xref: /openbmc/linux/Documentation/ABI/testing/evm (revision 483f7d69)

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.