Re: [PATCH 1/4] mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race

[Nicholas Piggin]

Excerpts from pet...@infradead.org's message of August 28, 2020 9:15 pm:
> On Fri, Aug 28, 2020 at 08:00:19PM +1000, Nicholas Piggin wrote:
> 
>> 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().
> 
> ARM at least has activate_mm() := switch_mm(), so it could be made to
> work.
>

Yeah, so long as that post_lock_switch switch did the right thing with
respect to its TLB flushing. It should do because arm doesn't seem to
check ->mm or ->active_mm (and if it was broken, the scheduler context
switch would be suspect too). I don't think the fix would be hard, just
that I don't have a good way to test it and qemu isn't great for testing
this kind of thing.

um too I think could probably defer that lock until after interrupts are
enabled again. I might throw a bunch of arch conversion patches over the
wall if this gets merged and try to move things along.

Thanks,
Nick

Re: [PATCH 1/4] mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race

[peterz]

On Fri, Aug 28, 2020 at 08:00:19PM +1000, Nicholas Piggin wrote:

> 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().

ARM at least has activate_mm() := switch_mm(), so it could be made to
work.

[PATCH 1/4] mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race

[Nicholas Piggin]

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 38cf307c1f20 ("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 
---
 arch/Kconfig |  7 +++
 fs/exec.c| 17 +++--
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index af14a567b493..94821e3f94d1 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -414,6 +414,13 @@ config MMU_GATHER_NO_GATHER
bool
depends on MMU_GATHER_TABLE_FREE
 
+config ARCH_WANT_IRQS_OFF_ACTIVATE_MM
+   bool
+   help
+ Temporary select until all architectures can be converted to have
+ irqs disabled over activate_mm. Architectures that do IPI based TLB
+ shootdowns should enable this.
+
 config ARCH_HAVE_NMI_SAFE_CMPXCHG
bool
 
diff --git a/fs/exec.c b/fs/exec.c
index a91003e28eaa..d4fb18baf1fb 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1130,11 +1130,24 @@ static int exec_mmap(struct mm_struct *mm)
}
 
task_lock(tsk);
-   active_mm = tsk->active_mm;
membarrier_exec_mmap(mm);
-   tsk->mm = mm;
+
+   local_irq_disable();
+   active_mm = tsk->active_mm;
tsk->active_mm = mm;
+   tsk->mm = mm;
+   /*
+* This prevents preemption while active_mm is being loaded and
+* it and mm are being updated, which could cause problems for
+* lazy tlb mm refcounting when these are updated by context
+* switches. Not all architectures can handle irqs off over
+* activate_mm yet.
+*/
+   if (!IS_ENABLED(CONFIG_ARCH_WANT_IRQS_OFF_ACTIVATE_MM))
+   local_irq_enable();
activate_mm(active_mm, mm);
+   if (IS_ENABLED(CONFIG_ARCH_WANT_IRQS_OFF_ACTIVATE_MM))
+   local_irq_enable();
tsk->mm->vmacache_seqnum = 0;
vmacache_flush(tsk);
task_unlock(tsk);
-- 
2.23.0