Re: [PATCH] mm/gup_test: fix race with PIN_LONGTERM_TEST ioctls

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/12/26 06:20, John Hubbard wrote:
> On 6/7/26 7:50 PM, Yunhui Cui wrote:
>> The PIN_LONGTERM_TEST helpers keep their state in global variables that
>> are protected by pin_longterm_test_mutex when accessed from ioctl().
>> However, gup_test_release() calls pin_longterm_test_stop() without
>> holding that mutex.
>>
>> This can race with PIN_LONGTERM_TEST_STOP and let two callers operate on
>> the same pages array concurrently, corrupting the test state and possibly
>> freeing it twice:
> 
> Let's add here that there are *no* such callers in the kernel, today.
> 
>>
>>  CPU 0                              CPU 1
>>  -----                              -----
>>  ioctl(PIN_LONGTERM_TEST_STOP)
>>    mutex_lock(&pin_longterm_test_mutex)
>>    pin_longterm_test_stop()
>>      if (pin_longterm_test_pages)
>>        kvfree(pin_longterm_test_pages)
>>
>>                                     close()
>>                                       gup_test_release()
>>                                         pin_longterm_test_stop()
>>                                           if (pin_longterm_test_pages)
>>                                             kvfree(pin_longterm_test_pages)
>>
>>      pin_longterm_test_pages = NULL
>>    mutex_unlock(&pin_longterm_test_mutex)
>>
>> Protect the release path with the same mutex so that stop and release
>> cannot run pin_longterm_test_stop() concurrently.
>>
>> Fixes: c77369b437f9 ("mm/gup_test: start/stop/read functionality for PIN LONGTERM test")
>> Cc: stable@vger.kernel.org
> 
> umm, no, to "Cc: stable". This is the sort of thing that gives AI
> a bad name. Specifically:
> 
> * Nothing in tree can possibly hit this race condition.
> 
> * This fix is purely static code analysis hygiene: correcting
>   a theoretical problem that does not actually provide any
>   sort of vulnerability fix in the kernel.
> 

Yes, all rather useless churn that consumes our capacity.

-- 
Cheers,

David