails. On success, the resulting key ID will be returned.


  *  Read the payload data from a key::

	long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
		    size_t buflen);

     This function attempts to read the payload data from the specified key
     into the buffer. The process must have read permission on the key to
     succeed.

     The returned data will be processed for presentation by the key type. For
     instance, a keyring will return an array of key_serial_t entries
     representing the IDs of all the keys to which it is subscribed. The user
     defined key type will return its data as is. If a key type does not
     implement this function, error EOPNOTSUPP will result.

     If the specified buffer is too small, then the size of the buffer required
     will be returned.  Note that in this case, the contents of the buffer may
     have been overwritten in some undefined way.

     Otherwise, on success, the function will return the amount of data copied
     into the buffer.

  *  Instantiate a partially constructed key::

	long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
		    const void *payload, size_t plen,
		    key_serial_t keyring);
	long keyctl(KEYCTL_INSTANTIATE_IOV, key_serial_t key,
		    const struct iovec *payload_iov, unsigned ioc,
		    key_serial_t keyring);

     If the kernel calls back to userspace to complete the instantiation of a
     key, userspace should use this call to supply data for the key before the
     invoked process returns, or else the key will be marked negative
     automatically.

     The process must have write access on the key to be able to instantiate
     it, and the key must be uninstantiated.

     If a keyring is specified (non-zero), the key will also be linked into
     that keyring, however all the constraints applying in KEYCTL_LINK apply in
     this case too.

     The payload and plen arguments describe the payload data as for add_key().

     The payload_iov and ioc arguments describe the payload data in an iovec
     array instead of a single buffer.


  *  Negatively instantiate a partially constructed key::

	long keyctl(KEYCTL_NEGATE, key_serial_t key,
		    unsigned timeout, key_serial_t keyring);
	long keyctl(KEYCTL_REJECT, key_serial_t key,
		    unsigned timeout, unsigned error, key_serial_t keyring);

     If the kernel calls back to userspace to complete the instantiation of a
     key, userspace should use this call mark the key as negative before the
     invoked process returns if it is unable to fulfill the request.

     The process must have write access on the key to be able to instantiate
     it, and the key must be uninstantiated.

     If a keyring is specified (non-zero), the key will also be linked into
     that keyring, however all the constraints applying in KEYCTL_LINK apply in
     this case too.

     If the key is rejected, future searches for it will return the specified
     error code until the rejected key expires.  Negating the key is the same
     as rejecting the key with ENOKEY as the error code.


  *  Set the default request-key destination keyring::

	long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl);

     This sets the default keyring to which implicitly requested keys will be
     attached for this thread. reqkey_defl should be one of these constants::

	CONSTANT				VALUE	NEW DEFAULT KEYRING
	======================================	======	=======================
	KEY_REQKEY_DEFL_NO_CHANGE		-1	No change
	KEY_REQKEY_DEFL_DEFAULT			0	Default[1]
	KEY_REQKEY_DEFL_THREAD_KEYRING		1	Thread keyring
	KEY_REQKEY_DEFL_PROCESS_KEYRING		2	Process keyring
	KEY_REQKEY_DEFL_SESSION_KEYRING		3	Session keyring
	KEY_REQKEY_DEFL_USER_KEYRING		4	User keyring
	KEY_REQKEY_DEFL_USER_SESSION_KEYRING	5	User session keyring
	KEY_REQKEY_DEFL_GROUP_KEYRING		6	Group keyring

     The old default will be returned if successful and error EINVAL will be
     returned if reqkey_defl is not one of the above values.

     The default keyring can be overridden by the keyring indicated to the
     request_key() system call.

     Note that this setting is inherited across fork/exec.

     [1] The default is: the thread keyring if there is one, otherwise
     the process keyring if there is one, otherwise the session keyring if
     there is one, otherwise the user default session keyring.


  *  Set the timeout on a key::

	long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout);

     This sets or clears the timeout on a key. The timeout can be 0 to clear
     the timeout or a number of seconds to set the expiry time that far into
     the future.

     The process must have attribute modification access on a key to set its
     timeout. Timeouts may not be set with this function on negative, revoked
     or expired keys.


  *  Assume the authority granted to instantiate a key::

	long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key);

     This assumes or divests the authority required to instantiate the
     specified key. Authority can only be assumed if the thread has the
     authorisation key associated with the specified key in its keyrings
     somewhere.

     Once authority is assumed, searches for keys will also search the
     requester's keyrings using the requester's security label, UID, GID and
     groups.

     If the requested authority is unavailable, error EPERM will be returned,
     likewise if the authority has been revoked because the target key is
     already instantiated.

     If the specified key is 0, then any assumed authority will be divested.

     The assumed authoritative key is inherited across fork and exec.


  *  Get the LSM security context attached to a key::

	long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer,
		    size_t buflen)

     This function returns a string that represents the LSM security context
     attached to a key in the buffer provided.

     Unless there's an error, it always returns the amount of data it could
     produce, even if that's too big for the buffer, but it won't copy more
     than requested to userspace. If the buffer pointer is NULL then no copy
     will take place.

