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
