[PATCH nf v2] netfilter: bridge: use DEBUG_NET_WARN_ON_ONCE in nf_ct_bridge_refrag()

[PATCH nf v2] netfilter: bridge: use DEBUG_NET_WARN_ON_ONCE in nf_ct_bridge_refrag()

[Laveesh Bansal]

nf_ct_bridge_refrag() warns when skb->protocol is neither IPv4 nor IPv6.
A vlan filtering bridge can get there with a vlan ethertype: if the
protocol of the tag of a frame differs from the vlan_protocol of the
bridge, br_allowed_ingress() moves the tag back into the packet after
NF_BR_PRE_ROUTING, where bridge conntrack has already defragmented it.

Anyone who can send fragments into a port of such a bridge can trigger the
warning, which takes the machine down with panic_on_warn. The packet is
dropped either way, so only warn on kernels built with CONFIG_DEBUG_NET.

This only avoids the warning. Such fragmented packets are still dropped.
syzbot reported the warning. It keeps triggering on CONFIG_DEBUG_NET
kernels until the vlan tag is handled, which is left for a follow-up:

  https://syzkaller.appspot.com/bug?extid=a8ba738fe2db6b4bb27f

Fixes: 3c171f496ef5 ("netfilter: bridge: add connection tracking system")
Assisted-by: LLM
Signed-off-by: Laveesh Bansal <laveeshb@laveeshbansal.com>
---
v2:
- Reduced to DEBUG_NET_WARN_ON_ONCE(), as suggested by Pablo. Handling the
  vlan tag and the selftest will follow for nf-next.
- Dropped Reported-by, Closes and Cc: stable. This does not fix the syzbot
  report, its config has CONFIG_DEBUG_NET=y.
- Only the commit that added the warning is listed in Fixes:.

v1: https://lore.kernel.org/r/20261020005403.38212-1-laveeshb@laveeshbansal.com

 net/bridge/netfilter/nf_conntrack_bridge.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/bridge/netfilter/nf_conntrack_bridge.c b/net/bridge/netfilter/nf_conntrack_bridge.c
index b5444335b86f..8c83dc05114b 100644
--- a/net/bridge/netfilter/nf_conntrack_bridge.c
+++ b/net/bridge/netfilter/nf_conntrack_bridge.c
@@ -358,7 +358,7 @@ nf_ct_bridge_refrag(struct sk_buff *skb, const struct nf_hook_state *state,
 		nf_br_ip6_fragment(state->net, state->sk, skb, &data, output);
 		break;
 	default:
-		WARN_ON_ONCE(1);
+		DEBUG_NET_WARN_ON_ONCE(1);
 		return NF_DROP;
 	}
 
-- 
2.43.0