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+What: /sys/kernel/security/evm
+What: /sys/kernel/security/*/evm
+Date: March 2011
+Contact: Mimi Zohar <zohar@us.ibm.com>
+Description:
+ EVM protects a file's security extended attributes(xattrs)
+ against integrity attacks. The initial method maintains an
+ HMAC-sha1 value across the extended attributes, storing the
+ value as the extended attribute 'security.evm'.
+
+ EVM supports two classes of security.evm. The first is
+ an HMAC-sha1 generated locally with a
+ trusted/encrypted key stored in the Kernel Key
+ Retention System. The second is a digital signature
+ generated either locally or remotely using an
+ asymmetric key. These keys are loaded onto root's
+ keyring using keyctl, and EVM is then enabled by
+ echoing a value to <securityfs>/evm made up of the
+ following bits:
+
+ === ==================================================
+ Bit Effect
+ === ==================================================
+ 0 Enable HMAC validation and creation
+ 1 Enable digital signature validation
+ 2 Permit modification of EVM-protected metadata at
+ runtime. Not supported if HMAC validation and
+ creation is enabled (deprecated).
+ 31 Disable further runtime modification of EVM policy
+ === ==================================================
+
+ For example::
+
+ echo 1 ><securityfs>/evm
+
+ will enable HMAC validation and creation
+
+ ::
+
+ echo 0x80000003 ><securityfs>/evm
+
+ will enable HMAC and digital signature validation and
+ HMAC creation and disable all further modification of policy.
+
+ ::
+
+ echo 0x80000006 ><securityfs>/evm
+
+ will enable digital signature validation, permit
+ modification of EVM-protected metadata and
+ disable all further modification of policy. This option is now
+ deprecated in favor of::
+
+ echo 0x80000002 ><securityfs>/evm
+
+ as the outstanding issues that prevent the usage of EVM portable
+ signatures have been solved.
+
+ Echoing a value is additive, the new value is added to the
+ existing initialization flags.
+
+ For example, after::
+
+ echo 2 ><securityfs>/evm
+
+ another echo can be performed::
+
+ echo 1 ><securityfs>/evm
+
+ and the resulting value will be 3.
+
+ Note that once an HMAC key has been loaded, it will no longer
+ be possible to enable metadata modification. Signaling that an
+ HMAC key has been loaded will clear the corresponding flag.
+ For example, if the current value is 6 (2 and 4 set)::
+
+ echo 1 ><securityfs>/evm
+
+ will set the new value to 3 (4 cleared).
+
+ Loading an HMAC key is the only way to disable metadata
+ modification.
+
+ Until key loading has been signaled EVM can not create
+ or validate the 'security.evm' xattr, but returns
+ INTEGRITY_UNKNOWN. Loading keys and signaling EVM
+ should be done as early as possible. Normally this is
+ done in the initramfs, which has already been measured
+ as part of the trusted boot. For more information on
+ creating and loading existing trusted/encrypted keys,
+ refer to:
+ Documentation/security/keys/trusted-encrypted.rst. Both
+ dracut (via 97masterkey and 98integrity) and systemd (via
+ core/ima-setup) have support for loading keys at boot
+ time.
+
+What: /sys/kernel/security/*/evm/evm_xattrs
+Date: April 2018
+Contact: Matthew Garrett <mjg59@google.com>
+Description:
+ Shows the set of extended attributes used to calculate or
+ validate the EVM signature, and allows additional attributes
+ to be added at runtime. Any signatures generated after
+ additional attributes are added (and on files possessing those
+ additional attributes) will only be valid if the same
+ additional attributes are configured on system boot. Writing
+ a single period (.) will lock the xattr list from any further
+ modification.