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EVM: Allow userland to permit modification of EVM-protected metadata

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When EVM is enabled it forbids modification of metadata protected by
EVM unless there is already a valid EVM signature. If any modification
is made, the kernel will then generate a new EVM HMAC. However, this
does not map well on use cases which use only asymmetric EVM signatures,
as in this scenario the kernel is unable to generate new signatures.

This patch extends the /sys/kernel/security/evm interface to allow
userland to request that modification of these xattrs be permitted. This
is only permitted if no keys have already been loaded. In this
configuration, modifying the metadata will invalidate the EVM appraisal
on the file in question. This allows packaging systems to write out new
files, set the relevant extended attributes and then move them into
place.

There's also some refactoring of the use of evm_initialized in order to
avoid heading down codepaths that assume there's a key available.

Signed-off-by: Matthew Garrett <mjg59@google.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
This commit is contained in:
Matthew Garrett
2017-11-07 07:18:35 -08:00
committed by Mimi Zohar
parent b7e27bc1d4
commit ae1ba1676b
4 changed files with 88 additions and 31 deletions
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54
Documentation/ABI/testing/evm
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@@ -14,30 +14,46 @@ Description:
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:
echoing a value to <securityfs>/evm made up of the
following bits:
1: enable HMAC validation and creation
2: enable digital signature validation
3: enable HMAC and digital signature validation and HMAC
creation
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.
31 Disable further runtime modification of EVM policy
Further writes will be blocked if HMAC support is enabled or
if bit 32 is set:
For example:
echo 0x80000002 ><securityfs>/evm
echo 1 ><securityfs>/evm
will enable digital signature validation and block
further writes to <securityfs>/evm.
will enable HMAC validation and creation
Until this is done, 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:
echo 0x80000003 ><securityfs>/evm
Documentation/security/keys/trusted-encrypted.rst. Both dracut
(via 97masterkey and 98integrity) and systemd (via
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
Note that once a key has been loaded, it will no longer be
possible to enable 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.