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

[Shuah Khan <skhan@linuxfoundation.org>]

On 8/7/26 08:18, Aleksandr Nogikh wrote:
> Hi Shuah,
> 
> Thanks for reviewing the patch!
> 
> On Tue, Aug 4, 2026 at 7:31 PM Shuah Khan <skhan@linuxfoundation.org> wrote:
>>
>> On 8/3/26 09:05, syzbot wrote:
>>> From: Aleksandr Nogikh <nogikh@google.com>
>>>
>>> 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.

Thanks.

> 
>>
>> 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.
> 

Yes I agree with you on rewinding being hard.

Care to explain the scope of this assist?

Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot

thanks,
-- Shuah