Re: [PATCH v2] thunderbolt: Fix use-after-free in tb_dp_dprx_work

[Mika Westerberg <mika.westerberg@linux.intel.com>]

Hi,

On Tue, Sep 23, 2025 at 01:13:57PM +0800, Duoming Zhou wrote:
> The original code relies on cancel_delayed_work() in tb_dp_dprx_stop(),
> which does not ensure that the delayed work item tunnel->dprx_work has
> fully completed if it was already running. This leads to use-after-free
> scenarios where tb_tunnel is deallocated by tb_tunnel_put(), while
> tunnel->dprx_work remains active and attempts to dereference tb_tunnel
> in tb_dp_dprx_work().
> 
> A typical race condition is illustrated below:
> 
> CPU 0                            | CPU 1
> tb_dp_tunnel_active()            |
>   tb_deactivate_and_free_tunnel()| tb_dp_dprx_start()
>     tb_tunnel_deactivate()       |   queue_delayed_work()
>       tb_dp_activate()           |
>         tb_dp_dprx_stop()        | tb_dp_dprx_work() //delayed worker
>           cancel_delayed_work()  |
>     tb_tunnel_put(tunnel);       |
>                                  |   tunnel = container_of(...); //UAF
>                                  |   tunnel-> //UAF
> 
> Replacing cancel_delayed_work() with cancel_delayed_work_sync() is
> not feasible as it would introduce a deadlock: both tb_dp_dprx_work()
> and the cleanup path acquire tb->lock, and cancel_delayed_work_sync()
> would wait indefinitely for the work item that cannot proceed.
> 
> Instead, implement proper reference counting:
> - If cancel_delayed_work() returns true (work is pending), we release
>   the reference in the stop function.
> - If it returns false (work is executing or already completed), the
>   reference is released in delayed work function itself.
> 
> This ensures the tb_tunnel remains valid during work item execution
> while preventing memory leaks.
> 
> This bug was found by static analysis.
> 
> Fixes: d6d458d42e1e ("thunderbolt: Handle DisplayPort tunnel activation asynchronously")
> Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>

Because it is pretty late in the cycle, I would like to get some exposure
for this one before it goes to stable trees. I applied this to my
thunderbolt.git/next now. Once it is pulled into mainline this will be
picked up by the stable machinery too.

Thanks!