tls: fix skb_to_sgvec returning unhandled error.

The current code does not inspect the return value of skb_to_sgvec. This
can cause a nullptr kernel panic when the malformed sgvec is passed into
the crypto request.

Checking the return value of skb_to_sgvec and skipping decryption if it
is negative fixes this problem.

Fixes: c46234eb ("tls: RX path for ktls")
Acked-by: Dave Watson <davejwatson@fb.com>
Signed-off-by: Doron Roberts-Kedes <doronrk@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tls/tls_sw.c

@@ -701,6 +701,10 @@ static int decrypt_skb(struct sock *sk, struct sk_buff *skb,
 	nsg = skb_to_sgvec(skb, &sgin[1],
 			   rxm->offset + tls_ctx->rx.prepend_size,
 			   rxm->full_len - tls_ctx->rx.prepend_size);
+	if (nsg < 0) {
+		ret = nsg;
+		goto out;
+	}
 
 	tls_make_aad(ctx->rx_aad_ciphertext,
 		     rxm->full_len - tls_ctx->rx.overhead_size,
@@ -712,6 +716,7 @@ static int decrypt_skb(struct sock *sk, struct sk_buff *skb,
 				rxm->full_len - tls_ctx->rx.overhead_size,
 				skb, sk->sk_allocation);
 
+out:
 	if (sgin != &sgin_arr[0])
 		kfree(sgin);