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

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Date: Tue, 25 Jun 2024 08:52:54 +0800
From: Edward Adam Davis <eadavis@...com>
To: luiz.dentz@...il.com
Cc: eadavis@...com,
	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 Luiz Augusto von Dentz,

On Mon, 24 Jun 2024 09:36:14 -0400, Luiz Augusto von Dentz wrote:
> > > 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.
Yes, we can fix l2cap_conless_channel, but key point is that "chan->lock"
cannot be used to synchronize l2cap_conless_channel and l2cap_sock_release,
otherwise it will form an AA lock with l2cap_data_channel.

Why not fix it in l2cap_conless_channel but in l2cap_sock_recv_cb, because
l2cap_sock_recv_cb is on the same layer as l2cap_sock_kill, using a new 
lock for synchronization is more appropriate.
--
Edward