What: /sys/kernel/security/evm What: /sys/kernel/security/*/evm Date: March 2011 Contact: Mimi Zohar 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 /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 >/evm will enable HMAC validation and creation :: echo 0x80000003 >/evm will enable HMAC and digital signature validation and HMAC creation and disable all further modification of policy. :: echo 0x80000006 >/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 >/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 >/evm another echo can be performed:: echo 1 >/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 >/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 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.