Commit 9acc89d3 authored by Roberto Sassu's avatar Roberto Sassu Committed by Mimi Zohar
Browse files

evm: Refuse EVM_ALLOW_METADATA_WRITES only if an HMAC key is loaded 



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: ae1ba167 ("EVM: Allow userland to permit modification of EVM-protected metadata")
Signed-off-by: default avatarRoberto Sassu <roberto.sassu@huawei.com>
Cc: stable@vger.kernel.org # 4.16.x
Signed-off-by: default avatarMimi Zohar <zohar@linux.ibm.com>
parent aa2ead71

Documentation/ABI/testing/evm

+24 −2
Original line number Diff line number Diff line
@@ -49,8 +49,30 @@ Description: 
		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.

		Echoing a value is additive, the new value is added to the

		existing initialization flags.



		For example, after::



		  echo 2 ><securityfs>/evm



		another echo can be performed::



		  echo 1 ><securityfs>/evm



		and the resulting value will be 3.



		Note that once an HMAC key has been loaded, it will no longer

		be possible to enable metadata modification. Signaling that an

		HMAC key has been loaded will clear the corresponding flag.

		For example, if the current value is 6 (2 and 4 set)::



		  echo 1 ><securityfs>/evm



		will set the new value to 3 (4 cleared).



		Loading an HMAC key is the only way to disable metadata

		modification.



		Until key loading has been signaled EVM can not create

		or validate the 'security.evm' xattr, but returns

security/integrity/evm/evm_secfs.c

+4 −4
Original line number Diff line number Diff line
@@ -81,12 +81,12 @@ static ssize_t evm_write_key(struct file *file, const char __user *buf, 
	if (!i || (i & ~EVM_INIT_MASK) != 0)

		return -EINVAL;



	/* Don't allow a request to freshly enable metadata writes if

	 * keys are loaded.

	/*

	 * Don't allow a request to enable metadata writes if

	 * an HMAC key is loaded.

	 */

	if ((i & EVM_ALLOW_METADATA_WRITES) &&

	    ((evm_initialized & EVM_KEY_MASK) != 0) &&

	    !(evm_initialized & EVM_ALLOW_METADATA_WRITES))

	    (evm_initialized & EVM_INIT_HMAC) != 0)

		return -EPERM;



	if (i & EVM_INIT_HMAC) {