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		Note that once a key has been loaded, it will no longer be
46		possible to enable metadata modification.
47
48		Until key loading has been signaled EVM can not create
49		or validate the 'security.evm' xattr, but returns
50		INTEGRITY_UNKNOWN.  Loading keys and signaling EVM
51		should be done as early as possible.  Normally this is
52		done in the initramfs, which has already been measured
53		as part of the trusted boot.  For more information on
54		creating and loading existing trusted/encrypted keys,
55		refer to:
56		Documentation/security/keys/trusted-encrypted.rst. Both
57		dracut (via 97masterkey and 98integrity) and systemd (via
58		core/ima-setup) have support for loading keys at boot
59		time.
60
61What:		security/integrity/evm/evm_xattrs
62Date:		April 2018
63Contact:	Matthew Garrett <mjg59@google.com>
64Description:
65		Shows the set of extended attributes used to calculate or
66		validate the EVM signature, and allows additional attributes
67		to be added at runtime. Any signatures generated after
68		additional attributes are added (and on files posessing those
69		additional attributes) will only be valid if the same
70		additional attributes are configured on system boot. Writing
71		a single period (.) will lock the xattr list from any further
72		modification.