Re: [PATCH v3] mm: Fix kthread_use_mm() vs TLB invalidate

[Nicholas Piggin]

Excerpts from peterz@xxxxxxxxxxxxx's message of August 21, 2020 11:04 pm:
> On Fri, Aug 21, 2020 at 11:09:51AM +0530, Aneesh Kumar K.V wrote:
>> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
>> 
>> > For SMP systems using IPI based TLB invalidation, looking at
>> > current->active_mm is entirely reasonable. This then presents the
>> > following race condition:
>> >
>> >
>> >   CPU0			CPU1
>> >
>> >   flush_tlb_mm(mm)	use_mm(mm)
>> >     <send-IPI>
>> > 			  tsk->active_mm = mm;
>> > 			  <IPI>
>> > 			    if (tsk->active_mm == mm)
>> > 			      // flush TLBs
>> > 			  </IPI>
>> > 			  switch_mm(old_mm,mm,tsk);
>> >
>> >
>> > Where it is possible the IPI flushed the TLBs for @old_mm, not @mm,
>> > because the IPI lands before we actually switched.
>> >
>> > Avoid this by disabling IRQs across changing ->active_mm and
>> > switch_mm().
>> >
>> > [ There are all sorts of reasons this might be harmless for various
>> > architecture specific reasons, but best not leave the door open at
>> > all. ]
>> 
>> 
>> Do we have similar race with exec_mmap()? I am looking at exec_mmap()
>> runnning parallel to do_exit_flush_lazy_tlb(). We can get
>> 
>> 	if (current->active_mm == mm) {
>> 
>> true and if we don't disable irq around updating tsk->mm/active_mm we
>> can end up doing mmdrop on wrong mm?
> 
> exec_mmap() is called after de_thread(), there should not be any mm
> specific invalidations around I think.
> 
> Then again, CLONE_VM without CLONE_THREAD might still be possible, so
> yeah, we probably want IRQs disabled there too, just for consistency and
> general paranoia if nothing else.

The problem is probably not this TLB flushing race, but I think there
is a lazy tlb race.

  call_usermodehelper()
    kernel_execve()
      old_mm = current->mm;
      active_mm = current->active_mm;
      *** preempt *** ---------------------->schedule()
                                               prev->active_mm = NULL;
					       mmdrop(prev active mm)
					     ... 
                      <----------------------schedule()
      current->mm = mm;
      current->active_mm = mm;
      if (!old_mm)
          mmdrop(active_mm); /* double free! */

There's possibly other problematic interleavings. powerpc also has an
issue with switching away a lazy tlb mm via IPI which is basically the
same problem so I just illustrate the more general issue.

I think we just make it a rule that these always get updated under
local_irq_disable, to be safe.

Trouble is we can't just do it, because some architectures can't do
activate_mm with irqs disabled. ARM and UM, at least. UM can't even
do preempt_disabled. We can probably change them to make them work,
I'm not sure what the best way to go is, my first attempt is to require
activate_mm to do the mm switching and the irq disable as well, but
I'll need some help from the archs

I'll send out rfcs in a minute.

Thanks,
Nick