     A NUL character is included at the end of the string if the buffer is
     sufficiently big.  This is included in the returned count.  If no LSM is
     in force then an empty string will be returned.

     A process must have view permission on the key for this function to be
     successful.


  *  Install the calling process's session keyring on its parent::

	long keyctl(KEYCTL_SESSION_TO_PARENT);

     This functions attempts to install the calling process's session keyring
     on to the calling process's parent, replacing the parent's current session
     keyring.

     The calling process must have the same ownership as its parent, the
     keyring must have the same ownership as the calling process, the calling
     process must have LINK permission on the keyring and the active LSM module
     mustn't deny permission, otherwise error EPERM will be returned.

     Error ENOMEM will be returned if there was insufficient memory to complete
     the operation, otherwise 0 will be returned to indicate success.

     The keyring will be replaced next time the parent process leaves the
     kernel and resumes executing userspace.


  *  Invalidate a key::

	long keyctl(KEYCTL_INVALIDATE, key_serial_t key);

     This function marks a key as being invalidated and then wakes up the
     garbage collector.  The garbage collector immediately removes invalidated
     keys from all keyrings and deletes the key when its reference count
     reaches zero.

     Keys that are marked invalidated become invisible to normal key operations
     immediately, though they are still visible in /proc/keys until deleted
     (they're marked with an 'i' flag).

     A process must have search permission on the key for this function to be
     successful.

  *  Compute a Diffie-Hellman shared secret or public key::

	long keyctl(KEYCTL_DH_COMPUTE, struct keyctl_dh_params *params,
		    char *buffer, size_t buflen, struct keyctl_kdf_params *kdf);

     The params struct contains serial numbers for three keys::

	 - The prime, p, known to both parties
	 - The local private key
	 - The base integer, which is either a shared generator or the
	   remote public key

     The value computed is::

	result = base ^ private (mod prime)

     If the base is the shared generator, the result is the local
     public key.  If the base is the remote public key, the result is
     the shared secret.

     If the parameter kdf is NULL, the following applies:

	 - The buffer length must be at least the length of the prime, or zero.

	 - If the buffer length is nonzero, the length of the result is
	   returned when it is successfully calculated and copied in to the
	   buffer. When the buffer length is zero, the minimum required
	   buffer length is returned.

     The kdf parameter allows the caller to apply a key derivation function
     (KDF) on the Diffie-Hellman computation where only the result
     of the KDF is returned to the caller. The KDF is characterized with
     struct keyctl_kdf_params as follows:

	 - ``char *hashname`` specifies the NUL terminated string identifying
	   the hash used from the kernel crypto API and applied for the KDF
	   operation. The KDF implemenation complies with SP800-56A as well
	   as with SP800-108 (the counter KDF).

	 - ``char *otherinfo`` specifies the OtherInfo data as documented in
	   SP800-56A section 5.8.1.2. The length of the buffer is given with
	   otherinfolen. The format of OtherInfo is defined by the caller.
	   The otherinfo pointer may be NULL if no OtherInfo shall be used.

     This function will return error EOPNOTSUPP if the key type is not
     supported, error ENOKEY if the key could not be found, or error
     EACCES if the key is not readable by the caller. In addition, the
     function will return EMSGSIZE when the parameter kdf is non-NULL
     and either the buffer length or the OtherInfo length exceeds the
     allowed length.


  *  Restrict keyring linkage::

	long keyctl(KEYCTL_RESTRICT_KEYRING, key_serial_t keyring,
		    const char *type, const char *restriction);

     An existing keyring can restrict linkage of additional keys by evaluating
     the contents of the key according to a restriction scheme.

     "keyring" is the key ID for an existing keyring to apply a restriction
     to. It may be empty or may already have keys linked. Existing linked keys
     will remain in the keyring even if the new restriction would reject them.

     "type" is a registered key type.

     "restriction" is a string describing how key linkage is to be restricted.
     The format varies depending on the key type, and the string is passed to
     the lookup_restriction() function for the requested type.  It may specify
     a method and relevant data for the restriction such as signature
     verification or constraints on key payload. If the requested key type is
     later unregistered, no keys may be added to the keyring after the key type
     is removed.

     To apply a keyring restriction the process must have Set Attribute
     permission and the keyring must not be previously restricted.

     One application of restricted keyrings is to verify X.509 certificate
     chains or individual certificate signatures using the asymmetric key type.
     See Documentation/crypto/asymmetric-keys.rst for specific restrictions
     applicable to the asymmetric key type.


