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

[peterz]

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@xxxxxxxxx>

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

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
>