Re: [PATCH v4 bpf 1/2] bpf: fix skb_do_redirect return values

[Yan Zhai]

On Mon, Jul 31, 2023 at 5:11 PM Martin KaFai Lau <martin.lau@xxxxxxxxx> wrote:
>
> On 7/31/23 2:35 PM, Yan Zhai wrote:
> > On Fri, Jul 28, 2023 at 5:02 PM Martin KaFai Lau <martin.lau@xxxxxxxxx> wrote:
> >>
> >> On 7/25/23 6:08 PM, Yan Zhai wrote:
> >>> skb_do_redirect returns various of values: error code (negative),
> >>> 0 (success), and some positive status code, e.g. NET_XMIT_CN,
> >>> NET_RX_DROP. Commit 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel
> >>> infrastructure") didn't check the return code correctly, so positive
> >>> values are propagated back along call chain:
> >>>
> >>>     ip_finish_output2
> >>>       -> bpf_xmit
> >>>         -> run_lwt_bpf
> >>>           -> skb_do_redirect
> >>
> >>   From looking at skb_do_redirect, the skb_do_redirect should have consumed the
> >> skb except for the -EAGAIN return value. afaik, -EAGAIN could only happen by
> >> using the bpf_redirect_peer helper. lwt does not have the bpf_redirect_peer
> >> helper available, so there is no -EAGAIN case in lwt. iow, skb_do_redirect
> >> should have always consumed the skb in lwt. or did I miss something?
> >>
> >> If that is the case, it feels like the fix should be in run_lwt_bpf() and the
> >> "if (ret == 0)" test in run_lwt_bpf() is unnecessary?
> >>
> >>                          ret = skb_do_redirect(skb);
> >>                          if (ret == 0)
> >>                                  ret = BPF_REDIRECT;
> >>
> >>
> > Just fixing skb redirect return code won't be sufficient. I realized
> > there are other return paths that need to be treated, e.g. bpf reroute
> > path also directly returns dev_queue_xmit status. I plan to check for
> > LWTUNNEL_XMIT_CONTINUE (and change it to a value that does not
> > conflict with NET_RX_DROP and NET_XMIT_DROP) in the next revision. On
> > the other hand, the return value of NETDEV_TX_BUSY is another hassle.
>
> I suspect we are talking about different things or I am still missing something.
>
> I was thinking skb_do_redirect() should have always consumed the skb and
> bpf_xmit should always return LWTUNNEL_XMIT_DONE also (instead of
> LWTUNNEL_XMIT_CONTINUE described in the this patch commit message). It is what
> sch_handle_egress() is doing also. Could you explain how is it different from
> the skb_do_redirect usage in sch_handle_egress() or you are suggesting the
> current sch_handle_egress() has the issue too also?
>
I think we were not on the same page. You are absolutely right that
skb_do_redirect should consume the packet anyway. The difference
between your proposal and this patch is that this patch returns errno
or LWTUNNEL_XMIT_DONE, and yours does not even return errno. Both
approaches fix the issue of "redirect to down device crashes the
kernel".

What I commented was an exact same issue at different location: BPF
reroute may trigger the crash as well, since it also returns
dev_queue_xmit status in bpf_xmit. Need to fix this, or instead fixing
LWTUNNEL_XMIT_CONTINUE value and correct the behavior at lwtunnel_xmit
rather than bpf_xmit.

Yan

>
> > As Dan suggested, packets might not have been freed when this is
> > returned from drivers. The caller of dev_queue_xmit might need to free
> > skb when this happens.
> >
> > Yan
>