linux-kernel - Re: [PATCH] thunderbolt: Fix use-after-free in tb_dp_dprx

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Message-ID: <153e445.ee16.19974e9d7dd.Coremail.duoming@zju.edu.cn>
Date: Tue, 23 Sep 2025 12:51:46 +0800 (GMT+08:00)
From: duoming@....edu.cn
To: "Mika Westerberg" <mika.westerberg@...ux.intel.com>
Cc: linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	YehezkelShB@...il.com, westeri@...nel.org, michael.jamet@...el.com,
	andreas.noever@...il.com
Subject: Re: [PATCH] thunderbolt: Fix use-after-free in tb_dp_dprx_work

On Mon, 22 Sep 2025 14:54:43 +0200 Mika Westerberg 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@....edu.cn>
> > ---
> >  drivers/thunderbolt/tunnel.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/thunderbolt/tunnel.c b/drivers/thunderbolt/tunnel.c
> > index d52efe3f658c..89fa0c626d3e 100644
> > --- a/drivers/thunderbolt/tunnel.c
> > +++ b/drivers/thunderbolt/tunnel.c
> > @@ -1073,6 +1073,7 @@ static void tb_dp_dprx_work(struct work_struct *work)
> >  
> >  	if (tunnel->callback)
> >  		tunnel->callback(tunnel, tunnel->callback_data);
> > +	tb_tunnel_put(tunnel);
> >  }
> >  
> >  static int tb_dp_dprx_start(struct tb_tunnel *tunnel)
> > @@ -1097,11 +1098,14 @@ static int tb_dp_dprx_start(struct tb_tunnel *tunnel)
> >  
> >  static void tb_dp_dprx_stop(struct tb_tunnel *tunnel)
> >  {
> > +	bool ret;
> > +
> 
> Why you need variable here?
> 
> >  	if (tunnel->dprx_started) {
> >  		tunnel->dprx_started = false;
> >  		tunnel->dprx_canceled = true;
> > -		cancel_delayed_work(&tunnel->dprx_work);
> > -		tb_tunnel_put(tunnel);
> > +		ret = cancel_delayed_work(&tunnel->dprx_work);
> > +		if (ret)
> > +			tb_tunnel_put(tunnel);
> 
> Just do:
> 
> 	if (cancel_delayed_work(...))
> 		tb_tunnel_put(tunnel);
> 
Thank you for your suggestions, I will modify the code
and send the patch v2.

Best regards,
Duoming Zhou