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

[Peter Zijlstra]

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. ]

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);