What:		/sys/kernel/security/evmWhat:		/sys/kernel/security/*/evmDate:		March 2011Contact:	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).		3	  Require asymmetric signatures to be version 3		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_xattrsDate:		April 2018Contact:	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.