xref: /openbmc/linux/Documentation/ABI/testing/evm (revision e15a5365)
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		Note that once a key has been loaded, it will no longer be
53		possible to enable metadata modification.
54
55		Until key loading has been signaled EVM can not create
56		or validate the 'security.evm' xattr, but returns
57		INTEGRITY_UNKNOWN.  Loading keys and signaling EVM
58		should be done as early as possible.  Normally this is
59		done in the initramfs, which has already been measured
60		as part of the trusted boot.  For more information on
61		creating and loading existing trusted/encrypted keys,
62		refer to:
63		Documentation/security/keys/trusted-encrypted.rst. Both
64		dracut (via 97masterkey and 98integrity) and systemd (via
65		core/ima-setup) have support for loading keys at boot
66		time.
67
68What:		security/integrity/evm/evm_xattrs
69Date:		April 2018
70Contact:	Matthew Garrett <mjg59@google.com>
71Description:
72		Shows the set of extended attributes used to calculate or
73		validate the EVM signature, and allows additional attributes
74		to be added at runtime. Any signatures generated after
75		additional attributes are added (and on files possessing those
76		additional attributes) will only be valid if the same
77		additional attributes are configured on system boot. Writing
78		a single period (.) will lock the xattr list from any further
79		modification.
80