crypto: algif_hash - Fix NULL hash crash with shash

Recently algif_hash has been changed to allow null hashes.  This
triggers a bug when used with an shash algorithm whereby it will
cause a crash during the digest operation.

This patch fixes it by avoiding the digest operation and instead
doing an init followed by a final which avoids the buggy code in
shash.

This patch also ensures that the result buffer is freed after an
error so that it is not returned as a genuine hash result on the
next recv call.

The shash/ahash wrapper code will be fixed later to handle this
case correctly.

Fixes: 493b2ed3 ("crypto: algif_hash - Handle NULL hashes correctly")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Laura Abbott <labbott@redhat.com>

crypto/algif_hash.c

@@ -214,23 +214,26 @@ static int hash_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 
 	ahash_request_set_crypt(&ctx->req, NULL, ctx->result, 0);
 
-	if (ctx->more) {
+	if (!result) {
+		err = af_alg_wait_for_completion(
+				crypto_ahash_init(&ctx->req),
+				&ctx->completion);
+		if (err)
+			goto unlock;
+	}
+
+	if (!result || ctx->more) {
 		ctx->more = 0;
 		err = af_alg_wait_for_completion(crypto_ahash_final(&ctx->req),
 						 &ctx->completion);
 		if (err)
 			goto unlock;
-	} else if (!result) {
-		err = af_alg_wait_for_completion(
-				crypto_ahash_digest(&ctx->req),
-				&ctx->completion);
 	}
 
 	err = memcpy_to_msg(msg, ctx->result, len);
 
-	hash_free_result(sk, ctx);
-
 unlock:
+	hash_free_result(sk, ctx);
 	release_sock(sk);
 
 	return err ?: len;