From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Subject: 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().
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.
[akpm@xxxxxxxxxxxxxxxxxxxx: add comment]
Link: http://lkml.kernel.org/r/20200721154106.GE10769@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reported-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/kthread.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
--- a/kernel/kthread.c~mm-fix-kthread_use_mm-vs-tlb-invalidate
+++ a/kernel/kthread.c
@@ -1241,13 +1241,16 @@ void kthread_use_mm(struct mm_struct *mm
WARN_ON_ONCE(tsk->mm);
task_lock(tsk);
+ /* Hold off tlb flush IPIs while switching mm's */
+ 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 +1279,11 @@ 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);
_
