Re: [PATCH v1 net-next] mptcp: Initialise rcv_mss before calling tcp_send_active_reset() in mptcp_do_fastclose().

Kuniyuki Iwashima
2025-11-26T19:46:36Z

On Wed, Nov 26, 2025 at 8:36 AM Matthieu Baerts <matttbe@kernel.org> wrote:
>
> Hi Kuniyuki,
>
> (+Cc MPTCP ML)
>
> On 25/11/2025 20:53, Kuniyuki Iwashima wrote:
> > syzbot reported divide-by-zero in __tcp_select_window() by
> > MPTCP socket. [0]
>
> Thank you for having released this bug report, and even more for having
> sent the fix!
>
> > We had a similar issue for the bare TCP and fixed in commit
> > 499350a5a6e7 ("tcp: initialize rcv_mss to TCP_MIN_MSS instead
> > of 0").
> >
> > Let's apply the same fix to mptcp_do_fastclose().
> >
> > [0]:
> > Oops: divide error: 0000 [#1] SMP KASAN PTI
> > CPU: 0 UID: 0 PID: 6068 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
> > RIP: 0010:__tcp_select_window+0x824/0x1320 net/ipv4/tcp_output.c:3336
> > Code: ff ff ff 44 89 f1 d3 e0 89 c1 f7 d1 41 01 cc 41 21 c4 e9 a9 00 00 00 e8 ca 49 01 f8 e9 9c 00 00 00 e8 c0 49 01 f8 44 89 e0 99 <f7> 7c 24 1c 41 29 d4 48 bb 00 00 00 00 00 fc ff df e9 80 00 00 00
> > RSP: 0018:ffffc90003017640 EFLAGS: 00010293
> > RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff88807b469e40
> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
> > RBP: ffffc90003017730 R08: ffff888033268143 R09: 1ffff1100664d028
> > R10: dffffc0000000000 R11: ffffed100664d029 R12: 0000000000000000
> > R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> > FS: 000055557faa0500(0000) GS:ffff888126135000(0000) knlGS:0000000000000000
> > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007f64a1912ff8 CR3: 0000000072122000 CR4: 00000000003526f0
> > Call Trace:
> > <TASK>
> > tcp_select_window net/ipv4/tcp_output.c:281 [inline]
> > __tcp_transmit_skb+0xbc7/0x3aa0 net/ipv4/tcp_output.c:1568
> > tcp_transmit_skb net/ipv4/tcp_output.c:1649 [inline]
> > tcp_send_active_reset+0x2d1/0x5b0 net/ipv4/tcp_output.c:3836
> > mptcp_do_fastclose+0x27e/0x380 net/mptcp/protocol.c:2793
> > mptcp_disconnect+0x238/0x710 net/mptcp/protocol.c:3253
> > mptcp_sendmsg_fastopen+0x2f8/0x580 net/mptcp/protocol.c:1776
>
> Note: arf, sorry, I just noticed my small syzkaller instances found it
> too, and I should have caught it before. I thought this issue was linked
> to another one [1]. It is not: this one here is specific to TFO where
> mptcp_disconnect() is called directly in one error path not cover by the
> selftests.
>
> [1]
> https://lore.kernel.org/20251125-net-mptcp-clear-sched-rtx-v1-1-1cea4ad2165f@kernel.org
>
> > mptcp_sendmsg+0x1774/0x1980 net/mptcp/protocol.c:1855
> > sock_sendmsg_nosec net/socket.c:727 [inline]
> > __sock_sendmsg+0xe5/0x270 net/socket.c:742
> > __sys_sendto+0x3bd/0x520 net/socket.c:2244
> > __do_sys_sendto net/socket.c:2251 [inline]
> > __se_sys_sendto net/socket.c:2247 [inline]
> > __x64_sys_sendto+0xde/0x100 net/socket.c:2247
> > do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> > do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
> > entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > RIP: 0033:0x7f66e998f749
> > Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
> > RSP: 002b:00007ffff9acedb8 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
> > RAX: ffffffffffffffda RBX: 00007f66e9be5fa0 RCX: 00007f66e998f749
> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
> > RBP: 00007ffff9acee10 R08: 0000000000000000 R09: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
> > R13: 00007f66e9be5fa0 R14: 00007f66e9be5fa0 R15: 0000000000000006
> > </TASK>
> >
> > Fixes: ae155060247b ("mptcp: fix duplicate reset on fastclose")
>
> I see that this patch targets "net-next". I think it would be better to
> target "net". No need to send a new version, this patch applies on top
> of "net" without conflicts, and Paolo told me he can apply it on top of
> "net".

Ah sorry, I didn't find mptcp_do_fastclose() in my net worktree
and started working on net-next, but my net tree was just stale.

Thank you for the review and sync-up with Paolo!

>
> The patch looks good to me, and can be applied to "net" directly:
>
> Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
>
>
> While at it, just to help me to track the backports:
>
> Cc: stable@vger.kernel.org
>
>
> > Reported-by: syzbot+3a92d359bc2ec6255a33@syzkaller.appspotmail.com
> > Closes: https://lore.kernel.org/netdev/69260882.a70a0220.d98e3.00b4.GAE@google.com/
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
> > ---
> > net/mptcp/protocol.c | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> > diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> > index 75bb1199bed9..40a8bdd5422a 100644
> > --- a/net/mptcp/protocol.c
> > +++ b/net/mptcp/protocol.c
> > @@ -2790,6 +2790,12 @@ static void mptcp_do_fastclose(struct sock *sk)
> > goto unlock;
> >
> > subflow->send_fastclose = 1;
> > +
> > + /* Initialize rcv_mss to TCP_MIN_MSS to avoid division by 0
> > + * issue in __tcp_select_window(), see tcp_disconnect().
> > + */
> > + inet_csk(ssk)->icsk_ack.rcv_mss = TCP_MIN_MSS;
>
>
> I initially thought this should have been set in mptcp_sendmsg_fastopen,
> before calling mptcp_disconnect(), but doing that here is probably safer
> to catch other cases, and it aligns with tcp_disconnect(). So that's
> perfect, no need to change anything!
>
>
> > +
> > tcp_send_active_reset(ssk, ssk->sk_allocation,
> > SK_RST_REASON_TCP_ABORT_ON_CLOSE);
> > unlock:
>
> Cheers,
> Matt
> --
> Sponsored by the NGI0 Core fund.
>