* Re: [PATCH v3] mm: Fix kthread_use_mm() vs TLB invalidate
[not found] ` <20200821130445.GP1362448@hirez.programming.kicks-ass.net>
@ 2020-08-28 3:26 ` Nicholas Piggin
2020-08-28 6:55 ` Nicholas Piggin
0 siblings, 1 reply; 2+ messages in thread
From: Nicholas Piggin @ 2020-08-28 3:26 UTC (permalink / raw)
To: Aneesh Kumar K.V, peterz, linux-arch
Cc: Andrew Morton, axboe, hch, jannh, keescook, linux-kernel, luto,
mathieu.desnoyers, torvalds, will
Excerpts from peterz@infradead.org'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@infradead.org> 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
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [PATCH v3] mm: Fix kthread_use_mm() vs TLB invalidate
2020-08-28 3:26 ` [PATCH v3] mm: Fix kthread_use_mm() vs TLB invalidate Nicholas Piggin
@ 2020-08-28 6:55 ` Nicholas Piggin
0 siblings, 0 replies; 2+ messages in thread
From: Nicholas Piggin @ 2020-08-28 6:55 UTC (permalink / raw)
To: Aneesh Kumar K.V, linux-arch, peterz
Cc: Andrew Morton, axboe, hch, jannh, keescook, linux-kernel, luto,
mathieu.desnoyers, torvalds, will
Excerpts from Nicholas Piggin's message of August 28, 2020 1:26 pm:
> Excerpts from peterz@infradead.org'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@infradead.org> 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.
Hmm, is it possible for something to be holding the mm_users when we
exec? That could actually make it a problem for TLB flushing too.
Thanks,
Nick
^ permalink raw reply [flat|nested] 2+ messages in thread
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[not found] ` <20200821130445.GP1362448@hirez.programming.kicks-ass.net>
2020-08-28 3:26 ` [PATCH v3] mm: Fix kthread_use_mm() vs TLB invalidate Nicholas Piggin
2020-08-28 6:55 ` Nicholas Piggin
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