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SELinux stores the active policy in memory, so the changes to this data at runtime would have an impact on the security guarantees provided by SELinux. Measuring in-memory SELinux policy through IMA subsystem provides a secure way for the attestation service to remotely validate the policy contents at runtime. Measure the hash of the loaded policy by calling the IMA hook ima_measure_critical_data(). Since the size of the loaded policy can be large (several MB), measure the hash of the policy instead of the entire policy to avoid bloating the IMA log entry. To enable SELinux data measurement, the following steps are required: 1, Add "ima_policy=critical_data" to the kernel command line arguments to enable measuring SELinux data at boot time. For example, BOOT_IMAGE=/boot/vmlinuz-5.10.0-rc1+ root=UUID=fd643309-a5d2-4ed3-b10d-3c579a5fab2f ro nomodeset security=selinux ima_policy=critical_data 2, Add the following rule to /etc/ima/ima-policy measure func=CRITICAL_DATA label=selinux Sample measurement of the hash of SELinux policy: To verify the measured data with the current SELinux policy run the following commands and verify the output hash values match. sha256sum /sys/fs/selinux/policy | cut -d' ' -f 1 grep "selinux-policy-hash" /sys/kernel/security/integrity/ima/ascii_runtime_measurements | tail -1 | cut -d' ' -f 6 Note that the actual verification of SELinux policy would require loading the expected policy into an identical kernel on a pristine/known-safe system and run the sha256sum /sys/kernel/selinux/policy there to get the expected hash. Signed-off-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com> Suggested-by: Stephen Smalley <stephen.smalley.work@gmail.com> Acked-by: Paul Moore <paul@paul-moore.com> Reviewed-by: Tyler Hicks <tyhicks@linux.microsoft.com> Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
137 lines
4.4 KiB
Plaintext
137 lines
4.4 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=] [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]|[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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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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