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