Re: [PATCH nf] netfilter: flowtable: fix pmtu discovery in nf_flow_tunnel_ip6ip6_push()

Re: [PATCH nf] netfilter: flowtable: fix pmtu discovery in nf_flow_tunnel_ip6ip6_push()

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
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Re: [PATCH nf] netfilter: flowtable: fix pmtu discovery in nf_flow_tunnel_ip6ip6_push()

[Pablo Neira Ayuso]

On Mon, Oct 19, 2026 at 10:19:07AM +0000, netdev-bot+sinfo@kernel.org wrote:
> Hi!
> 
> This is an automated message. This series looks like a fix, but its
> commit messages seem to be missing some information:
> 
>  - How the issue was discovered, e.g. hit in production, hit during
>    development, syzbot report, manual code inspection, LLM or static
>    analysis tool scan.
> 
>  - Whether the issue was actually triggered, or is only theoretical
>    (e.g. found by code inspection). If it was triggered please include
>    the symptoms, like the stack trace or error messages.

Sashiko reported this.

[PATCH nf] netfilter: flowtable: fix pmtu discovery in nf_flow_tunnel_ip6ip6_push()

[Lorenzo Bianconi]

nf_flow_tunnel_ip6ip6_push() invokes skb_scrub_packet() before updating
the PMTU. skb_scrub_packet() drops the skb dst, so
skb_dst_update_pmtu_no_confirm() finds no dst and the PMTU update is
silently lost.

Move the PMTU update before skb_scrub_packet() so the inner route is
updated with the MTU advertised by the tunnel dst, as done in
ip6_tnl_xmit().

Fixes: 93cf357fa797 ("netfilter: flowtable: Add IP6IP6 tx sw acceleration")
Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@oss.qualcomm.com>
---
 net/netfilter/nf_flow_table_ip.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/netfilter/nf_flow_table_ip.c b/net/netfilter/nf_flow_table_ip.c
index c8c29a9a1684..12faf85ee374 100644
--- a/net/netfilter/nf_flow_table_ip.c
+++ b/net/netfilter/nf_flow_table_ip.c
@@ -664,10 +664,10 @@ static int nf_flow_tunnel_ip6ip6_push(struct net *net, struct sk_buff *skb,
 	if (err)
 		return err;
 
-	skb_scrub_packet(skb, true);
 	mtu = dst_mtu(&rt->dst) - sizeof(*ip6h);
 	mtu = max(mtu, IPV6_MIN_MTU);
 	skb_dst_update_pmtu_no_confirm(skb, mtu);
+	skb_scrub_packet(skb, true);
 
 	skb_push(skb, sizeof(*ip6h));
 	skb_reset_network_header(skb);

---
base-commit: ee2d4c2d7cc18a2cd504044a75ea614e720248eb
change-id: 20261019-nf_flow_tunnel_ip6ip6_push-fix-pmtu-ff81656e32fc

Best regards,
-- 
Lorenzo Bianconi <lorenzo.bianconi@oss.qualcomm.com>