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

Pablo Neira Ayuso
2026-05-01T12:22:58Z

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