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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] Reported-by: Andy Lutomirski <luto@amacapital.net> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> 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: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Cc: <stable@vger.kernel.org> Link: http://lkml.kernel.org/r/20200721154106.GE10769@hirez.programming.kicks-ass.net Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -1241,13 +1241,16 @@ void kthread_use_mm(struct mm_struct *mm)
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WARN_ON_ONCE(tsk->mm);
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task_lock(tsk);
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/* Hold off tlb flush IPIs while switching mm's */
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local_irq_disable();
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active_mm = tsk->active_mm;
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if (active_mm != mm) {
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mmgrab(mm);
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tsk->active_mm = mm;
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}
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tsk->mm = mm;
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switch_mm(active_mm, mm, tsk);
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switch_mm_irqs_off(active_mm, mm, tsk);
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local_irq_enable();
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task_unlock(tsk);
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#ifdef finish_arch_post_lock_switch
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finish_arch_post_lock_switch();
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@ -1276,9 +1279,11 @@ void kthread_unuse_mm(struct mm_struct *mm)
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task_lock(tsk);
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sync_mm_rss(mm);
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local_irq_disable();
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tsk->mm = NULL;
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/* active_mm is still 'mm' */
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enter_lazy_tlb(mm, tsk);
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local_irq_enable();
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task_unlock(tsk);
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}
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EXPORT_SYMBOL_GPL(kthread_unuse_mm);
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