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

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Date:   Tue, 21 Jul 2020 17:41:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     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: [PATCH v3] mm: Fix kthread_use_mm() vs TLB invalidate


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: 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);