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EVM: Allow runtime modification of the set of verified xattrs

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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>
This commit is contained in:
Matthew Garrett
2018-05-15 10:38:26 -07:00
committed by Mimi Zohar
parent 21af766314
commit fa516b66a1
6 changed files with 202 additions and 4 deletions
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13
Documentation/ABI/testing/evm
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@@ -57,3 +57,16 @@ Description:
dracut (via 97masterkey and 98integrity) and systemd (via
core/ima-setup) have support for loading keys at boot
time.
What: security/integrity/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 posessing 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.