Re: [PATCH 0/2] HID: appletb-kbd: fix UAF and mutex-in-atomic in inactivity timer

[Jiri Kosina]

On Mon, 20 Apr 2026, Aditya Garg wrote:

> > This series addresses two defects in hid-appletb-kbd's inactivity
> > timer subsystem.  The two patches target different bugs and are
> > logically independent; they are sent together because they touch the
> > same tear-down code and because the same maintainer will review both.
> > 
> > Patch 1 fixes a slab use-after-free with two related tear-down windows
> > introduced by commit 38224c472a03 ("HID: appletb-kbd: fix slab
> > use-after-free bug in appletb_kbd_probe"):
> > 
> >    A) Within "if (kbd->backlight_dev)" the order was
> >       put_device() then timer_delete_sync().  A concurrent
> >       hid_appletb_bl unbind between those two calls can drop the last
> >       devm reference and free the backlight_device; the still-armed
> >       inactivity timer softirq then dereferences the freed object
> >       through backlight_device_set_brightness() -> mutex_lock(&ops_lock).
> > 
> >    B) The "if (kbd->backlight_dev)" block ran before
> >       hid_hw_close()/hid_hw_stop(), so even after window A is closed a
> >       late ".event" callback from the HID core (USB URB completion on
> >       real hardware) can arrive between timer_delete_sync() and
> >       put_device(), reach reset_inactivity_timer(), re-arm the timer
> >       via mod_timer(), and reopen the same UAF.
> > 
> > Both windows produce the same KASAN slab-use-after-free on the object
> > allocated by devm_backlight_device_register().  Patch 1 closes them
> > together by moving hid_hw_close()/hid_hw_stop() before the backlight
> > cleanup and, inside that cleanup block, calling timer_delete_sync()
> > before put_device().  Shipping both as one commit avoids leaving
> > stable kernels in a half-fixed state where only window A is closed.
> > 
> > Patch 2 fixes a separate "sleeping function called from invalid
> > context" bug in the same subsystem.  The inactivity timer is a
> > struct timer_list, so the callback runs in softirq context and calls
> > backlight_device_set_brightness() -> mutex_lock() from atomic
> > context; reset_inactivity_timer() has the same issue on the
> > brightness-restore path (it is called from appletb_kbd_hid_event()
> > and appletb_kbd_inp_event(), which run in softirq/IRQ context on
> > real USB hardware).  Convert the inactivity timer to a delayed_work
> > and defer the brightness-restore call to a dedicated work_struct so
> > both sleeping calls run in process context.
> > 
> > Sangyun Kim (2):
> >    HID: appletb-kbd: fix UAF in inactivity-timer cleanup path
> >    HID: appletb-kbd: run inactivity autodim from workqueues
> > 
> >   drivers/hid/hid-appletb-kbd.c | 56 ++++++++++++++++++++++-------------
> >   1 file changed, 36 insertions(+), 20 deletions(-)
> > 
> 
> I had a very weird bug just once. And that was when I pressed fn key, upon
> releasing, the touchbar mode did not restore to normal.

You mean with this patch applied, correct?

-- 
Jiri Kosina
SUSE Labs