[patch 004/163] mm: fix kthread_use_mm() vs TLB invalidate

Andrew Morton
2020-08-07T06:17:20Z

From: Peter Zijlstra <peterz@infradead.org>
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@linux-foundation.org: add comment]
Link: http://lkml.kernel.org/r/20200721154106.GE10769@hirez.programming.kicks-ass.net
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reported-by: Andy Lutomirski <luto@amacapital.net>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Kees Cook <keescook@chromium.org>
Cc: Jann Horn <jannh@google.com>
Cc: Will Deacon <will@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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);
_