  *  Query an asymmetric key::

	long keyctl(KEYCTL_PKEY_QUERY,
		    key_serial_t key_id, unsigned long reserved,
		    const char *params,
		    struct keyctl_pkey_query *info);

     Get information about an asymmetric key.  Specific algorithms and
     encodings may be queried by using the ``params`` argument.  This is a
     string containing a space- or tab-separated string of key-value pairs.
     Currently supported keys include ``enc`` and ``hash``.  The information
     is returned in the keyctl_pkey_query struct::

	__u32	supported_ops;
	__u32	key_size;
	__u16	max_data_size;
	__u16	max_sig_size;
	__u16	max_enc_size;
	__u16	max_dec_size;
	__u32	__spare[10];

     ``supported_ops`` contains a bit mask of flags indicating which ops are
     supported.  This is constructed from a bitwise-OR of::

	KEYCTL_SUPPORTS_{ENCRYPT,DECRYPT,SIGN,VERIFY}

     ``key_size`` indicated the size of the key in bits.

     ``max_*_size`` indicate the maximum sizes in bytes of a blob of data to be
     signed, a signature blob, a blob to be encrypted and a blob to be
     decrypted.

     ``__spare[]`` must be set to 0.  This is intended for future use to hand
     over one or more passphrases needed unlock a key.

     If successful, 0 is returned.  If the key is not an asymmetric key,
     EOPNOTSUPP is returned.


  *  Encrypt, decrypt, sign or verify a blob using an asymmetric key::

	long keyctl(KEYCTL_PKEY_ENCRYPT,
		    const struct keyctl_pkey_params *params,
		    const char *info,
		    const void *in,
		    void *out);

	long keyctl(KEYCTL_PKEY_DECRYPT,
		    const struct keyctl_pkey_params *params,
		    const char *info,
		    const void *in,
		    void *out);

	long keyctl(KEYCTL_PKEY_SIGN,
		    const struct keyctl_pkey_params *params,
		    const char *info,
		    const void *in,
		    void *out);

	long keyctl(KEYCTL_PKEY_VERIFY,
		    const struct keyctl_pkey_params *params,
		    const char *info,
		    const void *in,
		    const void *in2);

     Use an asymmetric key to perform a public-key cryptographic operation a
     blob of data.  For encryption and verification, the asymmetric key may
     only need the public parts to be available, but for decryption and signing
     the private parts are required also.

     The parameter block pointed to by params contains a number of integer
     values::

	__s32		key_id;
	__u32		in_len;
	__u32		out_len;
	__u32		in2_len;

     ``key_id`` is the ID of the asymmetric key to be used.  ``in_len`` and
     ``in2_len`` indicate the amount of data in the in and in2 buffers and
     ``out_len`` indicates the size of the out buffer as appropriate for the
     above operations.

     For a given operation, the in and out buffers are used as follows::

	Operation ID		in,in_len	out,out_len	in2,in2_len
	=======================	===============	===============	===============
	KEYCTL_PKEY_ENCRYPT	Raw data	Encrypted data	-
	KEYCTL_PKEY_DECRYPT	Encrypted data	Raw data	-
	KEYCTL_PKEY_SIGN	Raw data	Signature	-
	KEYCTL_PKEY_VERIFY	Raw data	-		Signature

     ``info`` is a string of key=value pairs that supply supplementary
     information.  These include:

	``enc=<encoding>`` The encoding of the encrypted/signature blob.  This
			can be "pkcs1" for RSASSA-PKCS1-v1.5 or
			RSAES-PKCS1-v1.5; "pss" for "RSASSA-PSS"; "oaep" for
			"RSAES-OAEP".  If omitted or is "raw", the raw output
			of the encryption function is specified.

	``hash=<algo>``	If the data buffer contains the output of a hash
			function and the encoding includes some indication of
			which hash function was used, the hash function can be
			specified with this, eg. "hash=sha256".

     The ``__spare[]`` space in the parameter block must be set to 0.  This is
     intended, amongst other things, to allow the passing of passphrases
     required to unlock a key.

     If successful, encrypt, decrypt and sign all return the amount of data
     written into the output buffer.  Verification returns 0 on success.


  *  Watch a key or keyring for changes::

	long keyctl(KEYCTL_WATCH_KEY, key_serial_t key, int queue_fd,
		    const struct watch_notification_filter *filter);

     This will set or remove a watch for changes on the specified key or
     keyring.

     "key" is the ID of the key to be watched.

     "queue_fd" is a file descriptor referring to an open pipe which
     manages the buffer into which notifications will be delivered.

     "filter" is either NULL to remove a watch or a filter specification to
     indicate what events are required from the key.

     See Documentation/core-api/watch_queue.rst for more information.

     Note that only one watch may be emplaced for any particular { key,
     queue_fd } combination.

     Notification records look like::

	struct key_notification {
		struct watch_notification watch;
		__u32	key_id;
		__u32	aux;
	};

     In this, watch::type will be "WATCH_TYPE_KEY_NOTIFY" and subtype will be
     one of::

	NOTIFY_KEY_INSTANTIATED
	NOTIFY_KEY_UPDATED
	NOTIFY_KEY_LINKED
	NOTIFY_KEY_UNLINKED
	NOTIFY_KEY_CLEARED
	NOTIFY_KEY_REVOKED
	NOTIFY_KEY_INVALIDATED
	NOTIFY_KEY_SETATTR

     Where these indicate a key being instantiated/rejected, updated, a link
     being made in a keyring, a link being removed from a keyring, a keyring
     being cleared, a key being revoked, a key being invalidated or a key
     having one of its attributes changed (user, group, perm, timeout,
     restriction).

