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

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

[syzbot]

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. 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.

Fixes: bb7871ad99ea ("usbip: event handler as one thread")
Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: syzbot+f4518ac0a3dd3aafde38@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f4518ac0a3dd3aafde38
Link: https://syzkaller.appspot.com/ai_job?id=88101c8e-4da9-45de-b879-0549d6fe5649
Signed-off-by: Aleksandr Nogikh <nogikh@google.com>

---
diff --git a/drivers/usb/usbip/usbip_event.c b/drivers/usb/usbip/usbip_event.c
index 0e00c2d00..81d761891 100644
--- a/drivers/usb/usbip/usbip_event.c
+++ b/drivers/usb/usbip/usbip_event.c
@@ -115,7 +115,7 @@ void usbip_stop_eh(struct usbip_device *ud)
 	if (pending)
 		usbip_dbg_eh("usbip_eh waiting completion %lx\n", pending);
 
-	wait_event_interruptible(ud->eh_waitq, !(ud->event & ~USBIP_EH_BYE));
+	wait_event(ud->eh_waitq, !(ud->event & ~USBIP_EH_BYE));
 	usbip_dbg_eh("usbip_eh has stopped\n");
 }
 EXPORT_SYMBOL_GPL(usbip_stop_eh);


base-commit: 075b74841bd0065a3bda3440873c747938e69b68
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Re: [PATCH] usbip: fix use-after-free in event_handler()

[Shuah Khan]

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 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?

thanks,
-- Shuah