Re: [PATCH v2 net] bpf: do not return NET_XMIT_xxx values on bpf_redirect

[Yan Zhai]

On Tue, Jul 18, 2023 at 10:42 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Tue, Jul 18, 2023 at 8:30 PM Yan Zhai <yan@xxxxxxxxxxxxxx> wrote:
> >
> > skb_do_redirect handles returns error code from both rx and tx path. The
> > tx path codes are special, e.g. NET_XMIT_CN: they are non-negative, and
> > can conflict with LWTUNNEL_XMIT_xxx values. Directly returning such code
> > can cause unexpected behavior. We found at least one bug that will panic
> > the kernel through KASAN report when we are redirecting packets to a
> > down or carrier-down device at lwt xmit hook:
> >
> > https://gist.github.com/zhaiyan920/8fbac245b261fe316a7ef04c9b1eba48
> >
> > Above bug is hit because NET_XMIT_CN is returned by noop_qdisc of the
> > down device, and it propagates from dev_queue_xmit all way to the lwt
> > logic. The result is skb that has been freed by the qdisc continues to
> > neighbor subsystem and triggers the bug.
>
> I'm struggling to parse the above paragraph.
> Where bpf prog is installed?
> Is this lwt bpf prog that returns BPF_REDIRECT ?
> that redirects to netdev with noop_qdisc ?
> What is the topology?
>
Sorry for the confusion. Mentioning noop_qdisc is an explanation of
what happened. The actual trigger is simple: install a bpf program on
lwt route at xmit hook. It bpf_redirect packets to a device FOO. If
FOO is down or carrier-down, redirected packets will crash the kernel.

> Please add a selftest to make sure we don't regress.
>
> Also pls mark your patch as [PATCH v3 bpf] when you respin.
>
Ack

> > This change converts the tx code to proper errors that lwt can consume.
> >
> > Suggested-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
> > Reported-by: Jordan Griege <jgriege@xxxxxxxxxxxxxx>
> > Signed-off-by: Yan Zhai <yan@xxxxxxxxxxxxxx>
> > ---
> > v2: coding style fix; sent to netdev instead of bpf for bug fixing.
> >
> > ---
> >  net/core/filter.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 06ba0e56e369..8738c7a4701d 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -2129,6 +2129,9 @@ static inline int __bpf_tx_skb(struct net_device *dev, struct sk_buff *skb)
> >         ret = dev_queue_xmit(skb);
> >         dev_xmit_recursion_dec();
> >
> > +       if (unlikely(ret > 0))
> > +               ret = net_xmit_errno(ret);
> > +
> >         return ret;
> >  }
> >
> > --
> > 2.30.2
> >



-- 

Yan