Re: [PATCH v2] Bluetooth: Fix l2cap_disconnect_req deadlock

[Ying Hsu]

Hi Simon,

I understand your concern about the repeated code.
However, simply hiding the locking logic in another function
introduces hidden assumptions.
For this patch, I would like to fix the deadlock in a simple and easy
to understand way.
We can always refactor the l2cap_chan utility functions later.

Hi Luis,

I'll add a fixes tag in the next version.

Best regards,
Ying


On Wed, May 24, 2023 at 3:06 AM Luiz Augusto von Dentz
<luiz.dentz@xxxxxxxxx> wrote:
>
> Hi Simon, Ying,
>
> On Tue, May 23, 2023 at 3:04 AM Simon Horman <simon.horman@xxxxxxxxxxxx> wrote:
> >
> > On Mon, May 22, 2023 at 11:41:51PM +0000, Ying Hsu wrote:
> > > L2CAP assumes that the locks conn->chan_lock and chan->lock are
> > > acquired in the order conn->chan_lock, chan->lock to avoid
> > > potential deadlock.
> > > For example, l2sock_shutdown acquires these locks in the order:
> > >   mutex_lock(&conn->chan_lock)
> > >   l2cap_chan_lock(chan)
> > >
> > > However, l2cap_disconnect_req acquires chan->lock in
> > > l2cap_get_chan_by_scid first and then acquires conn->chan_lock
> > > before calling l2cap_chan_del. This means that these locks are
> > > acquired in unexpected order, which leads to potential deadlock:
> > >   l2cap_chan_lock(c)
> > >   mutex_lock(&conn->chan_lock)
> > >
> > > This patch uses __l2cap_get_chan_by_scid to replace
> > > l2cap_get_chan_by_scid and adjusts the locking order to avoid the
> > > potential deadlock.
>
> This needs the fixes tag so we can backport it properly.
>
> > > Signed-off-by: Ying Hsu <yinghsu@xxxxxxxxxxxx>
> > > ---
> > > This commit has been tested on a Chromebook device.
> > >
> > > Changes in v2:
> > > - Adding the prefix "Bluetooth:" to subject line.
> > >
> > >  net/bluetooth/l2cap_core.c | 26 ++++++++++++++++++++------
> > >  1 file changed, 20 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> > > index 376b523c7b26..8f08192b8fb1 100644
> > > --- a/net/bluetooth/l2cap_core.c
> > > +++ b/net/bluetooth/l2cap_core.c
> > > @@ -4651,8 +4651,16 @@ static inline int l2cap_disconnect_req(struct l2cap_conn *conn,
> > >
> > >       BT_DBG("scid 0x%4.4x dcid 0x%4.4x", scid, dcid);
> > >
> > > -     chan = l2cap_get_chan_by_scid(conn, dcid);
> > > +     mutex_lock(&conn->chan_lock);
> > > +     chan = __l2cap_get_chan_by_scid(conn, dcid);
> > > +     if (chan) {
> > > +             chan = l2cap_chan_hold_unless_zero(chan);
> > > +             if (chan)
> > > +                     l2cap_chan_lock(chan);
> > > +     }
> > > +
> > >       if (!chan) {
> > > +             mutex_unlock(&conn->chan_lock);
> > >               cmd_reject_invalid_cid(conn, cmd->ident, dcid, scid);
> > >               return 0;
> > >       }
> >
> > Hi Ying,
> >
> > The conditional setting of chan and calling l2cap_chan_lock()
> > is both non-trivial and repeated. It seems that it ought to be
> > in a helper.
> >
> > Something like this (I'm sure a better function name can be chosen):
> >
> >         chan = __l2cap_get_and_lock_chan_by_scid(conn, dcid);
> >         if (!chan) {
> >                 ...
> >         }
> >
> >         ...
>
> Or perhaps we could do something like l2cap_del_chan_by_scid:
>
> https://gist.github.com/Vudentz/e513859ecb31e79c947dfcb4b5c60453
>
> --
> Luiz Augusto von Dentz