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74e9d920f2
commit 9acc89d31f0c94c8e573ed61f3e4340bbd526d0c upstream.
EVM_ALLOW_METADATA_WRITES is an EVM initialization flag that can be set to
temporarily disable metadata verification until all xattrs/attrs necessary
to verify an EVM portable signature are copied to the file. This flag is
cleared when EVM is initialized with an HMAC key, to avoid that the HMAC is
calculated on unverified xattrs/attrs.
Currently EVM unnecessarily denies setting this flag if EVM is initialized
with a public key, which is not a concern as it cannot be used to trust
xattrs/attrs updates. This patch removes this limitation.
Fixes: ae1ba1676b
("EVM: Allow userland to permit modification of EVM-protected metadata")
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Cc: stable@vger.kernel.org # 4.16.x
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
95 lines
3.2 KiB
Plaintext
95 lines
3.2 KiB
Plaintext
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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Bit Effect
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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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For example:
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echo 1 ><securityfs>/evm
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will enable HMAC validation and creation
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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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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 posessing 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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