summaryrefslogtreecommitdiffstats
path: root/arch/x86/lib/retpoline.S
blob: 5b26a952e2fa65253d44a2117be7ff16b2d94125 (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
/* SPDX-License-Identifier: GPL-2.0 */

#include <linux/export.h>
#include <linux/stringify.h>
#include <linux/linkage.h>
#include <asm/dwarf2.h>
#include <asm/cpufeatures.h>
#include <asm/alternative.h>
#include <asm/asm-offsets.h>
#include <asm/nospec-branch.h>
#include <asm/unwind_hints.h>
#include <asm/percpu.h>
#include <asm/frame.h>
#include <asm/nops.h>

	.section .text..__x86.indirect_thunk


.macro POLINE reg
	ANNOTATE_INTRA_FUNCTION_CALL
	call    .Ldo_rop_\@
	int3
.Ldo_rop_\@:
	mov     %\reg, (%_ASM_SP)
	UNWIND_HINT_FUNC
.endm

.macro RETPOLINE reg
	POLINE \reg
	RET
.endm

.macro THUNK reg

	.align RETPOLINE_THUNK_SIZE
SYM_INNER_LABEL(__x86_indirect_thunk_\reg, SYM_L_GLOBAL)
	UNWIND_HINT_UNDEFINED
	ANNOTATE_NOENDBR

	ALTERNATIVE_2 __stringify(RETPOLINE \reg), \
		      __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *%\reg; int3), X86_FEATURE_RETPOLINE_LFENCE, \
		      __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), ALT_NOT(X86_FEATURE_RETPOLINE)

.endm

/*
 * Despite being an assembler file we can't just use .irp here
 * because __KSYM_DEPS__ only uses the C preprocessor and would
 * only see one instance of "__x86_indirect_thunk_\reg" rather
 * than one per register with the correct names. So we do it
 * the simple and nasty way...
 *
 * Worse, you can only have a single EXPORT_SYMBOL per line,
 * and CPP can't insert newlines, so we have to repeat everything
 * at least twice.
 */

#define __EXPORT_THUNK(sym)	_ASM_NOKPROBE(sym); EXPORT_SYMBOL(sym)

	.align RETPOLINE_THUNK_SIZE
SYM_CODE_START(__x86_indirect_thunk_array)

#define GEN(reg) THUNK reg
#include <asm/GEN-for-each-reg.h>
#undef GEN

	.align RETPOLINE_THUNK_SIZE
SYM_CODE_END(__x86_indirect_thunk_array)

#define GEN(reg) __EXPORT_THUNK(__x86_indirect_thunk_ ## reg)
#include <asm/GEN-for-each-reg.h>
#undef GEN

#ifdef CONFIG_CALL_DEPTH_TRACKING
.macro CALL_THUNK reg
	.align RETPOLINE_THUNK_SIZE

SYM_INNER_LABEL(__x86_indirect_call_thunk_\reg, SYM_L_GLOBAL)
	UNWIND_HINT_UNDEFINED
	ANNOTATE_NOENDBR

	CALL_DEPTH_ACCOUNT
	POLINE \reg
	ANNOTATE_UNRET_SAFE
	ret
	int3
.endm

	.align RETPOLINE_THUNK_SIZE
SYM_CODE_START(__x86_indirect_call_thunk_array)

#define GEN(reg) CALL_THUNK reg
#include <asm/GEN-for-each-reg.h>
#undef GEN

	.align RETPOLINE_THUNK_SIZE
SYM_CODE_END(__x86_indirect_call_thunk_array)

#define GEN(reg) __EXPORT_THUNK(__x86_indirect_call_thunk_ ## reg)
#include <asm/GEN-for-each-reg.h>
#undef GEN

.macro JUMP_THUNK reg
	.align RETPOLINE_THUNK_SIZE

SYM_INNER_LABEL(__x86_indirect_jump_thunk_\reg, SYM_L_GLOBAL)
	UNWIND_HINT_UNDEFINED
	ANNOTATE_NOENDBR
	POLINE \reg
	ANNOTATE_UNRET_SAFE
	ret
	int3
.endm

	.align RETPOLINE_THUNK_SIZE
SYM_CODE_START(__x86_indirect_jump_thunk_array)

#define GEN(reg) JUMP_THUNK reg
#include <asm/GEN-for-each-reg.h>
#undef GEN

	.align RETPOLINE_THUNK_SIZE
SYM_CODE_END(__x86_indirect_jump_thunk_array)

#define GEN(reg) __EXPORT_THUNK(__x86_indirect_jump_thunk_ ## reg)
#include <asm/GEN-for-each-reg.h>
#undef GEN
#endif

#ifdef CONFIG_RETHUNK

/*
 * Be careful here: that label cannot really be removed because in
 * some configurations and toolchains, the JMP __x86_return_thunk the
 * compiler issues is either a short one or the compiler doesn't use
 * relocations for same-section JMPs and that breaks the returns
 * detection logic in apply_returns() and in objtool.
 */
	.section .text..__x86.return_thunk

