Re: [PATCH v2 1/4] mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race

peterz
2020-09-14T10:57:07Z

On Mon, Sep 14, 2020 at 02:52:16PM +1000, Nicholas Piggin wrote:
> 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@gmail.com>

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

I'm thinking we want this selected on x86 as well. Andy?

> ---
> 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
>