crypto: testmgr - add guard to dst buffer for ahash_export

Add a guard to 'state' buffer and warn if its consistency after
call to crypto_ahash_export() changes, so that any write that
goes beyond advertised statesize (and thus causing potential
memory corruption [1]) is more visible.

[1] https://marc.info/?l=linux-crypto-vger&m=147467656516085

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Marcelo Cerri <marcelo.cerri@canonical.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/testmgr.c

@@ -209,16 +209,19 @@ static int ahash_partial_update(struct ahash_request **preq,
 	char *state;
 	struct ahash_request *req;
 	int statesize, ret = -EINVAL;
+	const char guard[] = { 0x00, 0xba, 0xad, 0x00 };
 
 	req = *preq;
 	statesize = crypto_ahash_statesize(
 			crypto_ahash_reqtfm(req));
-	state = kmalloc(statesize, GFP_KERNEL);
+	state = kmalloc(statesize + sizeof(guard), GFP_KERNEL);
 	if (!state) {
 		pr_err("alt: hash: Failed to alloc state for %s\n", algo);
 		goto out_nostate;
 	}
+	memcpy(state + statesize, guard, sizeof(guard));
 	ret = crypto_ahash_export(req, state);
+	WARN_ON(memcmp(state + statesize, guard, sizeof(guard)));
 	if (ret) {
 		pr_err("alt: hash: Failed to export() for %s\n", algo);
 		goto out;