linux-kernel - Re: WARNING: refcount bug in l2cap_chan

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Message-ID: <CABBYNZJzxO=aTnYOL7UpDzK+PDNcAa-DuH_5N+FZV6OZp52uFA@mail.gmail.com>
Date:   Mon, 24 Feb 2020 17:18:56 -0800
From:   Luiz Augusto von Dentz <luiz.dentz@...il.com>
To:     Abhishek Pandit-Subedi <abhishekpandit@...omium.org>
Cc:     syzbot <syzbot+198362c76088d1515529@...kaller.appspotmail.com>,
        "David S. Miller" <davem@...emloft.net>,
        Johan Hedberg <johan.hedberg@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Bluez mailing list <linux-bluetooth@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Marcel Holtmann <marcel@...tmann.org>,
        netdev <netdev@...r.kernel.org>, syzkaller-bugs@...glegroups.com
Subject: Re: WARNING: refcount bug in l2cap_chan_put

Hi Abhishek,

On Mon, Feb 24, 2020 at 11:33 AM Abhishek Pandit-Subedi
<abhishekpandit@...omium.org> wrote:
>
> (Resent in plain text; sorry for double send)
>
> I took a brief look at this error and uncovered that 6lowpan uses zero
> locks when using l2cap (should be using the channel lock).
>
> It seems like it would be better just to convert its direct use of
> l2cap channel into using an l2cap socket.

I recall having some thought on that, I think having a socket like
RFCOMM does would be better but I don't remember why I haven't
follow-up on that, well we wanted to discontinue the bt specific
6lowpan on the kernel side though.

> On Mon, Feb 24, 2020 at 12:28 AM syzbot
> <syzbot+198362c76088d1515529@...kaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot found the following crash on:
> >
> > HEAD commit:    bee46b30 Add linux-next specific files for 20200221
> > git tree:       linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1244ea7ee00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=10693880b4976691
> > dashboard link: https://syzkaller.appspot.com/bug?extid=198362c76088d1515529
> > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=160a03d9e00000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=10f8e1dde00000
> >
> > Bisection is inconclusive: the bug happens on the oldest tested release.
> >
> > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=13f03a7ee00000
> > final crash:    https://syzkaller.appspot.com/x/report.txt?x=10083a7ee00000
> > console output: https://syzkaller.appspot.com/x/log.txt?x=17f03a7ee00000
> >
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+198362c76088d1515529@...kaller.appspotmail.com
> >
> > ------------[ cut here ]------------
> > refcount_t: underflow; use-after-free.
> > WARNING: CPU: 1 PID: 2940 at lib/refcount.c:28 refcount_warn_saturate+0x1dc/0x1f0 lib/refcount.c:28
> > Kernel panic - not syncing: panic_on_warn set ...
> > CPU: 1 PID: 2940 Comm: kworker/1:12 Not tainted 5.6.0-rc2-next-20200221-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > Workqueue: events do_enable_set
> > Call Trace:
> >  __dump_stack lib/dump_stack.c:77 [inline]
> >  dump_stack+0x197/0x210 lib/dump_stack.c:118
> >  panic+0x2e3/0x75c kernel/panic.c:221
> >  __warn.cold+0x2f/0x3e kernel/panic.c:582
> >  report_bug+0x289/0x300 lib/bug.c:195
> >  fixup_bug arch/x86/kernel/traps.c:175 [inline]
> >  fixup_bug arch/x86/kernel/traps.c:170 [inline]
> >  do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:267
> >  do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:286
> >  invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1027
> > RIP: 0010:refcount_warn_saturate+0x1dc/0x1f0 lib/refcount.c:28
> > Code: e9 d8 fe ff ff 48 89 df e8 81 81 10 fe e9 85 fe ff ff e8 07 54 d1 fd 48 c7 c7 00 c8 91 88 c6 05 6b f6 fc 06 01 e8 23 74 a1 fd <0f> 0b e9 ac fe ff ff 0f 1f 00 66 2e 0f 1f 84 00 00 00 00 00 55 48
> > RSP: 0018:ffffc9000952fbd8 EFLAGS: 00010286
> > RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> > RDX: 0000000000000000 RSI: ffffffff815ee766 RDI: fffff520012a5f6d
> > RBP: ffffc9000952fbe8 R08: ffff88809e82e600 R09: ffffed1015d26661
> > R10: ffffed1015d26660 R11: ffff8880ae933307 R12: 0000000000000003
> > R13: ffff888095b3f018 R14: dead000000000122 R15: ffffc9000952fc98
> >  refcount_sub_and_test include/linux/refcount.h:261 [inline]
> >  refcount_dec_and_test include/linux/refcount.h:281 [inline]
> >  kref_put include/linux/kref.h:64 [inline]
> >  l2cap_chan_put+0x1d9/0x240 net/bluetooth/l2cap_core.c:501
> >  do_enable_set+0x54b/0x960 net/bluetooth/6lowpan.c:1075
> >  process_one_work+0xa05/0x17a0 kernel/workqueue.c:2266
> >  worker_thread+0x98/0xe40 kernel/workqueue.c:2412
> >  kthread+0x361/0x430 kernel/kthread.c:255
> >  ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
> > Kernel Offset: disabled
> > Rebooting in 86400 seconds..
> >
> >
> > ---
> > This bug is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@...glegroups.com.
> >
> > syzbot will keep track of this bug report. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> > syzbot can test patches for this bug, for details see:
> > https://goo.gl/tpsmEJ#testing-patches



-- 
Luiz Augusto von Dentz