gso: fix dodgy bit handling for GSO_UDP_L4

Commit 1fd54773c2 ("udp: allow header check for dodgy GSO_UDP_L4
packets.") checks DODGY bit for UDP, but for packets that can be fed
directly to the device after gso_segs reset, it actually falls through
to fragmentation:

https://lore.kernel.org/all/CAJPywTKDdjtwkLVUW6LRA2FU912qcDmQOQGt2WaDo28KzYDg+A@mail.gmail.com/

This change restores the expected behavior of GSO_UDP_L4 packets.

Fixes: 1fd54773c2 ("udp: allow header check for dodgy GSO_UDP_L4 packets.")
Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Signed-off-by: Yan Zhai <yan@cloudflare.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp_offload.c

@@ -274,13 +274,20 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 	__sum16 check;
 	__be16 newlen;
 
-	if (skb_shinfo(gso_skb)->gso_type & SKB_GSO_FRAGLIST)
-		return __udp_gso_segment_list(gso_skb, features, is_ipv6);
-
 	mss = skb_shinfo(gso_skb)->gso_size;
 	if (gso_skb->len <= sizeof(*uh) + mss)
 		return ERR_PTR(-EINVAL);
 
+	if (skb_gso_ok(gso_skb, features | NETIF_F_GSO_ROBUST)) {
+		/* Packet is from an untrusted source, reset gso_segs. */
+		skb_shinfo(gso_skb)->gso_segs = DIV_ROUND_UP(gso_skb->len - sizeof(*uh),
+							     mss);
+		return NULL;
+	}
+
+	if (skb_shinfo(gso_skb)->gso_type & SKB_GSO_FRAGLIST)
+		return __udp_gso_segment_list(gso_skb, features, is_ipv6);
+
 	skb_pull(gso_skb, sizeof(*uh));
 
 	/* clear destructor to avoid skb_segment assigning it to tail */
@@ -388,8 +395,7 @@ static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
 	if (!pskb_may_pull(skb, sizeof(struct udphdr)))
 		goto out;
 
-	if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
-	    !skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST))
+	if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4)
 		return __udp_gso_segment(skb, features, false);
 
 	mss = skb_shinfo(skb)->gso_size;

net/ipv6/udp_offload.c

@@ -43,8 +43,7 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb,
 		if (!pskb_may_pull(skb, sizeof(struct udphdr)))
 			goto out;
 
-		if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
-		    !skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST))
+		if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4)
 			return __udp_gso_segment(skb, features, true);
 
 		mss = skb_shinfo(skb)->gso_size;