evm: skip replacing EVM signature with HMAC on read-only filesystem

If filesystem is mounted read-only or file is immutable, updating
xattr will fail. This is a usual case during early boot until
filesystem is remount read-write. This patch verifies conditions
to skip unnecessary attempt to calculate HMAC and set xattr.

Changes in v2:
* indention changed according to Lindent (requested by Mimi)
Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

security/integrity/evm/evm_main.c

@@ -162,8 +162,13 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
 					(const char *)xattr_data, xattr_len,
 					calc.digest, sizeof(calc.digest));
 		if (!rc) {
-			/* we probably want to replace rsa with hmac here */
-			evm_update_evmxattr(dentry, xattr_name, xattr_value,
+			/* Replace RSA with HMAC if not mounted readonly and
+			 * not immutable
+			 */
+			if (!IS_RDONLY(dentry->d_inode) &&
+			    !IS_IMMUTABLE(dentry->d_inode))
+				evm_update_evmxattr(dentry, xattr_name,
+						    xattr_value,
 						    xattr_value_len);
 		}
 		break;