Peter Zijlstra <pet...@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?
>
> Cc: sta...@kernel.org
> Reported-by: Andy Lutomirski <l...@amacapital.net>
> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.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);
