USB: cdc-acm: fix unthrottle races

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.

Also note that the first race was fixed by 36e59e0d70d6 ("cdc-acm: fix race between callback and unthrottle") back in 2015, but the bug was reintroduced a year later.

Fixes 1aba579f3cf5 ("cdc-acm: handle read pipe errors")

Fixes 088c64f81284 ("USB: cdc-acm: re-write read processing")

Signed-off-by Johan Hovold <[email protected]>