security: keys: perform capable check only on privileged operations

If the current task fails the check for the queried capability via
`capable(CAP_SYS_ADMIN)` LSMs like SELinux generate a denial message.
Issuing such denial messages unnecessarily can lead to a policy author
granting more privileges to a subject than needed to silence them.

Reorder CAP_SYS_ADMIN checks after the check whether the operation is
actually privileged.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

Author: cgzones

security/keys/keyctl.c

@@ -980,14 +980,19 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
 	ret = -EACCES;
 	down_write(&key->sem);
 
-	if (!capable(CAP_SYS_ADMIN)) {
+	{
+		bool is_privileged_op = false;
+
 		/* only the sysadmin can chown a key to some other UID */
 		if (user != (uid_t) -1 && !uid_eq(key->uid, uid))
-			goto error_put;
+			is_privileged_op = true;
 
 		/* only the sysadmin can set the key's GID to a group other
 		 * than one of those that the current process subscribes to */
 		if (group != (gid_t) -1 && !gid_eq(gid, key->gid) && !in_group_p(gid))
+			is_privileged_op = true;
+
+		if (is_privileged_op && !capable(CAP_SYS_ADMIN))
 			goto error_put;
 	}
 
@@ -1088,7 +1093,7 @@ long keyctl_setperm_key(key_serial_t id, key_perm_t perm)
 	down_write(&key->sem);
 
 	/* if we're not the sysadmin, we can only change a key that we own */
-	if (capable(CAP_SYS_ADMIN) || uid_eq(key->uid, current_fsuid())) {
+	if (uid_eq(key->uid, current_fsuid()) || capable(CAP_SYS_ADMIN)) {
 		key->perm = perm;
 		notify_key(key, NOTIFY_KEY_SETATTR, 0);
 		ret = 0;