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 Echoing a value is additive, the new value is added to the 46 existing initialization flags. 47 48 For example, after:: 49 50 echo 2 ><securityfs>/evm 51 52 another echo can be performed:: 53 54 echo 1 ><securityfs>/evm 55 56 and the resulting value will be 3. 57 58 Note that once an HMAC key has been loaded, it will no longer 59 be possible to enable metadata modification. Signaling that an 60 HMAC key has been loaded will clear the corresponding flag. 61 For example, if the current value is 6 (2 and 4 set):: 62 63 echo 1 ><securityfs>/evm 64 65 will set the new value to 3 (4 cleared). 66 67 Loading an HMAC key is the only way to disable metadata 68 modification. 69 70 Until key loading has been signaled EVM can not create 71 or validate the 'security.evm' xattr, but returns 72 INTEGRITY_UNKNOWN. Loading keys and signaling EVM 73 should be done as early as possible. Normally this is 74 done in the initramfs, which has already been measured 75 as part of the trusted boot. For more information on 76 creating and loading existing trusted/encrypted keys, 77 refer to: 78 Documentation/security/keys/trusted-encrypted.rst. Both 79 dracut (via 97masterkey and 98integrity) and systemd (via 80 core/ima-setup) have support for loading keys at boot 81 time. 82 83What: security/integrity/evm/evm_xattrs 84Date: April 2018 85Contact: Matthew Garrett <mjg59@google.com> 86Description: 87 Shows the set of extended attributes used to calculate or 88 validate the EVM signature, and allows additional attributes 89 to be added at runtime. Any signatures generated after 90 additional attributes are added (and on files posessing those 91 additional attributes) will only be valid if the same 92 additional attributes are configured on system boot. Writing 93 a single period (.) will lock the xattr list from any further 94 modification. 95