net: Don't set transport offset to invalid value

If the socket was created with socket(AF_PACKET, SOCK_RAW, 0),
skb->protocol will be unset, __skb_flow_dissect() will fail, and
skb_probe_transport_header() will fall back to the offset_hint, making
the resulting skb_transport_offset incorrect.

If, however, there is no transport header in the packet,
transport_header shouldn't be set to an arbitrary value.

Fix it by leaving the transport offset unset if it couldn't be found, to
be explicit rather than to fill it with some wrong value. It changes the
behavior, but if some code relied on the old behavior, it would be
broken anyway, as the old one is incorrect.

Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 12 insertions, 3 deletions

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index 80aae3a32c2a..c801a832851c 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -1169,15 +1169,24 @@ static int xenvif_tx_submit(struct xenvif_queue *queue)
                         continue;
                 }
 
-                skb_probe_transport_header(skb, 0);
+                skb_probe_transport_header(skb);
 
                 /* If the packet is GSO then we will have just set up the
                  * transport header offset in checksum_setup so it's now
                  * straightforward to calculate gso_segs.
                  */
                 if (skb_is_gso(skb)) {
-                        int mss = skb_shinfo(skb)->gso_size;
-                        int hdrlen = skb_transport_header(skb) -
+                        int mss, hdrlen;
+
+                        /* GSO implies having the L4 header. */
+                        WARN_ON_ONCE(!skb_transport_header_was_set(skb));
+                        if (unlikely(!skb_transport_header_was_set(skb))) {
+                                kfree_skb(skb);
+                                continue;
+                        }
+
+                        mss = skb_shinfo(skb)->gso_size;
+                        hdrlen = skb_transport_header(skb) -
                                 skb_mac_header(skb) +
                                 tcp_hdrlen(skb);