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

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>

1 files changed, 35 insertions, 19 deletions

diff --git a/Documentation/ABI/testing/evm b/Documentation/ABI/testing/evm
index 9578247e1792..d12cb2eae9ee 100644
--- a/Documentation/ABI/testing/evm
+++ b/Documentation/ABI/testing/evm
@@ -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.