     If a watched key is deleted, a basic watch_notification will be issued
     with "type" set to WATCH_TYPE_META and "subtype" set to
     watch_meta_removal_notification.  The watchpoint ID will be set in the
     "info" field.

     This needs to be configured by enabling:

	"Provide key/keyring change notifications" (KEY_NOTIFICATIONS)


Kernel Services
===============

The kernel services for key management are fairly simple to deal with. They can
be broken down into two areas: keys and key types.

Dealing with keys is fairly straightforward. Firstly, the kernel service
registers its type, then it searches for a key of that type. It should retain
the key as long as it has need of it, and then it should release it. For a
filesystem or device file, a search would probably be performed during the open
call, and the key released upon close. How to deal with conflicting keys due to
two different users opening the same file is left to the filesystem author to
solve.

To access the key manager, the following header must be #included::

	<linux/key.h>

Specific key types should have a header file under include/keys/ that should be
used to access that type.  For keys of type "user", for example, that would be::

	<keys/user-type.h>

Note that there are two different types of pointers to keys that may be
encountered:

  *  struct key *

     This simply points to the key structure itself. Key structures will be at
     least four-byte aligned.

  *  key_ref_t

     This is equivalent to a ``struct key *``, but the least significant bit is set
     if the caller "possesses" the key. By "possession" it is meant that the
     calling processes has a searchable link to the key from one of its
     keyrings. There are three functions for dealing with these::

	key_ref_t make_key_ref(const struct key *key, bool possession);

	struct key *key_ref_to_ptr(const key_ref_t key_ref);

	bool is_key_possessed(const key_ref_t key_ref);

     The first function constructs a key reference from a key pointer and
     possession information (which must be true or false).

     The second function retrieves the key pointer from a reference and the
     third retrieves the possession flag.

When accessing a key's payload contents, certain precautions must be taken to
prevent access vs modification races. See the section "Notes on accessing
payload contents" for more information.

 *  To search for a key, call::

	struct key *request_key(const struct key_type *type,
				const char *description,
				const char *callout_info);

    This is used to request a key or keyring with a description that matches
    the description specified according to the key type's match_preparse()
    method. This permits approximate matching to occur. If callout_string is
    not NULL, then /sbin/request-key will be invoked in an attempt to obtain
    the key from userspace. In that case, callout_string will be passed as an
    argument to the program.

    Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
    returned.

    If successful, the key will have been attached to the default keyring for
    implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.

    See also Documentation/security/keys/request-key.rst.


 *  To search for a key in a specific domain, call::

	struct key *request_key_tag(const struct key_type *type,
				    const char *description,
				    struct key_tag *domain_tag,
				    const char *callout_info);

    This is identical to request_key(), except that a domain tag may be
    specifies that causes search algorithm to only match keys matching that
    tag.  The domain_tag may be NULL, specifying a global domain that is
    separate from any nominated domain.


 *  To search for a key, passing auxiliary data to the upcaller, call::

	struct key *request_key_with_auxdata(const struct key_type *type,
					     const char *description,
					     struct key_tag *domain_tag,
					     const void *callout_info,
					     size_t callout_len,
					     void *aux);

    This is identical to request_key_tag(), except that the auxiliary data is
    passed to the key_type->request_key() op if it exists, and the
    callout_info is a blob of length callout_len, if given (the length may be
    0).


 *  To search for a key under RCU conditions, call::

	struct key *request_key_rcu(const struct key_type *type,
				    const char *description,
				    struct key_tag *domain_tag);

    which is similar to request_key_tag() except that it does not check for
    keys that are under construction and it will not call out to userspace to
    construct a key if it can't find a match.


 *  When it is no longer required, the key should be released using::

	void key_put(struct key *key);

    Or::

	void key_ref_put(key_ref_t key_ref);

    These can be called from interrupt context. If CONFIG_KEYS is not set then
    the argument will not be parsed.


 *  Extra references can be made to a key by calling one of the following
    functions::

	struct key *__key_get(struct key *key);
	struct key *key_get(struct key *key);

    Keys so references will need to be disposed of by calling key_put() when
    they've been finished with.  The key pointer passed in will be returned.

    In the case of key_get(), if the pointer is NULL or CONFIG_KEYS is not set
    then the key will not be dereferenced and no increment will take place.


 *  A key's serial number can be obtained by calling::

	key_serial_t key_serial(struct key *key);

    If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the
    latter case without parsing the argument).


 *  If a keyring was found in the search, this can be further searched by::

	key_ref_t keyring_search(key_ref_t keyring_ref,
				 const struct key_type *type,
				 const char *description,
				 bool recurse)

    This searches the specified keyring only (recurse == false) or keyring tree
    (recurse == true) specified for a matching key. Error ENOKEY is returned
    upon failure (use IS_ERR/PTR_ERR to determine). If successful, the returned
    key will need to be released.

