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1What: /sys/kernel/security/evm
2What: /sys/kernel/security/*/evm
3Date: March 2011
4Contact: Mimi Zohar <[email protected]>
5Description:
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 3 Require asymmetric signatures to be version 3
30 31 Disable further runtime modification of EVM policy
31 === ==================================================
32
33 For example::
34
35 echo 1 ><securityfs>/evm
36
37 will enable HMAC validation and creation
38
39 ::
40
41 echo 0x80000003 ><securityfs>/evm
42
43 will enable HMAC and digital signature validation and
44 HMAC creation and disable all further modification of policy.
45
46 ::
47
48 echo 0x80000006 ><securityfs>/evm
49
50 will enable digital signature validation, permit
51 modification of EVM-protected metadata and
52 disable all further modification of policy. This option is now
53 deprecated in favor of::
54
55 echo 0x80000002 ><securityfs>/evm
56
57 as the outstanding issues that prevent the usage of EVM portable
58 signatures have been solved.
59
60 Echoing a value is additive, the new value is added to the
61 existing initialization flags.
62
63 For example, after::
64
65 echo 2 ><securityfs>/evm
66
67 another echo can be performed::
68
69 echo 1 ><securityfs>/evm
70
71 and the resulting value will be 3.
72
73 Note that once an HMAC key has been loaded, it will no longer
74 be possible to enable metadata modification. Signaling that an
75 HMAC key has been loaded will clear the corresponding flag.
76 For example, if the current value is 6 (2 and 4 set)::
77
78 echo 1 ><securityfs>/evm
79
80 will set the new value to 3 (4 cleared).
81
82 Loading an HMAC key is the only way to disable metadata
83 modification.
84
85 Until key loading has been signaled EVM can not create
86 or validate the 'security.evm' xattr, but returns
87 INTEGRITY_UNKNOWN. Loading keys and signaling EVM
88 should be done as early as possible. Normally this is
89 done in the initramfs, which has already been measured
90 as part of the trusted boot. For more information on
91 creating and loading existing trusted/encrypted keys,
92 refer to:
93 Documentation/security/keys/trusted-encrypted.rst. Both
94 dracut (via 97masterkey and 98integrity) and systemd (via
95 core/ima-setup) have support for loading keys at boot
96 time.
97
98What: /sys/kernel/security/*/evm/evm_xattrs
99Date: April 2018
100Contact: Matthew Garrett <[email protected]>
101Description:
102 Shows the set of extended attributes used to calculate or
103 validate the EVM signature, and allows additional attributes
104 to be added at runtime. Any signatures generated after
105 additional attributes are added (and on files possessing those
106 additional attributes) will only be valid if the same
107 additional attributes are configured on system boot. Writing
108 a single period (.) will lock the xattr list from any further
109 modification.