Re: [RESEND 1/1] Bluetooth: Use chan_list_lock to protect the whole put/destroy invokation

[Luiz Augusto von Dentz <luiz.dentz@gmail.com>]

Hi Eric, Lee,

On Mon, Jun 27, 2022 at 4:39 PM Luiz Augusto von Dentz
<luiz.dentz@gmail.com> wrote:
>
> Hi Eric, Lee,
>
> On Mon, Jun 27, 2022 at 7:41 AM Eric Dumazet <edumazet@google.com> wrote:
> >
> > On Wed, Jun 22, 2022 at 10:27 AM Lee Jones <lee.jones@linaro.org> wrote:
> > >
> > > This change prevents a use-after-free caused by one of the worker
> > > threads starting up (see below) *after* the final channel reference
> > > has been put() during sock_close() but *before* the references to the
> > > channel have been destroyed.
> > >
> > >   refcount_t: increment on 0; use-after-free.
> > >   BUG: KASAN: use-after-free in refcount_dec_and_test+0x20/0xd0
> > >   Read of size 4 at addr ffffffc114f5bf18 by task kworker/u17:14/705
> > >
> > >   CPU: 4 PID: 705 Comm: kworker/u17:14 Tainted: G S      W       4.14.234-00003-g1fb6d0bd49a4-dirty #28
> > >   Hardware name: Qualcomm Technologies, Inc. SM8150 V2 PM8150 Google Inc. MSM sm8150 Flame DVT (DT)
> > >   Workqueue: hci0 hci_rx_work
> > >   Call trace:
> > >    dump_backtrace+0x0/0x378
> > >    show_stack+0x20/0x2c
> > >    dump_stack+0x124/0x148
> > >    print_address_description+0x80/0x2e8
> > >    __kasan_report+0x168/0x188
> > >    kasan_report+0x10/0x18
> > >    __asan_load4+0x84/0x8c
> > >    refcount_dec_and_test+0x20/0xd0
> > >    l2cap_chan_put+0x48/0x12c
> > >    l2cap_recv_frame+0x4770/0x6550
> > >    l2cap_recv_acldata+0x44c/0x7a4
> > >    hci_acldata_packet+0x100/0x188
> > >    hci_rx_work+0x178/0x23c
> > >    process_one_work+0x35c/0x95c
> > >    worker_thread+0x4cc/0x960
> > >    kthread+0x1a8/0x1c4
> > >    ret_from_fork+0x10/0x18
> > >
> > > Cc: stable@kernel.org
> >
> > When was the bug added ? (Fixes: tag please)
> >
> > > Cc: Marcel Holtmann <marcel@holtmann.org>
> > > Cc: Johan Hedberg <johan.hedberg@gmail.com>
> > > Cc: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
> > > Cc: "David S. Miller" <davem@davemloft.net>
> > > Cc: Eric Dumazet <edumazet@google.com>
> > > Cc: Jakub Kicinski <kuba@kernel.org>
> > > Cc: Paolo Abeni <pabeni@redhat.com>
> > > Cc: linux-bluetooth@vger.kernel.org
> > > Cc: netdev@vger.kernel.org
> > > Signed-off-by: Lee Jones <lee.jones@linaro.org>
> > > ---
> > >  net/bluetooth/l2cap_core.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> > > index ae78490ecd3d4..82279c5919fd8 100644
> > > --- a/net/bluetooth/l2cap_core.c
> > > +++ b/net/bluetooth/l2cap_core.c
> > > @@ -483,9 +483,7 @@ static void l2cap_chan_destroy(struct kref *kref)
> > >
> > >         BT_DBG("chan %p", chan);
> > >
> > > -       write_lock(&chan_list_lock);
> > >         list_del(&chan->global_l);
> > > -       write_unlock(&chan_list_lock);
> > >
> > >         kfree(chan);
> > >  }
> > > @@ -501,7 +499,9 @@ void l2cap_chan_put(struct l2cap_chan *c)
> > >  {
> > >         BT_DBG("chan %p orig refcnt %u", c, kref_read(&c->kref));
> > >
> > > +       write_lock(&chan_list_lock);
> > >         kref_put(&c->kref, l2cap_chan_destroy);
> > > +       write_unlock(&chan_list_lock);
> > >  }
> > >  EXPORT_SYMBOL_GPL(l2cap_chan_put);
> > >
> > > --
> > > 2.36.1.255.ge46751e96f-goog
> > >
> >
> > I do not think this patch is correct.
> >
> > a kref does not need to be protected by a write lock.
> >
> > This might shuffle things enough to work around a particular repro you have.
> >
> > If the patch was correct why not protect kref_get() sides ?
> >
> > Before the &hdev->rx_work is scheduled (queue_work(hdev->workqueue,
> > &hdev->rx_work),
> > a reference must be taken.
> >
> > Then this reference must be released at the end of hci_rx_work() or
> > when hdev->workqueue
> > is canceled.
> >
> > This refcount is not needed _if_ the workqueue is properly canceled at
> > device dismantle,
> > in a synchronous way.
> >
> > I do not see this hdev->rx_work being canceled, maybe this is the real issue.
> >
> > There is a call to drain_workqueue() but this is not enough I think,
> > because hci_recv_frame()
> > can re-arm
> >    queue_work(hdev->workqueue, &hdev->rx_work);
>
> I suspect this likely a refcount problem, we do l2cap_get_chan_by_scid:
>
> /* Find channel with given SCID.
>  * Returns locked channel. */
> static struct l2cap_chan *l2cap_get_chan_by_scid(struct l2cap_conn
> *conn, u16 cid)
>
> So we return a locked channel but that doesn't prevent another thread
> to call l2cap_chan_put which doesn't care about l2cap_chan_lock so
> perhaps we actually need to host a reference while we have the lock,
> at least we do something like that on l2cap_sock.c:
>
> l2cap_chan_hold(chan);
> l2cap_chan_lock(chan);
>
> __clear_chan_timer(chan);
> l2cap_chan_close(chan, ECONNRESET);
> l2cap_sock_kill(sk);
>
> l2cap_chan_unlock(chan);
> l2cap_chan_put(chan);

