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1What: /sys/kernel/security/evm2What: /sys/kernel/security/*/evm3Date: March 20114Contact: Mimi Zohar <zohar@us.ibm.com>5Description:6 EVM protects a file's security extended attributes(xattrs)7 against integrity attacks. The initial method maintains an8 HMAC-sha1 value across the extended attributes, storing the9 value as the extended attribute 'security.evm'.10 11 EVM supports two classes of security.evm. The first is12 an HMAC-sha1 generated locally with a13 trusted/encrypted key stored in the Kernel Key14 Retention System. The second is a digital signature15 generated either locally or remotely using an16 asymmetric key. These keys are loaded onto root's17 keyring using keyctl, and EVM is then enabled by18 echoing a value to <securityfs>/evm made up of the19 following bits:20 21 === ==================================================22 Bit Effect23 === ==================================================24 0 Enable HMAC validation and creation25 1 Enable digital signature validation26 2 Permit modification of EVM-protected metadata at27 runtime. Not supported if HMAC validation and28 creation is enabled (deprecated).29 31 Disable further runtime modification of EVM policy30 === ==================================================31 32 For example::33 34 echo 1 ><securityfs>/evm35 36 will enable HMAC validation and creation37 38 ::39 40 echo 0x80000003 ><securityfs>/evm41 42 will enable HMAC and digital signature validation and43 HMAC creation and disable all further modification of policy.44 45 ::46 47 echo 0x80000006 ><securityfs>/evm48 49 will enable digital signature validation, permit50 modification of EVM-protected metadata and51 disable all further modification of policy. This option is now52 deprecated in favor of::53 54 echo 0x80000002 ><securityfs>/evm55 56 as the outstanding issues that prevent the usage of EVM portable57 signatures have been solved.58 59 Echoing a value is additive, the new value is added to the60 existing initialization flags.61 62 For example, after::63 64 echo 2 ><securityfs>/evm65 66 another echo can be performed::67 68 echo 1 ><securityfs>/evm69 70 and the resulting value will be 3.71 72 Note that once an HMAC key has been loaded, it will no longer73 be possible to enable metadata modification. Signaling that an74 HMAC key has been loaded will clear the corresponding flag.75 For example, if the current value is 6 (2 and 4 set)::76 77 echo 1 ><securityfs>/evm78 79 will set the new value to 3 (4 cleared).80 81 Loading an HMAC key is the only way to disable metadata82 modification.83 84 Until key loading has been signaled EVM can not create85 or validate the 'security.evm' xattr, but returns86 INTEGRITY_UNKNOWN. Loading keys and signaling EVM87 should be done as early as possible. Normally this is88 done in the initramfs, which has already been measured89 as part of the trusted boot. For more information on90 creating and loading existing trusted/encrypted keys,91 refer to:92 Documentation/security/keys/trusted-encrypted.rst. Both93 dracut (via 97masterkey and 98integrity) and systemd (via94 core/ima-setup) have support for loading keys at boot95 time.96 97What: /sys/kernel/security/*/evm/evm_xattrs98Date: April 201899Contact: Matthew Garrett <mjg59@google.com>100Description:101 Shows the set of extended attributes used to calculate or102 validate the EVM signature, and allows additional attributes103 to be added at runtime. Any signatures generated after104 additional attributes are added (and on files possessing those105 additional attributes) will only be valid if the same106 additional attributes are configured on system boot. Writing107 a single period (.) will lock the xattr list from any further108 modification.109