netdev - Re: [PATCH v3 bpf-next 4/5] bpf: tcp: Stop bpf_setsockopt(TCP

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Date:   Thu, 29 Sep 2022 09:07:55 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Martin KaFai Lau <martin.lau@...ux.dev>
Cc:     bpf <bpf@...r.kernel.org>, netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        kernel-team <kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 4/5] bpf: tcp: Stop bpf_setsockopt(TCP_CONGESTION)
 in init ops to recur itself

On Thu, Sep 29, 2022 at 12:04 AM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>
> From: Martin KaFai Lau <martin.lau@...nel.org>
>
> When a bad bpf prog '.init' calls
> bpf_setsockopt(TCP_CONGESTION, "itself"), it will trigger this loop:
>
> .init => bpf_setsockopt(tcp_cc) => .init => bpf_setsockopt(tcp_cc) ...
> ... => .init => bpf_setsockopt(tcp_cc).
>
> It was prevented by the prog->active counter before but the prog->active
> detection cannot be used in struct_ops as explained in the earlier
> patch of the set.
>
> In this patch, the second bpf_setsockopt(tcp_cc) is not allowed
> in order to break the loop.  This is done by using a bit of
> an existing 1 byte hole in tcp_sock to check if there is
> on-going bpf_setsockopt(TCP_CONGESTION) in this tcp_sock.
>
> Note that this essentially limits only the first '.init' can
> call bpf_setsockopt(TCP_CONGESTION) to pick a fallback cc (eg. peer
> does not support ECN) and the second '.init' cannot fallback to
> another cc.  This applies even the second
> bpf_setsockopt(TCP_CONGESTION) will not cause a loop.
>
> Signed-off-by: Martin KaFai Lau <martin.lau@...nel.org>

Reviewed-by: Eric Dumazet <edumazet@...gle.com>