Re: [PATCH] usbip: fix use-after-free in event_handler()

[Aleksandr Nogikh]

Hi Shuah,

Thanks for reviewing the patch!

On Tue, Aug 4, 2026 at 7:31 PM Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On 8/3/26 09:05, syzbot wrote:
> > From: Aleksandr Nogikh <nogikh@xxxxxxxxxx>
> >
> > The event_handler function in drivers/usb/usbip/usbip_event.c is a
> > workqueue item responsible for processing events for a struct usbip_device.
> > During device teardown, usbip_stop_eh() is called to wait for the event
> > handler to finish processing. However, usbip_stop_eh() uses
> > wait_event_interruptible() and ignores its return value. If the process
> > unbinding the driver receives a signal, wait_event_interruptible() returns
> > immediately, causing the teardown process to falsely assume the event
> > handler has finished.
>
> Does this mean unbinding didn't happen?

The unbinding did happen (and finished freeing the memory), but
without actually waiting for event_handler() in
drivers/usb/usbip/usbip_event.c to finish processing the removal
event. As the device memory was prematurely freed, we got the
use-after-free crash in event_handler().

For reference, here's a C reproducer for the original bug:
https://syzkaller.appspot.com/text?tag=ReproC&x=16ffb7b9580000
It sets up a timer to deliver a signal and wakes up the
wait_event_interruptible() call.

>
> The teardown process then proceeds to free the
> > usbip_device memory. Meanwhile, the event_handler workqueue is still
> > running and attempts to access the freed usbip_device, resulting in a KASAN
> > slab-use-after-free crash.
> >
> > BUG: KASAN: slab-use-after-free in __mutex_lock_common
> > kernel/locking/rtmutex_api.c:559 [inline]
> > BUG: KASAN: slab-use-after-free in mutex_lock_nested+0x5a/0x1d0
> > kernel/locking/rtmutex_api.c:578
> > Read of size 1 at addr ffff8881145245b0 by task kworker/u8:5/6177
> >
> > Call Trace:
> >   <TASK>
> >   lock_acquire+0x84/0x350 kernel/locking/lockdep.c:5842
> >   __mutex_lock_common kernel/locking/rtmutex_api.c:559 [inline]
> >   mutex_lock_nested+0x5a/0x1d0 kernel/locking/rtmutex_api.c:578
> >   event_handler+0x1e3/0x4a0 drivers/usb/usbip/usbip_event.c:73
> >   process_one_work kernel/workqueue.c:3322 [inline]
> >   process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405
> >   worker_thread+0xa47/0xfb0 kernel/workqueue.c:3486
> >   kthread+0x388/0x470 kernel/kthread.c:436
> >   ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
> >   ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> >   </TASK>
> >
> > To fix this, change wait_event_interruptible() to wait_event() in
> > usbip_stop_eh(). This ensures that the teardown process strictly waits for
> > the event handler to finish its execution and drop all references to the
> > usbip_device before the memory is freed, preventing the use-after-free.
> > This change is safe from deadlocks because usbip_stop_eh() is never called
> > with locks held that the event_handler would need to acquire.
>
> Correct - usbip_stop_eh() is called without lock hold after updating
> the shutdown_busid status to true. However I am curious what happens
> to the unbinding? Should usbip_stop_eh() check the return value of
> wait_event_interruptible() and handle the error instead?

During unbinding, we must wait for the scheduled removal event to
finish before we can safely free the device structures. From what I
see in the code, once we have reached wait_event_interruptible(),
there's no way to abort the process or somehow gracefully handle the
error.

-- 
Aleksandr

>
> thanks,
> -- Shuah