CVE-2025-38378

In the Linux kernel, the following vulnerability has been resolved:

HID: appletb-kbd: fix slab use-after-free bug in appletb_kbd_probe

In probe appletb_kbd_probe() a struct appletb_kbd *kbd is allocated via devm_kzalloc() to store touch bar keyboard related data. Later on if backlight_device_get_by_name() finds a backlight device with name appletb_backlight a timer (kbd->inactivity_timer) is setup with appletb_inactivity_timer() and the timer is armed to run after appletb_tb_dim_timeout (60) seconds.

A use-after-free is triggered when failure occurs after the timer is armed. This ultimately means probe failure occurs and as a result the struct appletb_kbd *kbd which is device managed memory is freed. After 60 seconds the timer will have expired and __run_timers will attempt to access the timer (kbd->inactivity_timer) however the kdb structure has been freed causing a use-after free.

[ 71.636938] ================================================================== [ 71.637915] BUG: KASAN: slab-use-after-free in __run_timers+0x7ad/0x890 [ 71.637915] Write of size 8 at addr ffff8881178c5958 by task swapper/1/0 [ 71.637915] [ 71.637915] CPU: 1 UID: 0 PID: 0 Comm: swapper/1 Not tainted 6.16.0-rc2-00318-g739a6c93cc75-dirty #12 PREEMPT(voluntary) [ 71.637915] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014 [ 71.637915] Call Trace: [ 71.637915] [ 71.637915] dump_stack_lvl+0x53/0x70 [ 71.637915] print_report+0xce/0x670 [ 71.637915] ? __run_timers+0x7ad/0x890 [ 71.637915] kasan_report+0xce/0x100 [ 71.637915] ? __run_timers+0x7ad/0x890 [ 71.637915] __run_timers+0x7ad/0x890 [ 71.637915] ? __pfx___run_timers+0x10/0x10 [ 71.637915] ? update_process_times+0xfc/0x190 [ 71.637915] ? __pfx_update_process_times+0x10/0x10 [ 71.637915] ? _raw_spin_lock_irq+0x80/0xe0 [ 71.637915] ? _raw_spin_lock_irq+0x80/0xe0 [ 71.637915] ? __pfx__raw_spin_lock_irq+0x10/0x10 [ 71.637915] run_timer_softirq+0x141/0x240 [ 71.637915] ? __pfx_run_timer_softirq+0x10/0x10 [ 71.637915] ? __pfx___hrtimer_run_queues+0x10/0x10 [ 71.637915] ? kvm_clock_get_cycles+0x18/0x30 [ 71.637915] ? ktime_get+0x60/0x140 [ 71.637915] handle_softirqs+0x1b8/0x5c0 [ 71.637915] ? __pfx_handle_softirqs+0x10/0x10 [ 71.637915] irq_exit_rcu+0xaf/0xe0 [ 71.637915] sysvec_apic_timer_interrupt+0x6c/0x80 [ 71.637915] [ 71.637915] [ 71.637915] Allocated by task 39: [ 71.637915] kasan_save_stack+0x33/0x60 [ 71.637915] kasan_save_track+0x14/0x30 [ 71.637915] __kasan_kmalloc+0x8f/0xa0 [ 71.637915] __kmalloc_node_track_caller_noprof+0x195/0x420 [ 71.637915] devm_kmalloc+0x74/0x1e0 [ 71.637915] appletb_kbd_probe+0x37/0x3c0 [ 71.637915] hid_device_probe+0x2d1/0x680 [ 71.637915] really_probe+0x1c3/0x690 [ 71.637915] __driver_probe_device+0x247/0x300 [ 71.637915] driver_probe_device+0x49/0x210 […] [ 71.637915] [ 71.637915] Freed by task 39: [ 71.637915] kasan_save_stack+0x33/0x60 [ 71.637915] kasan_save_track+0x14/0x30 [ 71.637915] kasan_save_free_info+0x3b/0x60 [ 71.637915] __kasan_slab_free+0x37/0x50 [ 71.637915] kfree+0xcf/0x360 [ 71.637915] devres_release_group+0x1f8/0x3c0 [ 71.637915] hid_device_probe+0x315/0x680 [ 71.637915] really_probe+0x1c3/0x690 [ 71.637915] __driver_probe_device+0x247/0x300 [ 71.637915] driver_probe_device+0x49/0x210 […]

The root cause of the issue is that the timer is not disarmed on failure paths leading to it remaining active and accessing freed memory. To fix this call timer_delete_sync() to deactivate the timer.

Another small issue is that timer_delete_sync is called unconditionally in appletb_kbd_remove(), fix this by checking for a valid kbd->backlight_dev before calling timer_delete_sync.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/38224c472a038fa9ccd4085511dd9f3d6119dbf9
• https://git.kernel.org/stable/c/51720dee3a61ebace36c3dcdd0b4a488e0970f29