    The possession attribute from the keyring reference is used to control
    access through the permissions mask and is propagated to the returned key
    reference pointer if successful.


 *  A keyring can be created by::

	struct key *keyring_alloc(const char *description, uid_t uid, gid_t gid,
				  const struct cred *cred,
				  key_perm_t perm,
				  struct key_restriction *restrict_link,
				  unsigned long flags,
				  struct key *dest);

    This creates a keyring with the given attributes and returns it.  If dest
    is not NULL, the new keyring will be linked into the keyring to which it
    points.  No permission checks are made upon the destination keyring.

    Error EDQUOT can be returned if the keyring would overload the quota (pass
    KEY_ALLOC_NOT_IN_QUOTA in flags if the keyring shouldn't be accounted
    towards the user's quota).  Error ENOMEM can also be returned.

    If restrict_link is not NULL, it should point to a structure that contains
    the function that will be called each time an attempt is made to link a
    key into the new keyring.  The structure may also contain a key pointer
    and an associated key type.  The function is called to check whether a key
    may be added into the keyring or not.  The key type is used by the garbage
    collector to clean up function or data pointers in this structure if the
    given key type is unregistered.  Callers of key_create_or_update() within
    the kernel can pass KEY_ALLOC_BYPASS_RESTRICTION to suppress the check.
    An example of using this is to manage rings of cryptographic keys that are
    set up when the kernel boots where userspace is also permitted to add keys
    - provided they can be verified by a key the kernel already has.

    When called, the restriction function will be passed the keyring being
    added to, the key type, the payload of the key being added, and data to be
    used in the restriction check.  Note that when a new key is being created,
    this is called between payload preparsing and actual key creation.  The
    function should return 0 to allow the link or an error to reject it.

    A convenience function, restrict_link_reject, exists to always return
    -EPERM to in this case.


 *  To check the validity of a key, this function can be called::

	int validate_key(struct key *key);

    This checks that the key in question hasn't expired or and hasn't been
    revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will
    be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be
    returned (in the latter case without parsing the argument).


 *  To register a key type, the following function should be called::

	int register_key_type(struct key_type *type);

    This will return error EEXIST if a type of the same name is already
    present.


 *  To unregister a key type, call::

	void unregister_key_type(struct key_type *type);


Under some circumstances, it may be desirable to deal with a bundle of keys.
The facility provides access to the keyring type for managing such a bundle::

	struct key_type key_type_keyring;

This can be used with a function such as request_key() to find a specific
keyring in a process's keyrings.  A keyring thus found can then be searched
with keyring_search().  Note that it is not possible to use request_key() to
search a specific keyring, so using keyrings in this way is of limited utility.


Notes On Accessing Payload Contents
===================================

The simplest payload is just data stored in key->payload directly.  In this
case, there's no need to indulge in RCU or locking when accessing the payload.

More complex payload contents must be allocated and pointers to them set in the
key->payload.data[] array.  One of the following ways must be selected to
access the data:

  1) Unmodifiable key type.

     If the key type does not have a modify method, then the key's payload can
     be accessed without any form of locking, provided that it's known to be
     instantiated (uninstantiated keys cannot be "found").

  2) The key's semaphore.

     The semaphore could be used to govern access to the payload and to control
     the payload pointer. It must be write-locked for modifications and would
     have to be read-locked for general access. The disadvantage of doing this
     is that the accessor may be required to sleep.

  3) RCU.

     RCU must be used when the semaphore isn't already held; if the semaphore
     is held then the contents can't change under you unexpectedly as the
     semaphore must still be used to serialise modifications to the key. The
     key management code takes care of this for the key type.

     However, this means using::

	rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock()

     to read the pointer, and::

	rcu_dereference() ... rcu_assign_pointer() ... call_rcu()

     to set the pointer and dispose of the old contents after a grace period.
     Note that only the key type should ever modify a key's payload.

     Furthermore, an RCU controlled payload must hold a struct rcu_head for the
     use of call_rcu() and, if the payload is of variable size, the length of
     the payload. key->datalen cannot be relied upon to be consistent with the
     payload just dereferenced if the key's semaphore is not held.

     Note that key->payload.data[0] has a shadow that is marked for __rcu
     usage.  This is called key->payload.rcu_data0.  The following accessors
     wrap the RCU calls to this element:

     a) Set or change the first payload pointer::

		rcu_assign_keypointer(struct key *key, void *data);

     b) Read the first payload pointer with the key semaphore held::

		[const] void *dereference_key_locked([const] struct key *key);

	 Note that the return value will inherit its constness from the key
	 parameter.  Static analysis will give an error if it things the lock
	 isn't held.

     c) Read the first payload pointer with the RCU read lock held::

		const void *dereference_key_rcu(const struct key *key);


Defining a Key Type
===================

A kernel service may want to define its own key type. For instance, an AFS
filesystem might want to define a Kerberos 5 ticket key type. To do this, it
author fills in a key_type struct and registers it with the system.

