crypto: user - prevent operating on larval algorithms

Michal Suchanek reported [1] that running the pcrypt_aead01 test from
LTP [2] in a loop and holding Ctrl-C causes a NULL dereference of
alg->cra_users.next in crypto_remove_spawns(), via crypto_del_alg().
The test repeatedly uses CRYPTO_MSG_NEWALG and CRYPTO_MSG_DELALG.

The crash occurs when the instance that CRYPTO_MSG_DELALG is trying to
unregister isn't a real registered algorithm, but rather is a "test
larval", which is a special "algorithm" added to the algorithms list
while the real algorithm is still being tested.  Larvals don't have
initialized cra_users, so that causes the crash.  Normally pcrypt_aead01
doesn't trigger this because CRYPTO_MSG_NEWALG waits for the algorithm
to be tested; however, CRYPTO_MSG_NEWALG returns early when interrupted.

Everything else in the "crypto user configuration" API has this same bug
too, i.e. it inappropriately allows operating on larval algorithms
(though it doesn't look like the other cases can cause a crash).

Fix this by making crypto_alg_match() exclude larval algorithms.

[1] https://lkml.kernel.org/r/20190625071624.27039-1-msuchanek@suse.de
[2] https://github.com/linux-test-project/ltp/blob/20190517/testcases/kernel/crypto/pcrypt_aead01.c

Reported-by: Michal Suchanek <msuchanek@suse.de>
Fixes: a38f7907b9 ("crypto: Add userspace configuration API")
Cc: <stable@vger.kernel.org> # v3.2+
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/crypto_user_base.c

@@ -56,6 +56,9 @@ struct crypto_alg *crypto_alg_match(struct crypto_user_alg *p, int exact)
 	list_for_each_entry(q, &crypto_alg_list, cra_list) {
 		int match = 0;
 
+		if (crypto_is_larval(q))
+			continue;
+
 		if ((q->cra_flags ^ p->cru_type) & p->cru_mask)
 			continue;