crypto: ccp/gcm - use const time tag comparison.

Avoid leaking GCM tag through timing side channel.

Fixes: 36cf515b ("crypto: ccp - Enable support for AES GCM on v5 CCPs")
Cc: <stable@vger.kernel.org> # v4.12+
Signed-off-by: Cfir Cohen <cfir@google.com>
Acked-by: Gary R Hook <ghook@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/crypto/ccp/ccp-ops.c

@@ -850,7 +850,8 @@ static int ccp_run_aes_gcm_cmd(struct ccp_cmd_queue *cmd_q,
 		if (ret)
 			goto e_tag;
 
-		ret = memcmp(tag.address, final_wa.address, AES_BLOCK_SIZE);
+		ret = crypto_memneq(tag.address, final_wa.address,
+				    AES_BLOCK_SIZE) ? -EBADMSG : 0;
 		ccp_dm_free(&tag);
 	}