[PATCH net 2/5] vxlan: unclone skb head before modifying eth header in route_shortcircuit()

When route_shortcircuit() performs L3 short-circuit routing, it modifies
the Ethernet header of the skb in-place:
    memcpy(eth_hdr(skb)->h_source, eth_hdr(skb)->h_dest, dev->addr_len);
    memcpy(eth_hdr(skb)->h_dest, n->ha, dev->addr_len);

If the incoming skb is cloned (for example by packet sockets, tcpdump, or
dev_queue_xmit), modifying the Ethernet header without uncloning can corrupt
the packet header for other readers holding a reference to the cloned skb.

Ensure the skb header is writable and unshared by calling skb_cow_head(skb, 0)
prior to updating the Ethernet header. If skb_cow_head() fails, abort short-circuiting
and return false to allow standard packet processing fallback.

Fixes: e4f67addf158 ("add DOVE extensions for VXLAN")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 drivers/net/vxlan/vxlan_core.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c
index 9060a1259ac283d4db4b7a854ce74203496e9313..fb4a706cd5786d1937b1922acced31ffdb187c2f 100644
--- a/drivers/net/vxlan/vxlan_core.c
+++ b/drivers/net/vxlan/vxlan_core.c

@@ -2163,6 +2163,10 @@ static bool route_shortcircuit(struct net_device *dev, struct sk_buff *skb)
 
 		diff = !ether_addr_equal(eth_hdr(skb)->h_dest, n->ha);
 		if (diff) {
+			if (skb_cow_head(skb, 0)) {
+				neigh_release(n);
+				return false;
+			}
 			memcpy(eth_hdr(skb)->h_source, eth_hdr(skb)->h_dest,
 				dev->addr_len);
 			memcpy(eth_hdr(skb)->h_dest, n->ha, dev->addr_len);

-- 
2.55.0.229.g6434b31f56-goog