[PATCH net 02/14] netfilter: nft_fwd_netdev: add device and headroom validate with neigh forwarding

[Pablo Neira Ayuso <pablo@netfilter.org>]

The ttl field has been decremented already and evaluation of this rule
would proceed, just drop this packet instead if there is no destination
device to forwards this packet. This is exactly what nf_dup already does
in this case.

Moreover, check for headroom and call skb_expand_head() like in the IP
output path to ensure there is sufficient headroom when forwarding this
via neigh_xmit().

Fixes: d32de98ea70f ("netfilter: nft_fwd_netdev: allow to forward packets via neighbour layer")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
 net/netfilter/nft_fwd_netdev.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nft_fwd_netdev.c b/net/netfilter/nft_fwd_netdev.c
index 2cc809303ce8..605b1d42abce 100644
--- a/net/netfilter/nft_fwd_netdev.c
+++ b/net/netfilter/nft_fwd_netdev.c
@@ -102,6 +102,7 @@ static void nft_fwd_neigh_eval(const struct nft_expr *expr,
 	struct sk_buff *skb = pkt->skb;
 	int nhoff = skb_network_offset(skb);
 	struct net_device *dev;
+	unsigned int hh_len;
 	int neigh_table;
 
 	switch (priv->nfproto) {
@@ -153,8 +154,19 @@ static void nft_fwd_neigh_eval(const struct nft_expr *expr,
 	}
 
 	dev = dev_get_by_index_rcu(nft_net(pkt), oif);
-	if (dev == NULL)
-		return;
+	if (dev == NULL) {
+		verdict = NF_DROP;
+		goto out;
+	}
+
+	hh_len = LL_RESERVED_SPACE(dev);
+	if (unlikely(skb_headroom(skb) < hh_len && dev->header_ops)) {
+		skb = skb_expand_head(skb, hh_len);
+		if (!skb) {
+			verdict = NF_STOLEN;
+			goto out;
+		}
+	}
 
 	skb->dev = dev;
 	skb_clear_tstamp(skb);
-- 
2.47.3