linux-kernel - Re: [PATCH] tcp: Do not reset the icsk_ca_initialized in tcp_init

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Date:   Mon, 28 Jun 2021 12:24:19 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Phi Nguyen <phind.uet@...il.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>, kpsingh@...nel.org,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzbot+f1e24a0594d4e3a895d3@...kaller.appspotmail.com,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH] tcp: Do not reset the icsk_ca_initialized in tcp_init_transfer.

On Mon, Jun 28, 2021 at 12:18 PM Phi Nguyen <phind.uet@...il.com> wrote:
>
> On 6/28/2021 10:52 PM, Eric Dumazet wrote:
>
> > Unfortunately this patch might break things.
> >
> > We keep changing this CC switching, with eBPF being mixed in the equation.
> >
> > I would suggest you find a Fixes: tag first, so that we can continue
> > the discussion.
> >
> > Thank you.
>
> Thank for your feedback. I will resubmit it with a Fixes tag.
>
> Regard.

Thanks.

Can you also please provide a summary of the event sequence that
triggers the bug? Based on your Reported-by tag, I guess this is based
on the syzbot reproducer:

 https://groups.google.com/g/syzkaller-bugs/c/VbHoSsBz0hk/m/cOxOoTgPCAAJ

but perhaps you can give a summary of the event sequence that causes
the bug? Is it that the call:

setsockopt$inet_tcp_TCP_CONGESTION(r0, 0x6, 0xd,
&(0x7f0000000000)='cdg\x00', 0x4)

initializes the CC and happens before the connection is established,
and then when the connection is established, the line that sets:
  icsk->icsk_ca_initialized = 0;
is incorrect, causing the CC to be initialized again without first
calling the cleanup code that deallocates the CDG-allocated memory?

thanks,
neal