1What: security/evm 2Date: March 2011 3Contact: Mimi Zohar <zohar@us.ibm.com> 4Description: 5 EVM protects a file's security extended attributes(xattrs) 6 against integrity attacks. The initial method maintains an 7 HMAC-sha1 value across the extended attributes, storing the 8 value as the extended attribute 'security.evm'. 9 10 EVM supports two classes of security.evm. The first is 11 an HMAC-sha1 generated locally with a 12 trusted/encrypted key stored in the Kernel Key 13 Retention System. The second is a digital signature 14 generated either locally or remotely using an 15 asymmetric key. These keys are loaded onto root's 16 keyring using keyctl, and EVM is then enabled by 17 echoing a value to <securityfs>/evm made up of the 18 following bits: 19 20 Bit Effect 21 0 Enable HMAC validation and creation 22 1 Enable digital signature validation 23 2 Permit modification of EVM-protected metadata at 24 runtime. Not supported if HMAC validation and 25 creation is enabled. 26 31 Disable further runtime modification of EVM policy 27 28 For example: 29 30 echo 1 ><securityfs>/evm 31 32 will enable HMAC validation and creation 33 34 echo 0x80000003 ><securityfs>/evm 35 36 will enable HMAC and digital signature validation and 37 HMAC creation and disable all further modification of policy. 38 39 echo 0x80000006 ><securityfs>/evm 40 41 will enable digital signature validation, permit 42 modification of EVM-protected metadata and 43 disable all further modification of policy 44 45 Note that once a key has been loaded, it will no longer be 46 possible to enable metadata modification. 47 48 Until key loading has been signaled EVM can not create 49 or validate the 'security.evm' xattr, but returns 50 INTEGRITY_UNKNOWN. Loading keys and signaling EVM 51 should be done as early as possible. Normally this is 52 done in the initramfs, which has already been measured 53 as part of the trusted boot. For more information on 54 creating and loading existing trusted/encrypted keys, 55 refer to: 56 Documentation/security/keys/trusted-encrypted.rst. Both 57 dracut (via 97masterkey and 98integrity) and systemd (via 58 core/ima-setup) have support for loading keys at boot 59 time. 60