Source files that implement key types should include the following header file::

	<linux/key-type.h>

The structure has a number of fields, some of which are mandatory:

  *  ``const char *name``

     The name of the key type. This is used to translate a key type name
     supplied by userspace into a pointer to the structure.


  *  ``size_t def_datalen``

     This is optional - it supplies the default payload data length as
     contributed to the quota. If the key type's payload is always or almost
     always the same size, then this is a more efficient way to do things.

     The data length (and quota) on a particular key can always be changed
     during instantiation or update by calling::

	int key_payload_reserve(struct key *key, size_t datalen);

     With the revised data length. Error EDQUOT will be returned if this is not
     viable.


  *  ``int (*vet_description)(const char *description);``

     This optional method is called to vet a key description.  If the key type
     doesn't approve of the key description, it may return an error, otherwise
     it should return 0.


  *  ``int (*preparse)(struct key_preparsed_payload *prep);``

     This optional method permits the key type to attempt to parse payload
     before a key is created (add key) or the key semaphore is taken (update or
     instantiate key).  The structure pointed to by prep looks like::

	struct key_preparsed_payload {
		char		*description;
		union key_payload payload;
		const void	*data;
		size_t		datalen;
		size_t		quotalen;
		time_t		expiry;
	};

     Before calling the method, the caller will fill in data and datalen with
     the payload blob parameters; quotalen will be filled in with the default
     quota size from the key type; expiry will be set to TIME_T_MAX and the
     rest will be cleared.

     If a description can be proposed from the payload contents, that should be
     attached as a string to the description field.  This will be used for the
     key description if the caller of add_key() passes NULL or "".

     The method can attach anything it likes to payload.  This is merely passed
     along to the instantiate() or update() operations.  If set, the expiry
     time will be applied to the key if it is instantiated from this data.

     The method should return 0 if successful or a negative error code
     otherwise.


  *  ``void (*free_preparse)(struct key_preparsed_payload *prep);``

     This method is only required if the preparse() method is provided,
     otherwise it is unused.  It cleans up anything attached to the description
     and payload fields of the key_preparsed_payload struct as filled in by the
     preparse() method.  It will always be called after preparse() returns
     successfully, even if instantiate() or update() succeed.


  *  ``int (*instantiate)(struct key *key, struct key_preparsed_payload *prep);``

     This method is called to attach a payload to a key during construction.
     The payload attached need not bear any relation to the data passed to this
     function.

     The prep->data and prep->datalen fields will define the original payload
     blob.  If preparse() was supplied then other fields may be filled in also.

     If the amount of data attached to the key differs from the size in
     keytype->def_datalen, then key_payload_reserve() should be called.

     This method does not have to lock the key in order to attach a payload.
     The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
     anything else from gaining access to the key.

     It is safe to sleep in this method.

     generic_key_instantiate() is provided to simply copy the data from
     prep->payload.data[] to key->payload.data[], with RCU-safe assignment on
     the first element.  It will then clear prep->payload.data[] so that the
     free_preparse method doesn't release the data.


  *  ``int (*update)(struct key *key, const void *data, size_t datalen);``

     If this type of key can be updated, then this method should be provided.
     It is called to update a key's payload from the blob of data provided.

     The prep->data and prep->datalen fields will define the original payload
     blob.  If preparse() was supplied then other fields may be filled in also.

     key_payload_reserve() should be called if the data length might change
     before any changes are actually made. Note that if this succeeds, the type
     is committed to changing the key because it's already been altered, so all
     memory allocation must be done first.

     The key will have its semaphore write-locked before this method is called,
     but this only deters other writers; any changes to the key's payload must
     be made under RCU conditions, and call_rcu() must be used to dispose of
     the old payload.

     key_payload_reserve() should be called before the changes are made, but
     after all allocations and other potentially failing function calls are
     made.

     It is safe to sleep in this method.


  *  ``int (*match_preparse)(struct key_match_data *match_data);``

     This method is optional.  It is called when a key search is about to be
     performed.  It is given the following structure::

	struct key_match_data {
		bool (*cmp)(const struct key *key,
			    const struct key_match_data *match_data);
		const void	*raw_data;
		void		*preparsed;
		unsigned	lookup_type;
	};

     On entry, raw_data will be pointing to the criteria to be used in matching
     a key by the caller and should not be modified.  ``(*cmp)()`` will be pointing
     to the default matcher function (which does an exact description match
     against raw_data) and lookup_type will be set to indicate a direct lookup.

     The following lookup_type values are available:

       *  KEYRING_SEARCH_LOOKUP_DIRECT - A direct lookup hashes the type and
      	  description to narrow down the search to a small number of keys.

       *  KEYRING_SEARCH_LOOKUP_ITERATE - An iterative lookup walks all the
      	  keys in the keyring until one is matched.  This must be used for any
      	  search that's not doing a simple direct match on the key description.

