Patrick McHardy [off-list ref] wrote:

On 30.08.2011 12:57, Florian Westphal wrote:

quoted

When incoking iptables hooks from bridge netfilter, the assumption
that non-confirmed skb->nfct is never shared does no longer hold,
as bridge code clones skbs when e.g. forwarding packets to multiple
bridge ports.

When NFQUEUE is used, we can BUG because nf_nat_setup_info can be
invoked simultaneously for the same conntrack:

I'm wondering how this can happen, when flooding packets to multiple
ports, they are still processed by the same CPU one after another,
so for the second and further packets, nf_nat should notice that
the mappings are already set up.

Main problem is that we end up with same ->nfct in both
INPUT and POSTROUTING (br_pass_frame_up vs. br_forward).

its extremely unlikely but reproduceable with something like
hping2 -i u1200 -2 -p 138 -d 128 192.168.0.255

(assuming bridge interface has an address within that network).

Also, with recent change nf_reinject can be run in parallel.
(the original problem was observed on 2.6.32.24, but i can
 reproduce it with nf-next, too).