On Tue, Jul 21, 2020 at 02:06:23PM -0700, Andrew Morton wrote:
> On Tue, 21 Jul 2020 17:41:06 +0200 Peter Zijlstra <pet...@infradead.org>
> wrote:
>
> >
> > 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. ]
>
> Can we give the -stable maintainers (and others) more explanation of
> why they might choose to merge this?
Like so then?
---
Subject: mm: Fix kthread_use_mm() vs TLB invalidate
From: Peter Zijlstra <pet...@infradead.org>
Date: Tue, 11 Feb 2020 10:25:19 +0100
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().
Of the (SMP) architectures that have IPI based TLB invalidate:
Alpha - checks active_mm
ARC - ASID specific
IA64 - checks active_mm
MIPS - ASID specific flush
OpenRISC - shoots down world
PARISC - shoots down world
SH - ASID specific
SPARC - ASID specific
x86 - N/A
xtensa - checks active_mm
So at the very least Alpha, IA64 and Xtensa are suspect.
On top of this, for scheduler consistency we need at least preemption
disabled across changing tsk->mm and doing switch_mm(), which is
currently provided by task_lock(), but that's not sufficient for
PREEMPT_RT.
Reported-by: Andy Lutomirski <l...@amacapital.net>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Cc: sta...@kernel.org
---
kernel/kthread.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -1241,13 +1241,20 @@ void kthread_use_mm(struct mm_struct *mm
WARN_ON_ONCE(tsk->mm);
task_lock(tsk);
+ /*
+ * Serialize the tsk->mm store and switch_mm() against TLB invalidation
+ * IPIs. Also make sure we're non-preemptible on PREEMPT_RT to not race
+ * against the scheduler writing to these variables.
+ */
+ 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 +1283,11 @@ void kthread_unuse_mm(struct mm_struct *
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);
