mirror of https://gitee.com/openkylin/linux.git
mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race
Reading and modifying current->mm and current->active_mm and switching
mm should be done with irqs off, to prevent races seeing an intermediate
state.
This is similar to commit 38cf307c1f
("mm: fix kthread_use_mm() vs TLB
invalidate"). At exec-time when the new mm is activated, the old one
should usually be single-threaded and no longer used, unless something
else is holding an mm_users reference (which may be possible).
Absent other mm_users, there is also a race with preemption and lazy tlb
switching. Consider the kernel_execve case where the current thread is
using a lazy tlb active mm:
call_usermodehelper()
kernel_execve()
old_mm = current->mm;
active_mm = current->active_mm;
*** preempt *** --------------------> schedule()
prev->active_mm = NULL;
mmdrop(prev active_mm);
...
<-------------------- schedule()
current->mm = mm;
current->active_mm = mm;
if (!old_mm)
mmdrop(active_mm);
If we switch back to the kernel thread from a different mm, there is a
double free of the old active_mm, and a missing free of the new one.
Closing this race only requires interrupts to be disabled while ->mm
and ->active_mm are being switched, but the TLB problem requires also
holding interrupts off over activate_mm. Unfortunately not all archs
can do that yet, e.g., arm defers the switch if irqs are disabled and
expects finish_arch_post_lock_switch() to be called to complete the
flush; um takes a blocking lock in activate_mm().
So as a first step, disable interrupts across the mm/active_mm updates
to close the lazy tlb preempt race, and provide an arch option to
extend that to activate_mm which allows architectures doing IPI based
TLB shootdowns to close the second race.
This is a bit ugly, but in the interest of fixing the bug and backporting
before all architectures are converted this is a compromise.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200914045219.3736466-2-npiggin@gmail.com
This commit is contained in:
parent
d012a7190f
commit
d53c3dfb23
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@ -414,6 +414,13 @@ config MMU_GATHER_NO_GATHER
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bool
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depends on MMU_GATHER_TABLE_FREE
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config ARCH_WANT_IRQS_OFF_ACTIVATE_MM
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bool
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help
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Temporary select until all architectures can be converted to have
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irqs disabled over activate_mm. Architectures that do IPI based TLB
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shootdowns should enable this.
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config ARCH_HAVE_NMI_SAFE_CMPXCHG
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bool
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17
fs/exec.c
17
fs/exec.c
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@ -1130,11 +1130,24 @@ static int exec_mmap(struct mm_struct *mm)
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}
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task_lock(tsk);
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active_mm = tsk->active_mm;
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membarrier_exec_mmap(mm);
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tsk->mm = mm;
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local_irq_disable();
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active_mm = tsk->active_mm;
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tsk->active_mm = mm;
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tsk->mm = mm;
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/*
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* This prevents preemption while active_mm is being loaded and
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* it and mm are being updated, which could cause problems for
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* lazy tlb mm refcounting when these are updated by context
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* switches. Not all architectures can handle irqs off over
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* activate_mm yet.
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*/
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if (!IS_ENABLED(CONFIG_ARCH_WANT_IRQS_OFF_ACTIVATE_MM))
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local_irq_enable();
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activate_mm(active_mm, mm);
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if (IS_ENABLED(CONFIG_ARCH_WANT_IRQS_OFF_ACTIVATE_MM))
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local_irq_enable();
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tsk->mm->vmacache_seqnum = 0;
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vmacache_flush(tsk);
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task_unlock(tsk);
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