Re: [BUG] usb: cdc-wdm: Missing barriers in ad-hoc lockless buffer

[Greg KH]

On Wed, Mar 04, 2026 at 04:41:32PM +0800, Gui-Dong Han wrote:
> Hello maintainers,
> 
> I would like to report a potential concurrency bug in
> drivers/usb/class/cdc-wdm.c.
> 
> The driver implements an ad-hoc lockless buffer using desc->ubuf and
> desc->length. In wdm_read(), the read side checks
> READ_ONCE(desc->length) outside the spinlock. However, the write side
> in wdm_in_callback() updates the buffer and length without WRITE_ONCE
> and any memory barriers.
> 
> Due to compiler optimization or CPU out-of-order execution, the
> desc->length update can be reordered before the memmove. If this
> happens, wdm_read() can see the new length and call copy_to_user() on
> uninitialized memory. This also violates LKMM data race rules [1].
> 
> Additionally, the driver relies heavily on set_bit() and test_bit() on
> desc->flags for synchronization. These bit operations do not provide
> implicit barriers, which might lead to similar ordering issues.
> 
> Proposed solutions:
> 1. Short-term: Add WRITE_ONCE() and smp_wmb() on the write side, and
> smp_rmb() on the read side.
> 2. Long-term: Replace the ad-hoc buffer with kfifo. This is a classic
> single-reader (holding desc->rlock) and single-writer (holding
> desc->iuspin) scenario, making it a perfect fit for kfifo.
> 
> I discovered this issue while studying the driver's code. The presence
> of a READ_ONCE() on the read side without a matching WRITE_ONCE() on
> the write side caught my attention as a potential data race under the
> LKMM. In my opinion, implementing ad-hoc lockless algorithms directly
> within individual drivers is highly error-prone. To avoid these subtle
> memory ordering and barrier bugs, drivers should rely on established,
> well-tested kernel libraries like kfifo to handle this type of
> concurrency.
> 
> I am currently trying to reproduce the issue via stress testing on
> ARM64, though the race window is tight. I will also attempt a kfifo
> refactoring. However, since I am not familiar with this specific
> driver, I welcome anyone else to take over the kfifo conversion to
> eliminate these potential bugs and simplify the code.
> 
> Thank you for your attention to this matter.

Patches to help resolve this would be great, and yes, moving to kfifo
might make this work much better overall.

thanks,

greg k-h