     The method may set cmp to point to a function of its choice that does some
     other form of match, may set lookup_type to KEYRING_SEARCH_LOOKUP_ITERATE
     and may attach something to the preparsed pointer for use by ``(*cmp)()``.
     ``(*cmp)()`` should return true if a key matches and false otherwise.

     If preparsed is set, it may be necessary to use the match_free() method to
     clean it up.

     The method should return 0 if successful or a negative error code
     otherwise.

     It is permitted to sleep in this method, but ``(*cmp)()`` may not sleep as
     locks will be held over it.

     If match_preparse() is not provided, keys of this type will be matched
     exactly by their description.


  *  ``void (*match_free)(struct key_match_data *match_data);``

     This method is optional.  If given, it called to clean up
     match_data->preparsed after a successful call to match_preparse().


  *  ``void (*revoke)(struct key *key);``

     This method is optional.  It is called to discard part of the payload
     data upon a key being revoked.  The caller will have the key semaphore
     write-locked.

     It is safe to sleep in this method, though care should be taken to avoid
     a deadlock against the key semaphore.


  *  ``void (*destroy)(struct key *key);``

     This method is optional. It is called to discard the payload data on a key
     when it is being destroyed.

     This method does not need to lock the key to access the payload; it can
     consider the key as being inaccessible at this time. Note that the key's
     type may have been changed before this function is called.

     It is not safe to sleep in this method; the caller may hold spinlocks.


  *  ``void (*describe)(const struct key *key, struct seq_file *p);``

     This method is optional. It is called during /proc/keys reading to
     summarise a key's description and payload in text form.

     This method will be called with the RCU read lock held. rcu_dereference()
     should be used to read the payload pointer if the payload is to be
     accessed. key->datalen cannot be trusted to stay consistent with the
     contents of the payload.

     The description will not change, though the key's state may.

     It is not safe to sleep in this method; the RCU read lock is held by the
     caller.


  *  ``long (*read)(const struct key *key, char __user *buffer, size_t buflen);``

     This method is optional. It is called by KEYCTL_READ to translate the
     key's payload into something a blob of data for userspace to deal with.
     Ideally, the blob should be in the same format as that passed in to the
     instantiate and update methods.

     If successful, the blob size that could be produced should be returned
     rather than the size copied.

     This method will be called with the key's semaphore read-locked. This will
     prevent the key's payload changing. It is not necessary to use RCU locking
     when accessing the key's payload. It is safe to sleep in this method, such
     as might happen when the userspace buffer is accessed.


  *  ``int (*request_key)(struct key_construction *cons, const char *op, void *aux);``

     This method is optional.  If provided, request_key() and friends will
     invoke this function rather than upcalling to /sbin/request-key to operate
     upon a key of this type.

     The aux parameter is as passed to request_key_async_with_auxdata() and
     similar or is NULL otherwise.  Also passed are the construction record for
     the key to be operated upon and the operation type (currently only
     "create").

     This method is permitted to return before the upcall is complete, but the
     following function must be called under all circumstances to complete the
     instantiation process, whether or not it succeeds, whether or not there's
     an error::

	void complete_request_key(struct key_construction *cons, int error);

     The error parameter should be 0 on success, -ve on error.  The
     construction record is destroyed by this action and the authorisation key
     will be revoked.  If an error is indicated, the key under construction
     will be negatively instantiated if it wasn't already instantiated.

     If this method returns an error, that error will be returned to the
     caller of request_key*().  complete_request_key() must be called prior to
     returning.

     The key under construction and the authorisation key can be found in the
     key_construction struct pointed to by cons:

      *  ``struct key *key;``

     	 The key under construction.

      *  ``struct key *authkey;``

     	 The authorisation key.


  *  ``struct key_restriction *(*lookup_restriction)(const char *params);``

     This optional method is used to enable userspace configuration of keyring
     restrictions. The restriction parameter string (not including the key type
     name) is passed in, and this method returns a pointer to a key_restriction
     structure containing the relevant functions and data to evaluate each
     attempted key link operation. If there is no match, -EINVAL is returned.


  *  ``asym_eds_op`` and ``asym_verify_signature``::

       int (*asym_eds_op)(struct kernel_pkey_params *params,
			  const void *in, void *out);
       int (*asym_verify_signature)(struct kernel_pkey_params *params,
				    const void *in, const void *in2);

     These methods are optional.  If provided the first allows a key to be
     used to encrypt, decrypt or sign a blob of data, and the second allows a
     key to verify a signature.

     In all cases, the following information is provided in the params block::

	struct kernel_pkey_params {
		struct key	*key;
		const char	*encoding;
		const char	*hash_algo;
		char		*info;
		__u32		in_len;
		union {
			__u32	out_len;
			__u32	in2_len;
		};
		enum kernel_pkey_operation op : 8;
	};

     This includes the key to be used; a string indicating the encoding to use
     (for instance, "pkcs1" may be used with an RSA key to indicate
     RSASSA-PKCS1-v1.5 or RSAES-PKCS1-v1.5 encoding or "raw" if no encoding);
     the name of the hash algorithm used to generate the data for a signature
     (if appropriate); the sizes of the input and output (or second input)
     buffers; and the ID of the operation to be performed.