#ifdef CONFIG_CPU_SRSO

/*
 * srso_alias_untrain_ret() and srso_alias_safe_ret() are placed at
 * special addresses:
 *
 * - srso_alias_untrain_ret() is 2M aligned
 * - srso_alias_safe_ret() is also in the same 2M page but bits 2, 8, 14
 * and 20 in its virtual address are set (while those bits in the
 * srso_alias_untrain_ret() function are cleared).
 *
 * This guarantees that those two addresses will alias in the branch
 * target buffer of Zen3/4 generations, leading to any potential
 * poisoned entries at that BTB slot to get evicted.
 *
 * As a result, srso_alias_safe_ret() becomes a safe return.
 */
	.pushsection .text..__x86.rethunk_untrain
SYM_CODE_START_NOALIGN(srso_alias_untrain_ret)
	UNWIND_HINT_FUNC
	ANNOTATE_NOENDBR
	ASM_NOP2
	lfence
	jmp srso_alias_return_thunk
SYM_FUNC_END(srso_alias_untrain_ret)
__EXPORT_THUNK(srso_alias_untrain_ret)
	.popsection

	.pushsection .text..__x86.rethunk_safe
SYM_CODE_START_NOALIGN(srso_alias_safe_ret)
	lea 8(%_ASM_SP), %_ASM_SP
	UNWIND_HINT_FUNC
	ANNOTATE_UNRET_SAFE
	ret
	int3
SYM_FUNC_END(srso_alias_safe_ret)

SYM_CODE_START_NOALIGN(srso_alias_return_thunk)
	UNWIND_HINT_FUNC
	ANNOTATE_NOENDBR
	call srso_alias_safe_ret
	ud2
SYM_CODE_END(srso_alias_return_thunk)
	.popsection

/*
 * SRSO untraining sequence for Zen1/2, similar to retbleed_untrain_ret()
 * above. On kernel entry, srso_untrain_ret() is executed which is a
 *
 * movabs $0xccccc30824648d48,%rax
 *
 * and when the return thunk executes the inner label srso_safe_ret()
 * later, it is a stack manipulation and a RET which is mispredicted and
 * thus a "safe" one to use.
 */
	.align 64
	.skip 64 - (srso_safe_ret - srso_untrain_ret), 0xcc
SYM_CODE_START_LOCAL_NOALIGN(srso_untrain_ret)
	ANNOTATE_NOENDBR
	.byte 0x48, 0xb8

/*
 * This forces the function return instruction to speculate into a trap
 * (UD2 in srso_return_thunk() below).  This RET will then mispredict
 * and execution will continue at the return site read from the top of
 * the stack.
 */
SYM_INNER_LABEL(srso_safe_ret, SYM_L_GLOBAL)
	lea 8(%_ASM_SP), %_ASM_SP
	ret
	int3
	int3
	/* end of movabs */
	lfence
	call srso_safe_ret
	ud2
SYM_CODE_END(srso_safe_ret)
SYM_FUNC_END(srso_untrain_ret)

SYM_CODE_START(srso_return_thunk)
	UNWIND_HINT_FUNC
	ANNOTATE_NOENDBR
	call srso_safe_ret
	ud2
SYM_CODE_END(srso_return_thunk)

#define JMP_SRSO_UNTRAIN_RET "jmp srso_untrain_ret"
#else /* !CONFIG_CPU_SRSO */
#define JMP_SRSO_UNTRAIN_RET "ud2"
/* Dummy for the alternative in CALL_UNTRAIN_RET. */
SYM_CODE_START(srso_alias_untrain_ret)
	ANNOTATE_UNRET_SAFE
	ANNOTATE_NOENDBR
	ret
	int3
SYM_FUNC_END(srso_alias_untrain_ret)
__EXPORT_THUNK(srso_alias_untrain_ret)
#endif /* CONFIG_CPU_SRSO */

#ifdef CONFIG_CPU_UNRET_ENTRY

/*
 * Some generic notes on the untraining sequences:
 *
 * They are interchangeable when it comes to flushing potentially wrong
 * RET predictions from the BTB.
 *
 * The SRSO Zen1/2 (MOVABS) untraining sequence is longer than the
 * Retbleed sequence because the return sequence done there
 * (srso_safe_ret()) is longer and the return sequence must fully nest
 * (end before) the untraining sequence. Therefore, the untraining
 * sequence must fully overlap the return sequence.
 *
 * Regarding alignment - the instructions which need to be untrained,
 * must all start at a cacheline boundary for Zen1/2 generations. That
 * is, instruction sequences starting at srso_safe_ret() and
 * the respective instruction sequences at retbleed_return_thunk()
 * must start at a cacheline boundary.
 */

