1 What: /sys/kernel/security/evm 2 What: /sys/kernel/security/*/evm 3 Date: March 2011 4 Contact: Mimi Zohar <zohar@us.ibm.com> 5 Description: 6 EVM protects a file's security extended attributes(xattrs) 7 against integrity attacks. The initial method maintains an 8 HMAC-sha1 value across the extended attributes, storing the 9 value as the extended attribute 'security.evm'. 10 11 EVM supports two classes of security.evm. The first is 12 an HMAC-sha1 generated locally with a 13 trusted/encrypted key stored in the Kernel Key 14 Retention System. The second is a digital signature 15 generated either locally or remotely using an 16 asymmetric key. These keys are loaded onto root's 17 keyring using keyctl, and EVM is then enabled by 18 echoing a value to <securityfs>/evm made up of the 19 following bits: 20 21 === ================================================== 22 Bit Effect 23 === ================================================== 24 0 Enable HMAC validation and creation 25 1 Enable digital signature validation 26 2 Permit modification of EVM-protected metadata at 27 runtime. Not supported if HMAC validation and 28 creation is enabled (deprecated). 29 31 Disable further runtime modification of EVM policy 30 === ================================================== 31 32 For example:: 33 34 echo 1 ><securityfs>/evm 35 36 will enable HMAC validation and creation 37 38 :: 39 40 echo 0x80000003 ><securityfs>/evm 41 42 will enable HMAC and digital signature validation and 43 HMAC creation and disable all further modification of policy. 44 45 :: 46 47 echo 0x80000006 ><securityfs>/evm 48 49 will enable digital signature validation, permit 50 modification of EVM-protected metadata and 51 disable all further modification of policy. This option is now 52 deprecated in favor of:: 53 54 echo 0x80000002 ><securityfs>/evm 55 56 as the outstanding issues that prevent the usage of EVM portable 57 signatures have been solved. 58 59 Echoing a value is additive, the new value is added to the 60 existing initialization flags. 61 62 For example, after:: 63 64 echo 2 ><securityfs>/evm 65 66 another echo can be performed:: 67 68 echo 1 ><securityfs>/evm 69 70 and the resulting value will be 3. 71 72 Note that once an HMAC key has been loaded, it will no longer 73 be possible to enable metadata modification. Signaling that an 74 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>/evm 78 79 will set the new value to 3 (4 cleared). 80 81 Loading an HMAC key is the only way to disable metadata 82 modification. 83 84 Until key loading has been signaled EVM can not create 85 or validate the 'security.evm' xattr, but returns 86 INTEGRITY_UNKNOWN. Loading keys and signaling EVM 87 should be done as early as possible. Normally this is 88 done in the initramfs, which has already been measured 89 as part of the trusted boot. For more information on 90 creating and loading existing trusted/encrypted keys, 91 refer to: 92 Documentation/security/keys/trusted-encrypted.rst. Both 93 dracut (via 97masterkey and 98integrity) and systemd (via 94 core/ima-setup) have support for loading keys at boot 95 time. 96 97 What: /sys/kernel/security/*/evm/evm_xattrs 98 Date: April 2018 99 Contact: Matthew Garrett <mjg59@google.com> 100 Description: 101 Shows the set of extended attributes used to calculate or 102 validate the EVM signature, and allows additional attributes 103 to be added at runtime. Any signatures generated after 104 additional attributes are added (and on files possessing those 105 additional attributes) will only be valid if the same 106 additional attributes are configured on system boot. Writing 107 a single period (.) will lock the xattr list from any further 108 modification.
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