mpls: drop skb's dst in mpls_forward()

Commit 394de110a733 ("net: Added pointer check for
dst->ops->neigh_lookup in dst_neigh_lookup_skb") added a test in
dst_neigh_lookup_skb() to avoid a NULL pointer dereference. The root
cause was the MPLS forwarding code, which doesn't call skb_dst_drop()
on incoming packets. That is, if the packet is received from a
collect_md device, it has a metadata_dst attached to it that doesn't
implement any dst_ops function.

To align the MPLS behaviour with IPv4 and IPv6, let's drop the dst in
mpls_forward(). This way, dst_neigh_lookup_skb() doesn't need to test
->neigh_lookup any more. Let's keep a WARN condition though, to
document the precondition and to ease detection of such problems in the
future.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/f8c2784c13faa54469a2aac339470b1049ca6b63.1604102750.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/net/mpls/af_mpls.c b/net/mpls/af_mpls.c
index f2868a8a50c3..47bab701555f 100644
--- a/net/mpls/af_mpls.c
+++ b/net/mpls/af_mpls.c
@@ -377,6 +377,8 @@ static int mpls_forward(struct sk_buff *skb, struct net_device *dev,
 	if (!pskb_may_pull(skb, sizeof(*hdr)))
 		goto err;
 
+	skb_dst_drop(skb);
+
 	/* Read and decode the label */
 	hdr = mpls_hdr(skb);
 	dec = mpls_entry_decode(hdr);