[PATCH 03/11] evm: Refuse EVM_ALLOW_METADATA_WRITES only if the HMAC key is loaded

[Roberto Sassu]

Granting metadata write is safe if the HMAC key is not loaded, as it won't
let an attacker obtain a valid HMAC from corrupted xattrs. evm_write_key()
however does not allow it if any key is loaded, including a public key,
which should not be a problem.

This patch allows setting EVM_ALLOW_METADATA_WRITES if the EVM_INIT_HMAC
flag is not set.

Cc: stable at vger.kernel.org # 4.16.x
Fixes: ae1ba1676b88e ("EVM: Allow userland to permit modification of EVM-protected metadata")
Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
---
 security/integrity/evm/evm_secfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c
index cfc3075769bb..92fe26ace797 100644
--- a/security/integrity/evm/evm_secfs.c
+++ b/security/integrity/evm/evm_secfs.c
@@ -84,7 +84,7 @@ static ssize_t evm_write_key(struct file *file, const char __user *buf,
 	 * keys are loaded.
 	 */
 	if ((i & EVM_ALLOW_METADATA_WRITES) &&
-	    ((evm_initialized & EVM_KEY_MASK) != 0) &&
+	    ((evm_initialized & EVM_INIT_HMAC) != 0) &&
 	    !(evm_initialized & EVM_ALLOW_METADATA_WRITES))
 		return -EPERM;
 
-- 
2.17.1