Re: [PATCH 1/5] USB: serial: fix unthrottle races

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, May 13, 2019 at 12:43:39PM +0200, Johan Hovold wrote:
> On Thu, Apr 25, 2019 at 06:05:36PM +0200, Johan Hovold wrote:
> > Fix two long-standing bugs which could potentially lead to memory
> > corruption or leave the port throttled until it is reopened (on weakly
> > ordered systems), respectively, when read-URB completion races with
> > unthrottle().
> > 
> > First, the URB must not be marked as free before processing is complete
> > to prevent it from being submitted by unthrottle() on another CPU.
> > 
> > 	CPU 1				CPU 2
> > 	================		================
> > 	complete()			unthrottle()
> > 	  process_urb();
> > 	  smp_mb__before_atomic();
> > 	  set_bit(i, free);		  if (test_and_clear_bit(i, free))
> > 	  					  submit_urb();
> > 
> > Second, the URB must be marked as free before checking the throttled
> > flag to prevent unthrottle() on another CPU from failing to observe that
> > the URB needs to be submitted if complete() sees that the throttled flag
> > is set.
> > 
> > 	CPU 1				CPU 2
> > 	================		================
> > 	complete()			unthrottle()
> > 	  set_bit(i, free);		  throttled = 0;
> > 	  smp_mb__after_atomic();	  smp_mb();
> > 	  if (throttled)		  if (test_and_clear_bit(i, free))
> > 	  	  return;			  submit_urb();
> > 
> > Note that test_and_clear_bit() only implies barriers when the test is
> > successful. To handle the case where the URB is still in use an explicit
> > barrier needs to be added to unthrottle() for the second race condition.
> > 
> > Fixes: d83b405383c9 ("USB: serial: add support for multiple read urbs")
> > Signed-off-by: Johan Hovold <johan@kernel.org>
> 
> Greg, I noticed you added a stable tag to the corresponding cdc-acm fix
> and think I should have added on one from the start to this one as well.
> 
> Would you mind queuing this one up for stable?
> 
> Upstream commit 3f5edd58d040bfa4b74fb89bc02f0bc6b9cd06ab.

Sure, now queued up for 4.9+

thanks,

greg k-h