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

[Stanislav Fomichev]

On Tue, Jul 18, 2023 at 11:22 AM 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
> accidentally redirect 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. Although skb has been freed by the qdisc, it still continues to
> neighbor subsystem and triggers the bug.
>
> This change converts the tx code to proper errors that lwt can consume.
>
> Reported-by: Jordan Griege <jgriege@xxxxxxxxxxxxxx>
> Signed-off-by: Yan Zhai <yan@xxxxxxxxxxxxxx>
> ---
>  net/core/filter.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 06ba0e56e369..c9cc501ecdc0 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2129,6 +2129,11 @@ static inline int __bpf_tx_skb(struct net_device *dev, struct sk_buff *skb)
>         ret = dev_queue_xmit(skb);
>         dev_xmit_recursion_dec();
>
> +       // We should not return NET_XMIT_xxx here since it will conflict with
> +       // LWTUNNEL_XMIT_xxx values. Convert the return value to errno instead.

C++ comments; should be /* */. But, also, maybe they are not really needed?

ret = dev_queue_xmit(skb);
if (ret)
        ret = net_xmit_errno(ret);

We have a bunch of places with the pattern like this, so probably can
do the same here?

> +       if (unlikely(ret != NET_XMIT_SUCCESS))
> +               ret = net_xmit_errno(ret);
> +
>         return ret;
>  }
>
> --
> 2.30.2
>