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From:   Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Date:   Tue, 30 May 2023 13:50:37 -0700
Message-ID: <CABBYNZLpNbYDrP9aZqx9dm=XMh2KdRDAy+2gXX0wexMBHiQQUA@mail.gmail.com>
Subject: Re: [PATCH v2] Bluetooth: Fix l2cap_disconnect_req deadlock
To:     Ying Hsu <yinghsu@chromium.org>
Cc:     Simon Horman <simon.horman@corigine.com>,
        linux-bluetooth@vger.kernel.org,
        chromeos-bluetooth-upstreaming@chromium.org,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Johan Hedberg <johan.hedberg@gmail.com>,
        Marcel Holtmann <marcel@holtmann.org>,
        Paolo Abeni <pabeni@redhat.com>, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org
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Hi Ying,

On Mon, May 29, 2023 at 10:08=E2=80=AFPM Ying Hsu <yinghsu@chromium.org> wr=
ote:
>
> Gentle ping, Luiz.
>
>
> On Thu, May 25, 2023 at 12:16=E2=80=AFPM Ying Hsu <yinghsu@chromium.org> =
wrote:
> >
> > Hi Luiz,
> >
> > The proposal solves the deadlock but might introduce other problems as
> > it breaks the order of l2cap_chan_del.
> > There are another way to resolve the deadlock:
> > ```
> > @@ -4663,7 +4663,9 @@ static inline int l2cap_disconnect_req(struct
> > l2cap_conn *conn,
> >
> >         chan->ops->set_shutdown(chan);
> >
> > +       l2cap_chan_unlock(chan);
> >         mutex_lock(&conn->chan_lock);
> > +       l2cap_chan_lock(chan);
> >         l2cap_chan_del(chan, ECONNRESET);
> >         mutex_unlock(&conn->chan_lock);
> >  ```

Yeah, I kind of like this better, that said I don't think changing the
order of l2cap_chan_del matters that much but it does change the
callback teardown sequence so perhaps we should stick to a simpler
solution for now.

Please submit an updated version so we can move forward with it.

> > If you're okay with it, I'll do some verification and post a full patch=
.
> >
> > Best regards,
> > Ying
> >
> > On Thu, May 25, 2023 at 2:56=E2=80=AFAM Luiz Augusto von Dentz
> > <luiz.dentz@gmail.com> wrote:
> > >
> > > Hi Ying,
> > >
> > > On Wed, May 24, 2023 at 3:54=E2=80=AFAM Ying Hsu <yinghsu@chromium.or=
g> wrote:
> > > >
> > > > 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 ea=
sy
> > > > 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.
> > >
> > > And how about doing this:
> > >
> > > https://gist.github.com/Vudentz/e513859ecb31e79c947dfcb4b5c60453
> > >
> > > > Best regards,
> > > > Ying
> > > >
> > > >
> > > > On Wed, May 24, 2023 at 3:06=E2=80=AFAM Luiz Augusto von Dentz
> > > > <luiz.dentz@gmail.com> wrote:
> > > > >
> > > > > Hi Simon, Ying,
> > > > >
> > > > > On Tue, May 23, 2023 at 3:04=E2=80=AFAM Simon Horman <simon.horma=
n@corigine.com> 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 a=
re
> > > > > > > acquired in the order conn->chan_lock, chan->lock to avoid
> > > > > > > potential deadlock.
> > > > > > > For example, l2sock_shutdown acquires these locks in the orde=
r:
> > > > > > >   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_loc=
k
> > > > > > > before calling l2cap_chan_del. This means that these locks ar=
e
> > > > > > > acquired in unexpected order, which leads to potential deadlo=
ck:
> > > > > > >   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@chromium.org>
> > > > > > > ---
> > > > > > > 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 =3D l2cap_get_chan_by_scid(conn, dcid);
> > > > > > > +     mutex_lock(&conn->chan_lock);
> > > > > > > +     chan =3D __l2cap_get_chan_by_scid(conn, dcid);
> > > > > > > +     if (chan) {
> > > > > > > +             chan =3D 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 cho=
sen):
> > > > > >
> > > > > >         chan =3D __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
> > >
> > >
> > >
> > > --
> > > Luiz Augusto von Dentz


--=20
Luiz Augusto von Dentz