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The kernel has the ability to restrict the set of hash algorithms it accepts for the security.ima xattr when it appraises files. Define a new IMA policy rule option "appraise_algos=", using the mentioned mechanism to expose a user-toggable policy knob to opt-in to that restriction and select the desired set of algorithms that must be accepted. When a policy rule uses the 'appraise_algos' option, appraisal of a file referenced by that rule will now fail if the digest algorithm employed to hash the file was not one of those explicitly listed in the option. In its absence, any hash algorithm compiled in the kernel will be accepted. For example, on a system where SELinux is properly deployed, the rule appraise func=BPRM_CHECK obj_type=iptables_exec_t \ appraise_algos=sha256,sha384 will block the execution of iptables if the xattr security.ima of its executables were not hashed with either sha256 or sha384. 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>
141 lines
4.7 KiB
Plaintext
141 lines
4.7 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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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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