linux-kernel - Re: [PATCH] bluetooth/l2cap: sync sock recv cb and release

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Date: Mon, 24 Jun 2024 09:36:14 -0400
From: Luiz Augusto von Dentz <luiz.dentz@...il.com>
To: Edward Adam Davis <eadavis@...com>
Cc: johan.hedberg@...il.com, linux-bluetooth@...r.kernel.org, 
	linux-kernel@...r.kernel.org, marcel@...tmann.org, 
	syzbot+b7f6f8c9303466e16c8a@...kaller.appspotmail.com, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] bluetooth/l2cap: sync sock recv cb and release

Hi Edward,

On Fri, Jun 21, 2024 at 11:46 PM Edward Adam Davis <eadavis@...com> wrote:
>
> Hi Luiz Augusto von Dentz,
>
> On Thu, 20 Jun 2024 12:53:19 -0400, Luiz Augusto von Dentz wrote:
> > >         release_sock(sk);
> > > +       l2cap_chan_unlock(chan);
> > > +       l2cap_chan_put(chan);
> > >
> > >         return err;
> > >  }
> > > --
> > > 2.43.0
> >
> > Looks like this was never really tested properly:
> >
> > ============================================
> > WARNING: possible recursive locking detected
> > 6.10.0-rc3-g4029dba6b6f1 #6823 Not tainted
> > --------------------------------------------
> > kworker/u5:0/35 is trying to acquire lock:
> > ffff888002ec2510 (&chan->lock#2/1){+.+.}-{3:3}, at:
> > l2cap_sock_recv_cb+0x44/0x1e0
> >
> > but task is already holding lock:
> > ffff888002ec2510 (&chan->lock#2/1){+.+.}-{3:3}, at:
> > l2cap_get_chan_by_scid+0xaf/0xd0
> >
> > other info that might help us debug this:
> >  Possible unsafe locking scenario:
> >
> >        CPU0
> >        ----
> >   lock(&chan->lock#2/1);
> >   lock(&chan->lock#2/1);
> >
> >  *** DEADLOCK ***
> >
> >  May be due to missing lock nesting notation
> >
> > 3 locks held by kworker/u5:0/35:
> >  #0: ffff888002b8a940 ((wq_completion)hci0#2){+.+.}-{0:0}, at:
> > process_one_work+0x750/0x930
> >  #1: ffff888002c67dd0 ((work_completion)(&hdev->rx_work)){+.+.}-{0:0},
> > at: process_one_work+0x44e/0x930
> >  #2: ffff888002ec2510 (&chan->lock#2/1){+.+.}-{3:3}, at:
> > l2cap_get_chan_by_scid+0xaf/0xd0
> >
> > l2cap_sock_recv_cb is assumed to be called with the chan_lock held so
> > perhaps we can just do:
> >
> >        sk = chan->data;
> >        if (!sk)
> >                return -ENXIO;
>
> If the release occurs after this judgment, the same problem will still occur.
> Recv and release must be synchronized using locks, which can be solved by
> adding new lock.
>
> Please use the new patch https://syzkaller.appspot.com/text?tag=Patch&x=15d2c48e980000, I have tested in
> https://syzkaller.appspot.com/bug?extid=b7f6f8c9303466e16c8a

Hmm, why don't we just fix l2cap_conless_channel? Btw,
l2cap_conless_channel is normally not used by any profiles thus why
there isn't any CI covering it, on the other hand l2cap_data_channel
is used by 99% of the profiles.

> --
> Edward
>


-- 
Luiz Augusto von Dentz