crypto: algif_hash - avoid zero-sized array

With this reproducer:
  struct sockaddr_alg alg = {
          .salg_family = 0x26,
          .salg_type = "hash",
          .salg_feat = 0xf,
          .salg_mask = 0x5,
          .salg_name = "digest_null",
  };
  int sock, sock2;

  sock = socket(AF_ALG, SOCK_SEQPACKET, 0);
  bind(sock, (struct sockaddr *)&alg, sizeof(alg));
  sock2 = accept(sock, NULL, NULL);
  setsockopt(sock, SOL_ALG, ALG_SET_KEY, "\x9b\xca", 2);
  accept(sock2, NULL, NULL);

==== 8< ======== 8< ======== 8< ======== 8< ====

one can immediatelly see an UBSAN warning:
UBSAN: Undefined behaviour in crypto/algif_hash.c:187:7
variable length array bound value 0 <= 0
CPU: 0 PID: 15949 Comm: syz-executor Tainted: G            E      4.4.30-0-default #1
...
Call Trace:
...
 [<ffffffff81d598fd>] ? __ubsan_handle_vla_bound_not_positive+0x13d/0x188
 [<ffffffff81d597c0>] ? __ubsan_handle_out_of_bounds+0x1bc/0x1bc
 [<ffffffffa0e2204d>] ? hash_accept+0x5bd/0x7d0 [algif_hash]
 [<ffffffffa0e2293f>] ? hash_accept_nokey+0x3f/0x51 [algif_hash]
 [<ffffffffa0e206b0>] ? hash_accept_parent_nokey+0x4a0/0x4a0 [algif_hash]
 [<ffffffff8235c42b>] ? SyS_accept+0x2b/0x40

It is a correct warning, as hash state is propagated to accept as zero,
but creating a zero-length variable array is not allowed in C.

Fix this as proposed by Herbert -- do "?: 1" on that site. No sizeof or
similar happens in the code there, so we just allocate one byte even
though we do not use the array.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net> (maintainer:CRYPTO API)
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/algif_hash.c

@@ -245,7 +245,7 @@ static int hash_accept(struct socket *sock, struct socket *newsock, int flags)
 	struct alg_sock *ask = alg_sk(sk);
 	struct hash_ctx *ctx = ask->private;
 	struct ahash_request *req = &ctx->req;
-	char state[crypto_ahash_statesize(crypto_ahash_reqtfm(req))];
+	char state[crypto_ahash_statesize(crypto_ahash_reqtfm(req)) ? : 1];
 	struct sock *sk2;
 	struct alg_sock *ask2;
 	struct hash_ctx *ctx2;