[PATCH net 11/14] netfilter: flowtable: ensure sufficient headroom in xmit path

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

Check for headroom and call skb_expand_head() like in the IP output
path to ensure there is sufficient headroom for the mac header when
forwarding this packet as suggested by sashiko.

Fixes: b5964aac51e0 ("netfilter: flowtable: consolidate xmit path")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
 net/netfilter/nf_flow_table_ip.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nf_flow_table_ip.c b/net/netfilter/nf_flow_table_ip.c
index dbd7644fdbeb..8d5fb7e940a1 100644
--- a/net/netfilter/nf_flow_table_ip.c
+++ b/net/netfilter/nf_flow_table_ip.c
@@ -471,8 +471,17 @@ struct nf_flow_xmit {
 static unsigned int nf_flow_queue_xmit(struct net *net, struct sk_buff *skb,
 				       struct nf_flow_xmit *xmit)
 {
-	skb->dev = xmit->outdev;
-	dev_hard_header(skb, skb->dev, ntohs(skb->protocol),
+	struct net_device *dev = xmit->outdev;
+	unsigned int 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)
+			return NF_STOLEN;
+	}
+
+	skb->dev = dev;
+	dev_hard_header(skb, dev, ntohs(skb->protocol),
 			xmit->dest, xmit->source, skb->len);
 	dev_queue_xmit(skb);
 
-- 
2.47.3