Re: [PATCH] mm: Warn mmput() from memory reclaim context.

[David Rientjes <rientjes@google.com>]

On Fri, 17 Jul 2020, Tetsuo Handa wrote:

> syzbot is reporting that mmput() from shrinker function has a risk of
> deadlock [1], for delayed_uprobe_add() from update_ref_ctr() calls
> kzalloc(GFP_KERNEL) with delayed_uprobe_lock held, and
> uprobe_clear_state() from __mmput() also holds delayed_uprobe_lock.
> 
> However, it took 18 months to hit this race for the third time, for
> mmput() invokes __mmput() only when ->mm_users dropped to 0. If we
> always warn like might_sleep(), we can detect the possibility of
> deadlock more easier.
> 
> For now, I inlined the check under CONFIG_PROVE_LOCKING. If we find
> more locations, we could introduce a macro like might_sleep().
> 

Hi Tetsuo,

It looks like this is one issue where mmput() interacted poorly while in 
direct reclaim because of a uprobes issue, I'm not sure that we can make a 
generalization that mmput() is *always* problematic when PF_MEMALLOC is 
set.  I'm also mindful of the (ab)use of PF_MEMALLOC outside just the 
direct reclaim path.  Or maybe there is a way you can show that mmput() 
while PF_MEMALLOC is set is always concerning?

> [1] https://syzkaller.appspot.com/bug?id=bc9e7303f537c41b2b0cc2dfcea3fc42964c2d45
> 

I wasn't familiar with this particular report, but it seems like the fix 
is simply to do the kzalloc() before taking delayed_uprobe_lock and 
freeing it if delayed_uprobe_check() already finds one for that uprobe?

> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
>  kernel/fork.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index efc5493203ae..8717ce50ff0d 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1109,6 +1109,10 @@ static inline void __mmput(struct mm_struct *mm)
>  void mmput(struct mm_struct *mm)
>  {
>  	might_sleep();
> +#ifdef CONFIG_PROVE_LOCKING
> +	/* Calling mmput() from shrinker context can deadlock. */
> +	WARN_ON(current->flags & PF_MEMALLOC);
> +#endif
>  
>  	if (atomic_dec_and_test(&mm->mm_users))
>  		__mmput(mm);