On Wed, Nov 04, 2020 at 12:14:06PM +1100, Michael Ellerman wrote:
> From: Nicholas Piggin <npiggin@xxxxxxxxx>
>
> commit d53c3dfb23c45f7d4f910c3a3ca84bf0a99c6143 upstream.
>
> 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 <npiggin@xxxxxxxxx>
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> [mpe: Manual backport to 4.19 due to membarrier_exec_mmap(mm) changes]
> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/20200914045219.3736466-2-npiggin@xxxxxxxxx
> ---
> arch/Kconfig | 7 +++++++
> fs/exec.c | 15 ++++++++++++++-
> 2 files changed, 21 insertions(+), 1 deletion(-)
Now queued up, thanks!
greg k-h
