Message-ID: <CAD-N9QWsdswh3JeOpe9+9gmv+KjznhaVQ5ujHa4yr-dQ_ZGXhQ@mail.gmail.com>
Date:   Mon, 19 Jul 2021 17:58:30 +0800
From:   Dongliang Mu <mudongliangabcd@...il.com>
To:     Sean Young <sean@...s.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Igor Matheus Andrade Torrente <igormtorrente@...il.com>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        linux-media@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] [media] em28xx-input: fix refcount bug in em28xx_usb_disconnect

On Mon, Jul 19, 2021 at 5:57 PM Dongliang Mu <mudongliangabcd@...il.com> wrote:
>
> This can also fix another crash report - KASAN: use-after-free Read in
> em28xx_close_extension [1]. It should be duplicated bug reports with
> the memory leak.

not memory leak, just the refcount bug mentioned in the commit message.

>
> [1] https://syzkaller.appspot.com/bug?id=a09553fd4df4c4a3824dc37a4040bf80d2600a50
>
>
> On Tue, Jul 13, 2021 at 5:49 PM Dongliang Mu <mudongliangabcd@...il.com> wrote:
> >
> > On Tue, Jul 13, 2021 at 4:48 PM Sean Young <sean@...s.org> wrote:
> > >
> > > On Wed, Jul 07, 2021 at 05:34:09PM +0800, Dongliang Mu wrote:
> > > > If em28xx_ir_init fails, it would decrease the refcount of dev. However,
> > > > in the em28xx_ir_fini, when ir is NULL, it goes to ref_put and decrease
> > > > the refcount of dev. This will lead to a refcount bug.
> > > >
> > > > Fix this bug by removing the kref_put in the error handling code
> > > > of em28xx_ir_init.
> > > >
> > > > refcount_t: underflow; use-after-free.
> > > > WARNING: CPU: 0 PID: 7 at lib/refcount.c:28 refcount_warn_saturate+0x18e/0x1a0 lib/refcount.c:28
> > > > Modules linked in:
> > > > CPU: 0 PID: 7 Comm: kworker/0:1 Not tainted 5.13.0 #3
> > > > Workqueue: usb_hub_wq hub_event
> > > > RIP: 0010:refcount_warn_saturate+0x18e/0x1a0 lib/refcount.c:28
> > > > Call Trace:
> > > >   kref_put.constprop.0+0x60/0x85 include/linux/kref.h:69
> > > >   em28xx_usb_disconnect.cold+0xd7/0xdc drivers/media/usb/em28xx/em28xx-cards.c:4150
> > > >   usb_unbind_interface+0xbf/0x3a0 drivers/usb/core/driver.c:458
> > > >   __device_release_driver drivers/base/dd.c:1201 [inline]
> > > >   device_release_driver_internal+0x22a/0x230 drivers/base/dd.c:1232
> > > >   bus_remove_device+0x108/0x160 drivers/base/bus.c:529
> > > >   device_del+0x1fe/0x510 drivers/base/core.c:3540
> > > >   usb_disable_device+0xd1/0x1d0 drivers/usb/core/message.c:1419
> > > >   usb_disconnect+0x109/0x330 drivers/usb/core/hub.c:2221
> > > >   hub_port_connect drivers/usb/core/hub.c:5151 [inline]
> > > >   hub_port_connect_change drivers/usb/core/hub.c:5440 [inline]
> > > >   port_event drivers/usb/core/hub.c:5586 [inline]
> > > >   hub_event+0xf81/0x1d40 drivers/usb/core/hub.c:5668
> > > >   process_one_work+0x2c9/0x610 kernel/workqueue.c:2276
> > > >   process_scheduled_works kernel/workqueue.c:2338 [inline]
> > > >   worker_thread+0x333/0x5b0 kernel/workqueue.c:2424
> > > >   kthread+0x188/0x1d0 kernel/kthread.c:319
> > > >   ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
> > > >
> > > > Reported-by: Dongliang Mu <mudongliangabcd@...il.com>
> > > > Fixes: ac5688637144 ("media: em28xx: Fix possible memory leak of em28xx struct")
> > > > Signed-off-by: Dongliang Mu <mudongliangabcd@...il.com>
> > > > ---
> > > > v1->v2: move kref_get to the original position
> > > >  drivers/media/usb/em28xx/em28xx-input.c | 1 -
> > > >  1 file changed, 1 deletion(-)
> > > >
> > > > diff --git a/drivers/media/usb/em28xx/em28xx-input.c b/drivers/media/usb/em28xx/em28xx-input.c
> > > > index 59529cbf9cd0..0b6d77c3bec8 100644
> > > > --- a/drivers/media/usb/em28xx/em28xx-input.c
> > > > +++ b/drivers/media/usb/em28xx/em28xx-input.c
> > > > @@ -842,7 +842,6 @@ static int em28xx_ir_init(struct em28xx *dev)
> > > >       kfree(ir);
> > > >  ref_put:
> > > >       em28xx_shutdown_buttons(dev);
> > > > -     kref_put(&dev->ref, em28xx_free_device);
> > > >       return err;
> > > >  }
> > >
> > > Ideally we want an init function to not have any side effects if it returns
> > > an error (or to do undo those side effects). With this change, the as long
> > > as is_audio_only is not set, we always do a kref_get(), even in the error
> > > case. As long as is_audio_only is not set, fini always does a kref_put().
> > >
> > > Now this works but it's not really very readable code, and it requires that
> > > the fini is called even if init returns an error.
> > >
> > > If an init function returns an error, it should undo any side effects like
> > > allocations or reference counts. So the best way to handle this to only
> > > do a kref_get() in the happy path of em28xx_ir_init().
> >
> > Hi Sean,
> >
> > In the v1 version, I moved kref_get from the beginning to the ending.
> > Do you mean this change?
> >
> > @@ -708,7 +708,6 @@ static int em28xx_ir_init(struct em28xx *dev)
> >   return 0;
> >   }
> >
> > - kref_get(&dev->ref);
> >   INIT_DELAYED_WORK(&dev->buttons_query_work, em28xx_query_buttons);
> >
> >   if (dev->board.buttons)
> > @@ -833,6 +832,9 @@ static int em28xx_ir_init(struct em28xx *dev)
> >
> >   dev_info(&dev->intf->dev, "Input extension successfully initialized\n");
> >
> > + /* Only increase refcount when this function is executed successfully */
> > + kref_get(&dev->ref);
> > +
> >   return 0;
> >
> > Another reason not to move kref_get is that Paval comments on my patch:
> >
> > > kref_get() should be before this call to not trigger UAF in em28xx_query_buttons()
> >
> > So I think it's safe to call kref_get at the beginning.
> >
> > >
> > >
> > > Thanks
> > >
> > > Sean

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