Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <ZLdY6JkWRccunvu0@debian.debian>
In-Reply-To: <ZLdY6JkWRccunvu0@debian.debian>
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Tue, 18 Jul 2023 20:42:23 -0700
Message-ID: <CAADnVQJNCEntFEh6pNY2HHwxoua0_2mRky2g2U5tj6XU2eoZog@mail.gmail.com>
Subject: Re: [PATCH v2 net] bpf: do not return NET_XMIT_xxx values on bpf_redirect
To:     Yan Zhai <yan@cloudflare.com>
Cc:     Network Development <netdev@vger.kernel.org>,
        kernel-team <kernel-team@cloudflare.com>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Daniel Borkmann <daniel@iogearbox.net>,
        John Fastabend <john.fastabend@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.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>, bpf <bpf@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Jordan Griege <jgriege@cloudflare.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Tue, Jul 18, 2023 at 8:30=E2=80=AFPM Yan Zhai <yan@cloudflare.com> 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 are redirecting 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. The result is skb that has been freed by the qdisc continues to
> neighbor subsystem and triggers the bug.

I'm struggling to parse the above paragraph.
Where bpf prog is installed?
Is this lwt bpf prog that returns BPF_REDIRECT ?
that redirects to netdev with noop_qdisc ?
What is the topology?

Please add a selftest to make sure we don't regress.

Also pls mark your patch as [PATCH v3 bpf] when you respin.

> This change converts the tx code to proper errors that lwt can consume.
>
> Suggested-by: Stanislav Fomichev <sdf@google.com>
> Reported-by: Jordan Griege <jgriege@cloudflare.com>
> Signed-off-by: Yan Zhai <yan@cloudflare.com>
> ---
> v2: coding style fix; sent to netdev instead of bpf for bug fixing.
>
> ---
>  net/core/filter.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 06ba0e56e369..8738c7a4701d 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2129,6 +2129,9 @@ static inline int __bpf_tx_skb(struct net_device *d=
ev, struct sk_buff *skb)
>         ret =3D dev_queue_xmit(skb);
>         dev_xmit_recursion_dec();
>
> +       if (unlikely(ret > 0))
> +               ret =3D net_xmit_errno(ret);
> +
>         return ret;
>  }
>
> --
> 2.30.2
>