keys: skip keys from another user namespace

When listing keys, do not return keys belonging to the
same uid in another user namespace.  Otherwise uid 500
in another user namespace will return keyrings called
uid.500 for another user namespace.
Signed-off-by: NSerge E. Hallyn <serue@us.ibm.com>
Acked-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NJames Morris <jmorris@namei.org>

security/keys/keyring.c

@@ -539,6 +539,9 @@ struct key *find_keyring_by_name(const char *name, bool skip_perm_check)
 				    &keyring_name_hash[bucket],
 				    type_data.link
 				    ) {
+			if (keyring->user->user_ns != current_user_ns())
+				continue;
+
 			if (test_bit(KEY_FLAG_REVOKED, &keyring->flags))
 				continue;