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

[Johan Hovold <johan@kernel.org>]

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.

Thanks,
Johan