Perhaps something like this:

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 09ecaf556de5..9050b6af3577 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -111,7 +111,7 @@ static struct l2cap_chan
*__l2cap_get_chan_by_scid(struct l2cap_conn *conn,
 }

 /* Find channel with given SCID.
- * Returns locked channel. */
+ * Returns a reference locked channel. */
 static struct l2cap_chan *l2cap_get_chan_by_scid(struct l2cap_conn *conn,
                                                 u16 cid)
 {
@@ -119,15 +119,17 @@ static struct l2cap_chan
*l2cap_get_chan_by_scid(struct l2cap_conn *conn,

        mutex_lock(&conn->chan_lock);
        c = __l2cap_get_chan_by_scid(conn, cid);
-       if (c)
+       if (c) {
+               l2cap_chan_hold(c);
                l2cap_chan_lock(c);
+       }
        mutex_unlock(&conn->chan_lock);

        return c;
 }

 /* Find channel with given DCID.
- * Returns locked channel.
+ * Returns a reference locked channel.
  */
 static struct l2cap_chan *l2cap_get_chan_by_dcid(struct l2cap_conn *conn,
                                                 u16 cid)
@@ -136,8 +138,10 @@ static struct l2cap_chan
*l2cap_get_chan_by_dcid(struct l2cap_conn *conn,

        mutex_lock(&conn->chan_lock);
        c = __l2cap_get_chan_by_dcid(conn, cid);
-       if (c)
+       if (c) {
+               l2cap_chan_hold(c);
                l2cap_chan_lock(c);
+       }
        mutex_unlock(&conn->chan_lock);

        return c;
@@ -4464,6 +4468,7 @@ static inline int l2cap_config_req(struct
l2cap_conn *conn,

 unlock:
        l2cap_chan_unlock(chan);
+       l2cap_chan_put(chan);
        return err;
 }

@@ -4578,6 +4583,7 @@ static inline int l2cap_config_rsp(struct
l2cap_conn *conn,

 done:
        l2cap_chan_unlock(chan);
+       l2cap_chan_put(chan);
        return err;
 }

@@ -5305,6 +5311,7 @@ static inline int l2cap_move_channel_req(struct
l2cap_conn *conn,
        l2cap_send_move_chan_rsp(chan, result);

        l2cap_chan_unlock(chan);
+       l2cap_chan_put(chan);

        return 0;
 }
@@ -5397,6 +5404,7 @@ static void l2cap_move_continue(struct
l2cap_conn *conn, u16 icid, u16 result)
        }

        l2cap_chan_unlock(chan);
+       l2cap_chan_put(chan);
 }

 static void l2cap_move_fail(struct l2cap_conn *conn, u8 ident, u16 icid,
@@ -5489,6 +5497,7 @@ static int l2cap_move_channel_confirm(struct
l2cap_conn *conn,
        l2cap_send_move_chan_cfm_rsp(conn, cmd->ident, icid);

        l2cap_chan_unlock(chan);
+       l2cap_chan_put(chan);

        return 0;
 }
@@ -5524,6 +5533,7 @@ static inline int
l2cap_move_channel_confirm_rsp(struct l2cap_conn *conn,
        }

        l2cap_chan_unlock(chan);
+       l2cap_chan_put(chan);

        return 0;
 }
@@ -5896,12 +5906,11 @@ static inline int l2cap_le_credits(struct
l2cap_conn *conn,
        if (credits > max_credits) {
                BT_ERR("LE credits overflow");
                l2cap_send_disconn_req(chan, ECONNRESET);
-               l2cap_chan_unlock(chan);

                /* Return 0 so that we don't trigger an unnecessary
                 * command reject packet.
                 */
-               return 0;
+               goto unlock;
        }

        chan->tx_credits += credits;
@@ -5912,7 +5921,9 @@ static inline int l2cap_le_credits(struct
l2cap_conn *conn,
        if (chan->tx_credits)
                chan->ops->resume(chan);

+unlock:
        l2cap_chan_unlock(chan);
+       l2cap_chan_put(chan);

        return 0;
 }
@@ -7598,6 +7609,7 @@ static void l2cap_data_channel(struct l2cap_conn
*conn, u16 cid,

 done:
        l2cap_chan_unlock(chan);
+       l2cap_chan_put(chan);
 }

 static void l2cap_conless_channel(struct l2cap_conn *conn, __le16 psm,


>
> --
> Luiz Augusto von Dentz



-- 
Luiz Augusto von Dentz