/*
 * Safety details here pertain to the AMD Zen{1,2} microarchitecture:
 * 1) The RET at retbleed_return_thunk must be on a 64 byte boundary, for
 *    alignment within the BTB.
 * 2) The instruction at retbleed_untrain_ret must contain, and not
 *    end with, the 0xc3 byte of the RET.
 * 3) STIBP must be enabled, or SMT disabled, to prevent the sibling thread
 *    from re-poisioning the BTB prediction.
 */
	.align 64
	.skip 64 - (retbleed_return_thunk - retbleed_untrain_ret), 0xcc
SYM_CODE_START_LOCAL_NOALIGN(retbleed_untrain_ret)
	ANNOTATE_NOENDBR
	/*
	 * As executed from retbleed_untrain_ret, this is:
	 *
	 *   TEST $0xcc, %bl
	 *   LFENCE
	 *   JMP retbleed_return_thunk
	 *
	 * Executing the TEST instruction has a side effect of evicting any BTB
	 * prediction (potentially attacker controlled) attached to the RET, as
	 * retbleed_return_thunk + 1 isn't an instruction boundary at the moment.
	 */
	.byte	0xf6

	/*
	 * As executed from retbleed_return_thunk, this is a plain RET.
	 *
	 * As part of the TEST above, RET is the ModRM byte, and INT3 the imm8.
	 *
	 * We subsequently jump backwards and architecturally execute the RET.
	 * This creates a correct BTB prediction (type=ret), but in the
	 * meantime we suffer Straight Line Speculation (because the type was
	 * no branch) which is halted by the INT3.
	 *
	 * With SMT enabled and STIBP active, a sibling thread cannot poison
	 * RET's prediction to a type of its choice, but can evict the
	 * prediction due to competitive sharing. If the prediction is
	 * evicted, retbleed_return_thunk will suffer Straight Line Speculation
	 * which will be contained safely by the INT3.
	 */
SYM_INNER_LABEL(retbleed_return_thunk, SYM_L_GLOBAL)
	ret
	int3
SYM_CODE_END(retbleed_return_thunk)

	/*
	 * Ensure the TEST decoding / BTB invalidation is complete.
	 */
	lfence

	/*
	 * Jump back and execute the RET in the middle of the TEST instruction.
	 * INT3 is for SLS protection.
	 */
	jmp retbleed_return_thunk
	int3
SYM_FUNC_END(retbleed_untrain_ret)

#define JMP_RETBLEED_UNTRAIN_RET "jmp retbleed_untrain_ret"
#else /* !CONFIG_CPU_UNRET_ENTRY */
#define JMP_RETBLEED_UNTRAIN_RET "ud2"
#endif /* CONFIG_CPU_UNRET_ENTRY */

#if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_SRSO)

SYM_FUNC_START(entry_untrain_ret)
	ALTERNATIVE JMP_RETBLEED_UNTRAIN_RET, JMP_SRSO_UNTRAIN_RET, X86_FEATURE_SRSO
SYM_FUNC_END(entry_untrain_ret)
__EXPORT_THUNK(entry_untrain_ret)

#endif /* CONFIG_CPU_UNRET_ENTRY || CONFIG_CPU_SRSO */

#ifdef CONFIG_CALL_DEPTH_TRACKING

	.align 64
SYM_FUNC_START(call_depth_return_thunk)
	ANNOTATE_NOENDBR
	/*
	 * Keep the hotpath in a 16byte I-fetch for the non-debug
	 * case.
	 */
	CALL_THUNKS_DEBUG_INC_RETS
	shlq	$5, PER_CPU_VAR(pcpu_hot + X86_call_depth)
	jz	1f
	ANNOTATE_UNRET_SAFE
	ret
	int3
1:
	CALL_THUNKS_DEBUG_INC_STUFFS
	.rept	16
	ANNOTATE_INTRA_FUNCTION_CALL
	call	2f
	int3
2:
	.endr
	add	$(8*16), %rsp

	CREDIT_CALL_DEPTH

	ANNOTATE_UNRET_SAFE
	ret
	int3
SYM_FUNC_END(call_depth_return_thunk)

#endif /* CONFIG_CALL_DEPTH_TRACKING */

/*
 * This function name is magical and is used by -mfunction-return=thunk-extern
 * for the compiler to generate JMPs to it.
 *
 * This code is only used during kernel boot or module init.  All
 * 'JMP __x86_return_thunk' sites are changed to something else by
 * apply_returns().
 *
 * This should be converted eventually to call a warning function which
 * should scream loudly when the default return thunk is called after
 * alternatives have been applied.
 *
 * That warning function cannot BUG() because the bug splat cannot be
 * displayed in all possible configurations, leading to users not really
 * knowing why the machine froze.
 */
SYM_CODE_START(__x86_return_thunk)
	UNWIND_HINT_FUNC
	ANNOTATE_NOENDBR
	ANNOTATE_UNRET_SAFE
	ret
	int3
SYM_CODE_END(__x86_return_thunk)
EXPORT_SYMBOL(__x86_return_thunk)

#endif /* CONFIG_RETHUNK */