Re: [PATCH v2] [media] em28xx-input: fix refcount bug in em28xx_usb_disconnect

[Dongliang Mu]

On Tue, Jul 13, 2021 at 4:48 PM Sean Young <sean@xxxxxxxx> 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@xxxxxxxxx>
> > Fixes: ac5688637144 ("media: em28xx: Fix possible memory leak of em28xx struct")
> > Signed-off-by: Dongliang Mu <mudongliangabcd@xxxxxxxxx>
> > ---
> > 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