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 === ================================================== 21 Bit Effect 22 === ================================================== 23 0 Enable HMAC validation and creation 24 1 Enable digital signature validation 25 2 Permit modification of EVM-protected metadata at 26 runtime. Not supported if HMAC validation and 27 creation is enabled. 28 31 Disable further runtime modification of EVM policy 29 === ================================================== 30 31 For example:: 32 33 echo 1 ><securityfs>/evm 34 35 will enable HMAC validation and creation 36 37 :: 38 39 echo 0x80000003 ><securityfs>/evm 40 41 will enable HMAC and digital signature validation and 42 HMAC creation and disable all further modification of policy. 43 44 :: 45 46 echo 0x80000006 ><securityfs>/evm 47 48 will enable digital signature validation, permit 49 modification of EVM-protected metadata and 50 disable all further modification of policy 51 52 Echoing a value is additive, the new value is added to the 53 existing initialization flags. 54 55 For example, after:: 56 57 echo 2 ><securityfs>/evm 58 59 another echo can be performed:: 60 61 echo 1 ><securityfs>/evm 62 63 and the resulting value will be 3. 64 65 Note that once an HMAC key has been loaded, it will no longer 66 be possible to enable metadata modification. Signaling that an 67 HMAC key has been loaded will clear the corresponding flag. 68 For example, if the current value is 6 (2 and 4 set):: 69 70 echo 1 ><securityfs>/evm 71 72 will set the new value to 3 (4 cleared). 73 74 Loading an HMAC key is the only way to disable metadata 75 modification. 76 77 Until key loading has been signaled EVM can not create 78 or validate the 'security.evm' xattr, but returns 79 INTEGRITY_UNKNOWN. Loading keys and signaling EVM 80 should be done as early as possible. Normally this is 81 done in the initramfs, which has already been measured 82 as part of the trusted boot. For more information on 83 creating and loading existing trusted/encrypted keys, 84 refer to: 85 Documentation/security/keys/trusted-encrypted.rst. Both 86 dracut (via 97masterkey and 98integrity) and systemd (via 87 core/ima-setup) have support for loading keys at boot 88 time. 89 90What: security/integrity/evm/evm_xattrs 91Date: April 2018 92Contact: Matthew Garrett <mjg59@google.com> 93Description: 94 Shows the set of extended attributes used to calculate or 95 validate the EVM signature, and allows additional attributes 96 to be added at runtime. Any signatures generated after 97 additional attributes are added (and on files possessing those 98 additional attributes) will only be valid if the same 99 additional attributes are configured on system boot. Writing 100 a single period (.) will lock the xattr list from any further 101 modification. 102