linux-kernel - Re: [PATCH v3] mm: Fix kthread_use

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Date:   Fri, 21 Aug 2020 11:09:51 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, luto@...capital.net, axboe@...nel.dk,
        keescook@...omium.org, torvalds@...ux-foundation.org,
        jannh@...gle.com, will@...nel.org, hch@....de, npiggin@...il.com,
        mathieu.desnoyers@...icios.com
Subject: Re: [PATCH v3] mm: Fix kthread_use_mm() vs TLB invalidate

Peter Zijlstra <peterz@...radead.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?

>
> Cc: stable@...nel.org
> Reported-by: Andy Lutomirski <luto@...capital.net>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>
> Sorry, I dropped the ball on this and only found it because I was
> looking at the whole membarrier things vs use_mm().
>
>
>  kernel/kthread.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 1d9e2fdfd67a..7221dcbffef3 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -1241,13 +1241,15 @@ void kthread_use_mm(struct mm_struct *mm)
>  	WARN_ON_ONCE(tsk->mm);
>  
>  	task_lock(tsk);
> +	local_irq_disable();
>  	active_mm = tsk->active_mm;
>  	if (active_mm != mm) {
>  		mmgrab(mm);
>  		tsk->active_mm = mm;
>  	}
>  	tsk->mm = mm;
> -	switch_mm(active_mm, mm, tsk);
> +	switch_mm_irqs_off(active_mm, mm, tsk);
> +	local_irq_enable();
>  	task_unlock(tsk);
>  #ifdef finish_arch_post_lock_switch
>  	finish_arch_post_lock_switch();
> @@ -1276,9 +1278,11 @@ void kthread_unuse_mm(struct mm_struct *mm)
>  
>  	task_lock(tsk);
>  	sync_mm_rss(mm);
> +	local_irq_disable();
>  	tsk->mm = NULL;
>  	/* active_mm is still 'mm' */
>  	enter_lazy_tlb(mm, tsk);
> +	local_irq_enable();
>  	task_unlock(tsk);
>  }
>  EXPORT_SYMBOL_GPL(kthread_unuse_mm);