netdev - Re: [PATCH v2] tcp: fix tcp_init_transfer() to not reset icsk_ca

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Date:   Fri, 2 Jul 2021 16:31:42 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Nguyen Dinh Phi <phind.uet@...il.com>
Cc:     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, 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 v2] tcp: fix tcp_init_transfer() to not reset icsk_ca_initialized

On Fri, Jul 2, 2021 at 3:41 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)
> 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: commit 8919a9b31eb4 ("tcp: Only init congestion control if not
> initialized already")

Please note that the patchwork tools have found a style/formatting
issue with your Fixes tag:

You can find them at:
https://patchwork.kernel.org/project/netdevbpf/list/
 ->
 https://patchwork.kernel.org/project/netdevbpf/patch/20210702194033.1370634-1-phind.uet@gmail.com/
  ->
   https://patchwork.hopto.org/static/nipa/510221/12356435/verify_fixes/stdout

The error is:
---
Fixes tag: Fixes: commit 8919a9b31eb4 ("tcp: Only init congestion control if not
Has these problem(s):
- leading word 'commit' unexpected
- Subject has leading but no trailing parentheses
- Subject has leading but no trailing quotes
---

Basically, please omit the "commit" and don't wrap the text (it's OK
if it's longer than 80 or 100 characters).

thanks,
neal