tls: Use size_add() in call to struct_size()

If, for any reason, the open-coded arithmetic causes a wraparound,
the protection that `struct_size()` adds against potential integer
overflows is defeated. Fix this by hardening call to `struct_size()`
with `size_add()`.

Fixes: b89fec5 ("tls: rx: wrap decrypt params in a struct")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tls/tls_sw.c

@@ -1487,7 +1487,7 @@ static int tls_decrypt_sg(struct sock *sk, struct iov_iter *out_iov,
 	 */
 	aead_size = sizeof(*aead_req) + crypto_aead_reqsize(ctx->aead_recv);
 	aead_size = ALIGN(aead_size, __alignof__(*dctx));
-	mem = kmalloc(aead_size + struct_size(dctx, sg, n_sgin + n_sgout),
+	mem = kmalloc(aead_size + struct_size(dctx, sg, size_add(n_sgin, n_sgout)),
 		      sk->sk_allocation);
 	if (!mem) {
 		err = -ENOMEM;