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

[Aditya Garg]

On 4/20/26 10:43, Sangyun Kim 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.

Although it was just once, and I was never able to reproduce it again.

Have you tested it on your Machine btw?