[PATCH] net: Fix resetting network_header in neigh_resolve_output and neigh_connected_output

[Abdelrhman Ahmed]

neigh_resolve_output and neigh_connected_output resets the skb to 
network_header because of the retry loop and this reset will pull down the data 
pointer to the network header in the first iteration then hardware header will 
be added, but it will overwrite any data which is inserted between network 
header and hardware header (for example by netfilter hooks) only for the first 
packet(s) before using cached hardware header as neigh_hh_output (which is 
called for using cached hardware header) does not reset to the network header.

The fix is to reset with reference to skb's data before loop instead of network 
header.

Fixes: e1f165032c8b ("net: Fix skb_under_panic oops in neigh_resolve_output")
Signed-off-by: Abdelrhman Ahmed <a...@abahmed.com>
---
 net/core/neighbour.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 29dd8cc..7aac242 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1293,15 +1293,19 @@ int neigh_resolve_output(struct neighbour *neigh, 
struct sk_buff *skb)
        int rc = 0;
 
        if (!neigh_event_send(neigh, skb)) {
-               int err;
+               int err, offset;
                struct net_device *dev = neigh->dev;
+               unsigned char *data;
                unsigned int seq;
 
                if (dev->header_ops->cache && !neigh->hh.hh_len)
                        neigh_hh_init(neigh);
 
+               data = skb->data;
+
                do {
-                       __skb_pull(skb, skb_network_offset(skb));
+                       offset = data - skb->data;
+                       __skb_pull(skb, offset);
                        seq = read_seqbegin(&neigh->ha_lock);
                        err = dev_hard_header(skb, dev, ntohs(skb->protocol),
                                              neigh->ha, NULL, skb->len);
@@ -1326,11 +1330,15 @@ EXPORT_SYMBOL(neigh_resolve_output);
 int neigh_connected_output(struct neighbour *neigh, struct sk_buff *skb)
 {
        struct net_device *dev = neigh->dev;
+       unsigned char *data;
        unsigned int seq;
-       int err;
+       int err, offset;
+
+       data = skb->data;
 
        do {
-               __skb_pull(skb, skb_network_offset(skb));
+               offset = data - skb->data;
+               __skb_pull(skb, offset);
                seq = read_seqbegin(&neigh->ha_lock);
                err = dev_hard_header(skb, dev, ntohs(skb->protocol),
                                      neigh->ha, NULL, skb->len);
-- 
1.9.1