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Message-ID: <266ab56e-ae83-7ddc-618e-3af228df81bd@linux.dev>
Date:   Fri, 28 Jul 2023 15:02:19 -0700
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Subject: Re: [PATCH v4 bpf 1/2] bpf: fix skb_do_redirect return values
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To:     Yan Zhai <yan@cloudflare.com>
Cc:     bpf@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Mykola Lysenko <mykolal@fb.com>, Shuah Khan <shuah@kernel.org>,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        linux-kselftest@vger.kernel.org, kernel-team@cloudflare.com,
        Jordan Griege <jgriege@cloudflare.com>,
        Markus Elfring <Markus.Elfring@web.de>,
        Jakub Sitnicki <jakub@cloudflare.com>
References: <cover.1690332693.git.yan@cloudflare.com>
 <e5d05e56bf41de82f10d33229b8a8f6b49290e98.1690332693.git.yan@cloudflare.com>
From:   Martin KaFai Lau <martin.lau@linux.dev>
In-Reply-To: <e5d05e56bf41de82f10d33229b8a8f6b49290e98.1690332693.git.yan@cloudflare.com>
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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