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Message-ID: <ZmDAQ6r49kSgwaMm@calendula>
Date: Wed, 5 Jun 2024 21:45:07 +0200
From: Pablo Neira Ayuso <pablo@...filter.org>
To: Florian Westphal <fw@...len.de>
Cc: Christoph Paasch <cpaasch@...le.com>,
Netfilter <netfilter-devel@...r.kernel.org>,
Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
daniel@...earbox.net, willemb@...gle.com
Subject: Re: [PATCH nf] netfilter: nf_reject: init skb->dev for reset packet
On Wed, Jun 05, 2024 at 09:08:33PM +0200, Florian Westphal wrote:
> Pablo Neira Ayuso <pablo@...filter.org> wrote:
>
> [ CC Willem ]
>
> > On Wed, Jun 05, 2024 at 08:14:50PM +0200, Florian Westphal wrote:
> > > Christoph Paasch <cpaasch@...le.com> wrote:
> > > > > Reported-by: Christoph Paasch <cpaasch@...le.com>
> > > > > Suggested-by: Paolo Abeni <pabeni@...hat.com>
> > > > > Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/494
> > > > > Signed-off-by: Florian Westphal <fw@...len.de>
> > > >
> > > > I just gave this one a shot in my syzkaller instances and am still hitting the issue.
> > >
> > > No, different bug, this patch is correct.
> > >
> > > I refuse to touch the flow dissector.
> >
> > I see callers of ip_local_out() in the tree which do not set skb->dev.
> >
> > I don't understand this:
> >
> > bool __skb_flow_dissect(const struct net *net,
> > const struct sk_buff *skb,
> > struct flow_dissector *flow_dissector,
> > void *target_container, const void *data,
> > __be16 proto, int nhoff, int hlen, unsigned int flags)
> > {
> > [...]
> > WARN_ON_ONCE(!net);
> > if (net) {
> >
> > it was added by 9b52e3f267a6 ("flow_dissector: handle no-skb use case")
> >
> > Is this WARN_ON_ONCE() bogus?
>
> When this was added (handle dissection from bpf prog, per netns), the correct
> solution would have been to pass 'struct net' explicitly via skb_get_hash()
> and all variants. As that was likely deemed to be too much code churn it
> tries to infer struct net via skb->{dev,sk}.
>
> So there are several options here:
> 1. remove the WARN_ON_ONCE and be done with it
> 2. remove the WARN_ON_ONCE and pretend net was init_net
> 3. also look at skb_dst(skb)->dev if skb->dev is unset, then back to 1)
> or 2)
> 4. stop using skb_get_hash() from netfilter (but there are likely other
> callers that might hit this).
> 5. fix up callers, one by one
> 6. assign skb->dev inside netfilter if its unset
>
> 3 and 2 combined are probably going to be the least invasive.
>
> 5 might take some time, we now know two, namely tcp resets generated
> from netfilter and igmp_send_report(). No idea if there are more.
Quickly browsing, synproxy and tee also calls ip_local_out() too.
icmp_send() which is used, eg. to send destination unreachable too to
reset.
There is also __skb_get_hash_symmetric() that could hit this from
nft_hash?
No idea what more callers need to be adjusted to remove this splat,
that was a cursory tree review.
And ip_output() sets on skb->dev from postrouting path, then if
callers are fixed, then skb->dev would be once then again from output path?
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