sunrpc: use constant time memory comparison for mac

Otherwise, we enable a MAC forgery via timing attack.
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Jeff Layton <jlayton@poochiereds.net>
Cc: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>
Cc: linux-nfs@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/auth_gss/gss_krb5_crypto.c

@@ -34,6 +34,7 @@
  * WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.
  */
 
+#include <crypto/algapi.h>
 #include <crypto/hash.h>
 #include <crypto/skcipher.h>
 #include <linux/err.h>
@@ -927,7 +928,7 @@ gss_krb5_aes_decrypt(struct krb5_ctx *kctx, u32 offset, struct xdr_buf *buf,
 	if (ret)
 		goto out_err;
 
-	if (memcmp(pkt_hmac, our_hmac, kctx->gk5e->cksumlength) != 0) {
+	if (crypto_memneq(pkt_hmac, our_hmac, kctx->gk5e->cksumlength) != 0) {
 		ret = GSS_S_BAD_SIG;
 		goto out_err;
 	}