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What: /sys/kernel/security/evm
What: /sys/kernel/security/*/evm
Date: March 2011
Contact: Mimi Zohar <zohar@us.ibm.com>
Description:
EVM protects a file's security extended attributes(xattrs)
against integrity attacks. The initial method maintains an
HMAC-sha1 value across the extended attributes, storing the
value as the extended attribute 'security.evm'.
EVM supports two classes of security.evm. The first is
an HMAC-sha1 generated locally with a
trusted/encrypted key stored in the Kernel Key
Retention System. The second is a digital signature
generated either locally or remotely using an
asymmetric key. These keys are loaded onto root's
keyring using keyctl, and EVM is then enabled by
echoing a value to <securityfs>/evm made up of the
following bits:
=== ==================================================
Bit Effect
=== ==================================================
0 Enable HMAC validation and creation
1 Enable digital signature validation
2 Permit modification of EVM-protected metadata at
runtime. Not supported if HMAC validation and
creation is enabled (deprecated).
31 Disable further runtime modification of EVM policy
=== ==================================================
For example::
echo 1 ><securityfs>/evm
will enable HMAC validation and creation
::
echo 0x80000003 ><securityfs>/evm
will enable HMAC and digital signature validation and
HMAC creation and disable all further modification of policy.
::
echo 0x80000006 ><securityfs>/evm
will enable digital signature validation, permit
modification of EVM-protected metadata and
disable all further modification of policy. This option is now
deprecated in favor of::
echo 0x80000002 ><securityfs>/evm
as the outstanding issues that prevent the usage of EVM portable
signatures have been solved.
Echoing a value is additive, the new value is added to the
existing initialization flags.
For example, after::
echo 2 ><securityfs>/evm
another echo can be performed::
echo 1 ><securityfs>/evm
and the resulting value will be 3.
Note that once an HMAC key has been loaded, it will no longer
be possible to enable metadata modification. Signaling that an
HMAC key has been loaded will clear the corresponding flag.
For example, if the current value is 6 (2 and 4 set)::
echo 1 ><securityfs>/evm
will set the new value to 3 (4 cleared).
Loading an HMAC key is the only way to disable metadata
modification.
Until key loading has been signaled EVM can not create
or validate the 'security.evm' xattr, but returns
INTEGRITY_UNKNOWN. Loading keys and signaling EVM
should be done as early as possible. Normally this is
done in the initramfs, which has already been measured
as part of the trusted boot. For more information on
creating and loading existing trusted/encrypted keys,
refer to:
Documentation/security/keys/trusted-encrypted.rst. Both
dracut (via 97masterkey and 98integrity) and systemd (via
core/ima-setup) have support for loading keys at boot
time.
What: /sys/kernel/security/*/evm/evm_xattrs
Date: April 2018
Contact: Matthew Garrett <mjg59@google.com>
Description:
Shows the set of extended attributes used to calculate or
validate the EVM signature, and allows additional attributes
to be added at runtime. Any signatures generated after
additional attributes are added (and on files possessing those
additional attributes) will only be valid if the same
additional attributes are configured on system boot. Writing
a single period (.) will lock the xattr list from any further
modification.
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