netdev - Re: [PATCH v5] tcp: fix tcp_init_transfer() to not reset icsk_ca

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Date:   Mon, 5 Jul 2021 14:52:23 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Nguyen Dinh Phi <phind.uet@...il.com>
Cc:     yhs@...com, edumazet@...gle.com, davem@...emloft.net,
        yoshfuji@...ux-ipv6.org, dsahern@...nel.org, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kafai@...com, songliubraving@...com, john.fastabend@...il.com,
        kpsingh@...nel.org, ycheng@...gle.com, yyd@...gle.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzbot+f1e24a0594d4e3a895d3@...kaller.appspotmail.com
Subject: Re: [PATCH v5] tcp: fix tcp_init_transfer() to not reset icsk_ca_initialized

On Mon, Jul 5, 2021 at 1:18 PM Nguyen Dinh Phi <phind.uet@...il.com> wrote:
>
> This commit fixes a bug (found by syzkaller) that could cause spurious
> double-initializations for congestion control modules, which could cause
> memory leaks orother problems for congestion control modules (like CDG)

nit: typo: "orother" -> "or other"

> that allocate memory in their init functions.
>
> The buggy scenario constructed by syzkaller was something like:
>
> (1) create a TCP socket
> (2) initiate a TFO connect via sendto()
> (3) while socket is in TCP_SYN_SENT, call setsockopt(TCP_CONGESTION),
>     which calls:
>        tcp_set_congestion_control() ->
>          tcp_reinit_congestion_control() ->
>            tcp_init_congestion_control()
> (4) receive ACK, connection is established, call tcp_init_transfer(),
>     set icsk_ca_initialized=0 (without first calling cc->release()),
>     call tcp_init_congestion_control() again.
>
> Note that in this sequence tcp_init_congestion_control() is called
> twice without a cc->release() call in between. Thus, for CC modules
> that allocate memory in their init() function, e.g, CDG, a memory leak
> may occur. The syzkaller tool managed to find a reproducer that
> triggered such a leak in CDG.
>
> The bug was introduced when that commit 8919a9b31eb4 ("tcp: Only init
> congestion control if not initialized already")
> introduced icsk_ca_initialized and set icsk_ca_initialized to 0 in
> tcp_init_transfer(), missing the possibility for a sequence like the
> one above, where a process could call setsockopt(TCP_CONGESTION) in
> state TCP_SYN_SENT (i.e. after the connect() or TFO open sendmsg()),
> which would call tcp_init_congestion_control(). It did not intend to
> reset any initialization that the user had already explicitly made;
> it just missed the possibility of that particular sequence (which
> syzkaller managed to find).
>
> Fixes: 8919a9b31eb4 (tcp: Only init congestion control if not initialized already)

Style nit: I believe this Fixes line should have double quotes around
the commit title, like:

  Fixes: 8919a9b31eb4 ("tcp: Only init congestion control if not
initialized already")

Otherwise, this looks good to me, and it passes our internal packetdrill tests.

thanks,
neal