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

[Alexei Starovoitov]

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?

Please add a selftest to make sure we don't regress.

Also pls mark your patch as [PATCH v3 bpf] when you respin.

> 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
>