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What: /sys/kernel/security/*/ima/policy
Date: May 2008
Contact: Mimi Zohar <zohar@us.ibm.com>
Description:
The Trusted Computing Group(TCG) runtime Integrity
Measurement Architecture(IMA) maintains a list of hash
values of executables and other sensitive system files
loaded into the run-time of this system. At runtime,
the policy can be constrained based on LSM specific data.
Policies are loaded into the securityfs file ima/policy
by opening the file, writing the rules one at a time and
then closing the file. The new policy takes effect after
the file ima/policy is closed.
IMA appraisal, if configured, uses these file measurements
for local measurement appraisal.
::
rule format: action [condition ...]
action: measure | dont_measure | appraise | dont_appraise |
audit | hash | dont_hash
condition:= base | lsm [option]
base: [[func=] [mask=] [fsmagic=] [fsuuid=] [fsname=]
[uid=] [euid=] [gid=] [egid=]
[fowner=] [fgroup=]]
lsm: [[subj_user=] [subj_role=] [subj_type=]
[obj_user=] [obj_role=] [obj_type=]]
option: [digest_type=] [template=] [permit_directio]
[appraise_type=] [appraise_flag=]
[appraise_algos=] [keyrings=]
base:
func:= [BPRM_CHECK][MMAP_CHECK][CREDS_CHECK][FILE_CHECK][MODULE_CHECK]
[FIRMWARE_CHECK]
[KEXEC_KERNEL_CHECK] [KEXEC_INITRAMFS_CHECK]
[KEXEC_CMDLINE] [KEY_CHECK] [CRITICAL_DATA]
[SETXATTR_CHECK]
mask:= [[^]MAY_READ] [[^]MAY_WRITE] [[^]MAY_APPEND]
[[^]MAY_EXEC]
fsmagic:= hex value
fsuuid:= file system UUID (e.g 8bcbe394-4f13-4144-be8e-5aa9ea2ce2f6)
uid:= decimal value
euid:= decimal value
gid:= decimal value
egid:= decimal value
fowner:= decimal value
fgroup:= decimal value
lsm: are LSM specific
option:
appraise_type:= [imasig] | [imasig|modsig] | [sigv3]
where 'imasig' is the original or the signature
format v2.
where 'modsig' is an appended signature,
where 'sigv3' is the signature format v3. (Currently
limited to fsverity digest based signatures
stored in security.ima xattr. Requires
specifying "digest_type=verity" first.)
appraise_flag:= [check_blacklist]
Currently, blacklist check is only for files signed with appended
signature.
digest_type:= verity
Require fs-verity's file digest instead of the
regular IMA file hash.
keyrings:= list of keyrings
(eg, .builtin_trusted_keys|.ima). Only valid
when action is "measure" and func is KEY_CHECK.
template:= name of a defined IMA template type
(eg, ima-ng). Only valid when action is "measure".
pcr:= decimal value
label:= [selinux]|[kernel_info]|[data_label]
data_label:= a unique string used for grouping and limiting critical data.
For example, "selinux" to measure critical data for SELinux.
appraise_algos:= comma-separated list of hash algorithms
For example, "sha256,sha512" to only accept to appraise
files where the security.ima xattr was hashed with one
of these two algorithms.
default policy:
# PROC_SUPER_MAGIC
dont_measure fsmagic=0x9fa0
dont_appraise fsmagic=0x9fa0
# SYSFS_MAGIC
dont_measure fsmagic=0x62656572
dont_appraise fsmagic=0x62656572
# DEBUGFS_MAGIC
dont_measure fsmagic=0x64626720
dont_appraise fsmagic=0x64626720
# TMPFS_MAGIC
dont_measure fsmagic=0x01021994
dont_appraise fsmagic=0x01021994
# RAMFS_MAGIC
dont_appraise fsmagic=0x858458f6
# DEVPTS_SUPER_MAGIC
dont_measure fsmagic=0x1cd1
dont_appraise fsmagic=0x1cd1
# BINFMTFS_MAGIC
dont_measure fsmagic=0x42494e4d
dont_appraise fsmagic=0x42494e4d
# SECURITYFS_MAGIC
dont_measure fsmagic=0x73636673
dont_appraise fsmagic=0x73636673
# SELINUX_MAGIC
dont_measure fsmagic=0xf97cff8c
dont_appraise fsmagic=0xf97cff8c
# CGROUP_SUPER_MAGIC
dont_measure fsmagic=0x27e0eb
dont_appraise fsmagic=0x27e0eb
# NSFS_MAGIC
dont_measure fsmagic=0x6e736673
dont_appraise fsmagic=0x6e736673
measure func=BPRM_CHECK
measure func=FILE_MMAP mask=MAY_EXEC
measure func=FILE_CHECK mask=MAY_READ uid=0
measure func=MODULE_CHECK
measure func=FIRMWARE_CHECK
appraise fowner=0
The default policy measures all executables in bprm_check,
all files mmapped executable in file_mmap, and all files
open for read by root in do_filp_open. The default appraisal
policy appraises all files owned by root.
Examples of LSM specific definitions:
SELinux::
dont_measure obj_type=var_log_t
dont_appraise obj_type=var_log_t
dont_measure obj_type=auditd_log_t
dont_appraise obj_type=auditd_log_t
measure subj_user=system_u func=FILE_CHECK mask=MAY_READ
measure subj_role=system_r func=FILE_CHECK mask=MAY_READ
Smack::
measure subj_user=_ func=FILE_CHECK mask=MAY_READ
Example of measure rules using alternate PCRs::
measure func=KEXEC_KERNEL_CHECK pcr=4
measure func=KEXEC_INITRAMFS_CHECK pcr=5
Example of appraise rule allowing modsig appended signatures:
appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig|modsig
Example of measure rule using KEY_CHECK to measure all keys:
measure func=KEY_CHECK
Example of measure rule using KEY_CHECK to only measure
keys added to .builtin_trusted_keys or .ima keyring:
measure func=KEY_CHECK keyrings=.builtin_trusted_keys|.ima
Example of the special SETXATTR_CHECK appraise rule, that
restricts the hash algorithms allowed when writing to the
security.ima xattr of a file:
appraise func=SETXATTR_CHECK appraise_algos=sha256,sha384,sha512
Example of a 'measure' rule requiring fs-verity's digests
with indication of type of digest in the measurement list.
measure func=FILE_CHECK digest_type=verity \
template=ima-ngv2
Example of 'measure' and 'appraise' rules requiring fs-verity
signatures (format version 3) stored in security.ima xattr.
The 'measure' rule specifies the 'ima-sigv3' template option,
which includes the indication of type of digest and the file
signature in the measurement list.
measure func=BPRM_CHECK digest_type=verity \
template=ima-sigv3
The 'appraise' rule specifies the type and signature format
version (sigv3) required.
appraise func=BPRM_CHECK digest_type=verity \
appraise_type=sigv3
All of these policy rules could, for example, be constrained
either based on a filesystem's UUID (fsuuid) or based on LSM
labels.
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