tls: rx: device: bound the frag walk

We can't do skb_walk_frags() on the input skbs, because
the input skbs is really just a pointer to the tcp read
queue. We need to bound the "is decrypted" check by the
amount of data in the message.

Note that the walk in tls_device_reencrypt() is after a
CoW so the skb there is safe to walk. Actually in the
current implementation it can't have frags at all, but
whatever, maybe one day it will.
Reported-by: Tariq Toukan <tariqt@nvidia.com>
Fixes: 84c61fe1 ("tls: rx: do not use the standard strparser")
Tested-by: Ran Rozenstein <ranro@nvidia.com>
Link: https://lore.kernel.org/r/20220809175544.354343-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/tls/tls_device.c

@@ -984,11 +984,17 @@ int tls_device_decrypted(struct sock *sk, struct tls_context *tls_ctx)
 	int is_decrypted = skb->decrypted;
 	int is_encrypted = !is_decrypted;
 	struct sk_buff *skb_iter;
+	int left;
 
+	left = rxm->full_len - skb->len;
 	/* Check if all the data is decrypted already */
-	skb_walk_frags(skb, skb_iter) {
+	skb_iter = skb_shinfo(skb)->frag_list;
+	while (skb_iter && left > 0) {
 		is_decrypted &= skb_iter->decrypted;
 		is_encrypted &= !skb_iter->decrypted;
+
+		left -= skb_iter->len;
+		skb_iter = skb_iter->next;
 	}
 
 	trace_tls_device_decrypted(sk, tcp_sk(sk)->copied_seq - rxm->full_len,