4f2946aa0c
While users can restrict the accepted hash algorithms for the security.ima xattr file signature when appraising said file, users cannot restrict the algorithms that can be set on that attribute: any algorithm built in the kernel is accepted on a write. Define a new value for the ima policy option 'func' that restricts globally the hash algorithms accepted when writing the security.ima xattr. When a policy contains a rule of the form appraise func=SETXATTR_CHECK appraise_algos=sha256,sha384,sha512 only values corresponding to one of these three digest algorithms will be accepted for writing the security.ima xattr. Attempting to write the attribute using another algorithm (or "free-form" data) will be denied with an audit log message. In the absence of such a policy rule, the default is still to only accept hash algorithms built in the kernel (with all the limitations that entails). Signed-off-by: THOBY Simon <Simon.THOBY@viveris.fr> Reviewed-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com> Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
148 lines
4.9 KiB
Plaintext
148 lines
4.9 KiB
Plaintext
What: security/ima/policy
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Date: May 2008
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Contact: Mimi Zohar <zohar@us.ibm.com>
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Description:
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The Trusted Computing Group(TCG) runtime Integrity
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Measurement Architecture(IMA) maintains a list of hash
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values of executables and other sensitive system files
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loaded into the run-time of this system. At runtime,
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the policy can be constrained based on LSM specific data.
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Policies are loaded into the securityfs file ima/policy
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by opening the file, writing the rules one at a time and
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then closing the file. The new policy takes effect after
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the file ima/policy is closed.
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IMA appraisal, if configured, uses these file measurements
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for local measurement appraisal.
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::
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rule format: action [condition ...]
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action: measure | dont_measure | appraise | dont_appraise |
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audit | hash | dont_hash
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condition:= base | lsm [option]
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base: [[func=] [mask=] [fsmagic=] [fsuuid=] [uid=]
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[euid=] [fowner=] [fsname=]]
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lsm: [[subj_user=] [subj_role=] [subj_type=]
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[obj_user=] [obj_role=] [obj_type=]]
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option: [[appraise_type=]] [template=] [permit_directio]
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[appraise_flag=] [appraise_algos=] [keyrings=]
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base:
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func:= [BPRM_CHECK][MMAP_CHECK][CREDS_CHECK][FILE_CHECK][MODULE_CHECK]
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[FIRMWARE_CHECK]
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[KEXEC_KERNEL_CHECK] [KEXEC_INITRAMFS_CHECK]
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[KEXEC_CMDLINE] [KEY_CHECK] [CRITICAL_DATA]
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[SETXATTR_CHECK]
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mask:= [[^]MAY_READ] [[^]MAY_WRITE] [[^]MAY_APPEND]
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[[^]MAY_EXEC]
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fsmagic:= hex value
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fsuuid:= file system UUID (e.g 8bcbe394-4f13-4144-be8e-5aa9ea2ce2f6)
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uid:= decimal value
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euid:= decimal value
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fowner:= decimal value
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lsm: are LSM specific
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option:
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appraise_type:= [imasig] [imasig|modsig]
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appraise_flag:= [check_blacklist]
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Currently, blacklist check is only for files signed with appended
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signature.
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keyrings:= list of keyrings
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(eg, .builtin_trusted_keys|.ima). Only valid
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when action is "measure" and func is KEY_CHECK.
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template:= name of a defined IMA template type
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(eg, ima-ng). Only valid when action is "measure".
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pcr:= decimal value
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label:= [selinux]|[kernel_info]|[data_label]
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data_label:= a unique string used for grouping and limiting critical data.
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For example, "selinux" to measure critical data for SELinux.
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appraise_algos:= comma-separated list of hash algorithms
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For example, "sha256,sha512" to only accept to appraise
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files where the security.ima xattr was hashed with one
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of these two algorithms.
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default policy:
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# PROC_SUPER_MAGIC
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dont_measure fsmagic=0x9fa0
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dont_appraise fsmagic=0x9fa0
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# SYSFS_MAGIC
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dont_measure fsmagic=0x62656572
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dont_appraise fsmagic=0x62656572
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# DEBUGFS_MAGIC
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dont_measure fsmagic=0x64626720
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dont_appraise fsmagic=0x64626720
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# TMPFS_MAGIC
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dont_measure fsmagic=0x01021994
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dont_appraise fsmagic=0x01021994
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# RAMFS_MAGIC
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dont_appraise fsmagic=0x858458f6
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# DEVPTS_SUPER_MAGIC
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dont_measure fsmagic=0x1cd1
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dont_appraise fsmagic=0x1cd1
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# BINFMTFS_MAGIC
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dont_measure fsmagic=0x42494e4d
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dont_appraise fsmagic=0x42494e4d
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# SECURITYFS_MAGIC
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dont_measure fsmagic=0x73636673
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dont_appraise fsmagic=0x73636673
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# SELINUX_MAGIC
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dont_measure fsmagic=0xf97cff8c
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dont_appraise fsmagic=0xf97cff8c
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# CGROUP_SUPER_MAGIC
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dont_measure fsmagic=0x27e0eb
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dont_appraise fsmagic=0x27e0eb
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# NSFS_MAGIC
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dont_measure fsmagic=0x6e736673
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dont_appraise fsmagic=0x6e736673
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measure func=BPRM_CHECK
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measure func=FILE_MMAP mask=MAY_EXEC
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measure func=FILE_CHECK mask=MAY_READ uid=0
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measure func=MODULE_CHECK
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measure func=FIRMWARE_CHECK
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appraise fowner=0
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The default policy measures all executables in bprm_check,
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all files mmapped executable in file_mmap, and all files
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open for read by root in do_filp_open. The default appraisal
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policy appraises all files owned by root.
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Examples of LSM specific definitions:
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SELinux::
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dont_measure obj_type=var_log_t
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dont_appraise obj_type=var_log_t
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dont_measure obj_type=auditd_log_t
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dont_appraise obj_type=auditd_log_t
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measure subj_user=system_u func=FILE_CHECK mask=MAY_READ
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measure subj_role=system_r func=FILE_CHECK mask=MAY_READ
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Smack::
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measure subj_user=_ func=FILE_CHECK mask=MAY_READ
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Example of measure rules using alternate PCRs::
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measure func=KEXEC_KERNEL_CHECK pcr=4
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measure func=KEXEC_INITRAMFS_CHECK pcr=5
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Example of appraise rule allowing modsig appended signatures:
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appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig|modsig
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Example of measure rule using KEY_CHECK to measure all keys:
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measure func=KEY_CHECK
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Example of measure rule using KEY_CHECK to only measure
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keys added to .builtin_trusted_keys or .ima keyring:
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measure func=KEY_CHECK keyrings=.builtin_trusted_keys|.ima
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Example of the special SETXATTR_CHECK appraise rule, that
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restricts the hash algorithms allowed when writing to the
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security.ima xattr of a file:
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appraise func=SETXATTR_CHECK appraise_algos=sha256,sha384,sha512
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