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.... 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); _