     For a given operation ID, the input and output buffers are used as
     follows::

	Operation ID		in,in_len	out,out_len	in2,in2_len
	=======================	===============	===============	===============
	kernel_pkey_encrypt	Raw data	Encrypted data	-
	kernel_pkey_decrypt	Encrypted data	Raw data	-
	kernel_pkey_sign	Raw data	Signature	-
	kernel_pkey_verify	Raw data	-		Signature

     asym_eds_op() deals with encryption, decryption and signature creation as
     specified by params->op.  Note that params->op is also set for
     asym_verify_signature().

     Encrypting and signature creation both take raw data in the input buffer
     and return the encrypted result in the output buffer.  Padding may have
     been added if an encoding was set.  In the case of signature creation,
     depending on the encoding, the padding created may need to indicate the
     digest algorithm - the name of which should be supplied in hash_algo.

     Decryption takes encrypted data in the input buffer and returns the raw
     data in the output buffer.  Padding will get checked and stripped off if
     an encoding was set.

     Verification takes raw data in the input buffer and the signature in the
     second input buffer and checks that the one matches the other.  Padding
     will be validated.  Depending on the encoding, the digest algorithm used
     to generate the raw data may need to be indicated in hash_algo.

     If successful, asym_eds_op() should return the number of bytes written
     into the output buffer.  asym_verify_signature() should return 0.

     A variety of errors may be returned, including EOPNOTSUPP if the operation
     is not supported; EKEYREJECTED if verification fails; ENOPKG if the
     required crypto isn't available.


  *  ``asym_query``::

       int (*asym_query)(const struct kernel_pkey_params *params,
			 struct kernel_pkey_query *info);

     This method is optional.  If provided it allows information about the
     public or asymmetric key held in the key to be determined.

     The parameter block is as for asym_eds_op() and co. but in_len and out_len
     are unused.  The encoding and hash_algo fields should be used to reduce
     the returned buffer/data sizes as appropriate.

     If successful, the following information is filled in::

	struct kernel_pkey_query {
		__u32		supported_ops;
		__u32		key_size;
		__u16		max_data_size;
		__u16		max_sig_size;
		__u16		max_enc_size;
		__u16		max_dec_size;
	};

     The supported_ops field will contain a bitmask indicating what operations
     are supported by the key, including encryption of a blob, decryption of a
     blob, signing a blob and verifying the signature on a blob.  The following
     constants are defined for this::

	KEYCTL_SUPPORTS_{ENCRYPT,DECRYPT,SIGN,VERIFY}

     The key_size field is the size of the key in bits.  max_data_size and
     max_sig_size are the maximum raw data and signature sizes for creation and
     verification of a signature; max_enc_size and max_dec_size are the maximum
     raw data and signature sizes for encryption and decryption.  The
     max_*_size fields are measured in bytes.

     If successful, 0 will be returned.  If the key doesn't support this,
     EOPNOTSUPP will be returned.


Request-Key Callback Service
============================

To create a new key, the kernel will attempt to execute the following command
line::

	/sbin/request-key create <key> <uid> <gid> \
		<threadring> <processring> <sessionring> <callout_info>

<key> is the key being constructed, and the three keyrings are the process
keyrings from the process that caused the search to be issued. These are
included for two reasons:

   1  There may be an authentication token in one of the keyrings that is
      required to obtain the key, eg: a Kerberos Ticket-Granting Ticket.

   2  The new key should probably be cached in one of these rings.

This program should set it UID and GID to those specified before attempting to
access any more keys. It may then look around for a user specific process to
hand the request off to (perhaps a path held in placed in another key by, for
example, the KDE desktop manager).

The program (or whatever it calls) should finish construction of the key by
calling KEYCTL_INSTANTIATE or KEYCTL_INSTANTIATE_IOV, which also permits it to
cache the key in one of the keyrings (probably the session ring) before
returning.  Alternatively, the key can be marked as negative with KEYCTL_NEGATE
or KEYCTL_REJECT; this also permits the key to be cached in one of the
keyrings.

If it returns with the key remaining in the unconstructed state, the key will
be marked as being negative, it will be added to the session keyring, and an
error will be returned to the key requestor.

Supplementary information may be provided from whoever or whatever invoked this
service. This will be passed as the <callout_info> parameter. If no such
information was made available, then "-" will be passed as this parameter
instead.


Similarly, the kernel may attempt to update an expired or a soon to expire key
by executing::

	/sbin/request-key update <key> <uid> <gid> \
		<threadring> <processring> <sessionring>

In this case, the program isn't required to actually attach the key to a ring;
the rings are provided for reference.


Garbage Collection
==================

Dead keys (for which the type has been removed) will be automatically unlinked
from those keyrings that point to them and deleted as soon as possible by a
background garbage collector.

Similarly, revoked and expired keys will be garbage collected, but only after a
certain amount of time has passed.  This time is set as a number of seconds in::

	/proc/sys/kernel/keys/gc_delay