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![Matthew Garrett](/assets/img/avatar_default.png)
Sites may wish to provide additional metadata alongside files in order to make more fine-grained security decisions[1]. The security of this is enhanced if this metadata is protected, something that EVM makes possible. However, the kernel cannot know about the set of extended attributes that local admins may wish to protect, and hardcoding this policy in the kernel makes it difficult to change over time and less convenient for distributions to enable. This patch adds a new /sys/kernel/security/integrity/evm/evm_xattrs node, which can be read to obtain the current set of EVM-protected extended attributes or written to in order to add new entries. Extending this list will not change the validity of any existing signatures provided that the file in question does not have any of the additional extended attributes - missing xattrs are skipped when calculating the EVM hash. [1] For instance, a package manager could install information about the package uploader in an additional extended attribute. Local LSM policy could then be associated with that extended attribute in order to restrict the privileges available to packages from less trusted uploaders. Signed-off-by: Matthew Garrett <mjg59@google.com> Reviewed-by: James Morris <james.morris@microsoft.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
72 lines
2.6 KiB
Text
72 lines
2.6 KiB
Text
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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Note that once a key has been loaded, it will no longer be
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possible to enable metadata 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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