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Date: Thu, 14 Jul 2022 10:46:18 -0700
From: Luiz Augusto von Dentz <luiz.dentz@...il.com>
To: Lee Jones <lee.jones@...aro.org>
Cc: Eric Dumazet <edumazet@...gle.com>,
LKML <linux-kernel@...r.kernel.org>, stable@...nel.org,
Marcel Holtmann <marcel@...tmann.org>,
Johan Hedberg <johan.hedberg@...il.com>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
"linux-bluetooth@...r.kernel.org" <linux-bluetooth@...r.kernel.org>,
netdev <netdev@...r.kernel.org>
Subject: Re: [RESEND 1/1] Bluetooth: Use chan_list_lock to protect the whole
put/destroy invokation
Hi Lee,
On Wed, Jul 6, 2022 at 1:58 PM Luiz Augusto von Dentz
<luiz.dentz@...il.com> wrote:
>
> Hi,
>
> On Wed, Jul 6, 2022 at 1:36 PM Luiz Augusto von Dentz
> <luiz.dentz@...il.com> wrote:
> >
> > Hi Lee,
> >
> > On Wed, Jul 6, 2022 at 3:53 AM Lee Jones <lee.jones@...aro.org> wrote:
> > >
> > > On Tue, 05 Jul 2022, Luiz Augusto von Dentz wrote:
> > >
> > > > Hi Lee,
> > > >
> > > > On Wed, Jun 29, 2022 at 8:28 AM Lee Jones <lee.jones@...aro.org> wrote:
> > > > >
> > > > > On Tue, 28 Jun 2022, Luiz Augusto von Dentz wrote:
> > > > >
> > > > > > Hi Eric, Lee,
> > > > > >
> > > > > > On Mon, Jun 27, 2022 at 4:39 PM Luiz Augusto von Dentz
> > > > > > <luiz.dentz@...il.com> wrote:
> > > > > > >
> > > > > > > Hi Eric, Lee,
> > > > > > >
> > > > > > > On Mon, Jun 27, 2022 at 7:41 AM Eric Dumazet <edumazet@...gle.com> wrote:
> > > > > > > >
> > > > > > > > On Wed, Jun 22, 2022 at 10:27 AM Lee Jones <lee.jones@...aro.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@...nel.org
> > > > > > > >
> > > > > > > > When was the bug added ? (Fixes: tag please)
> > > > > > > >
> > > > > > > > > Cc: Marcel Holtmann <marcel@...tmann.org>
> > > > > > > > > Cc: Johan Hedberg <johan.hedberg@...il.com>
> > > > > > > > > Cc: Luiz Augusto von Dentz <luiz.dentz@...il.com>
> > > > > > > > > Cc: "David S. Miller" <davem@...emloft.net>
> > > > > > > > > Cc: Eric Dumazet <edumazet@...gle.com>
> > > > > > > > > Cc: Jakub Kicinski <kuba@...nel.org>
> > > > > > > > > Cc: Paolo Abeni <pabeni@...hat.com>
> > > > > > > > > Cc: linux-bluetooth@...r.kernel.org
> > > > > > > > > Cc: netdev@...r.kernel.org
> > > > > > > > > Signed-off-by: Lee Jones <lee.jones@...aro.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);
> > > > > > > > >
> > > > > > > > >
> > > > > > > >
> > > > > > > > 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:
> > > > >
> > > > > I'm struggling to apply this for test:
> > > > >
> > > > > "error: corrupt patch at line 6"
> > > >
> > > > Check with the attached patch.
> > >
> > > With the patch applied:
> > >
> > > [ 188.825418][ T75] refcount_t: addition on 0; use-after-free.
> > > [ 188.825418][ T75] refcount_t: addition on 0; use-after-free.
> >
> > Looks like the changes just make the issue more visible since we are
> > trying to add a refcount when it is already 0 so this proves the
> > design is not quite right since it is removing the object from the
> > list only when destroying it while we probably need to do it before.
> >
> > How about we use kref_get_unless_zero as it appears it was introduced
> > exactly for such cases (patch attached.)
>
> Looks like I missed a few places like l2cap_global_chan_by_psm so here
> is another version.
Any feedback regarding these changes?
> > Luiz Augusto von Dentz
>
>
>
> --
> Luiz Augusto von Dentz
--
Luiz Augusto von Dentz
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