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





>
> Inside ip_finish_output2, redirected skb will continue to neighbor
> subsystem as if LWTUNNEL_XMIT_CONTINUE is returned, despite that this
> skb could have been freed. The bug can trigger use-after-free warning
> and crashes kernel afterwards:
>
> https://gist.github.com/zhaiyan920/8fbac245b261fe316a7ef04c9b1eba48

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