keys: Guard against null match function in keyring_search_aux()

The "dead" key type has no match operation, and a search for keys of
this type can cause a null dereference in keyring_search_iterator().
keyring_search() has a check for this, but request_keyring_and_link()
does not.  Move the check into keyring_search_aux(), covering both of
them.

This was fixed upstream by commit c06cfb08 ("KEYS: Remove
key_type::match in favour of overriding default by match_preparse"),
part of a series of large changes that are not suitable for
backporting.

CVE-2017-2647 / CVE-2017-6951
Reported-by: Igor Redko <redkoi@virtuozzo.com>
Reported-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
References: https://bugzilla.redhat.com/show_bug.cgi?id=CVE-2017-2647Reported-by: idl3r <idler1984@gmail.com>
References: https://www.spinics.net/lists/keyrings/msg01845.htmlSigned-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: David Howells <dhowells@redhat.com>

security/keys/keyring.c

@@ -848,6 +848,9 @@ key_ref_t keyring_search_aux(key_ref_t keyring_ref,
 			return ERR_PTR(err);
 	}
 
+	if (!ctx->match)
+		return ERR_PTR(-ENOKEY);
+
 	rcu_read_lock();
 	ctx->now = current_kernel_time();
 	if (search_nested_keyrings(keyring, ctx))
@@ -879,9 +882,6 @@ key_ref_t keyring_search(key_ref_t keyring,
 					   KEYRING_SEARCH_DO_STATE_CHECK),
 	};
 
-	if (!ctx.match)
-		return ERR_PTR(-ENOKEY);
-
 	return keyring_search_aux(keyring, &ctx);
 }
 EXPORT_SYMBOL(keyring_search);