Re: [PATCH] binder: Don't use mmput() from shrinker function.

[Michal Hocko <mhocko@kernel.org>]

On Thu 16-07-20 08:36:52, Tetsuo Handa wrote:
> syzbot is reporting that mmput() from shrinker function has a risk of
> deadlock [1]. Don't start synchronous teardown of mm when called from
> shrinker function.

Please add the actual lock dependency to the changelog.

Anyway is this deadlock real? Mayve I have missed some details but the
call graph points to these two paths.
uprobe_mmap					do_shrink_slab	
  uprobes_mmap_hash #lock
  install_breakpoint				  binder_shrink_scan
    set_swbp					    binder_alloc_free_page
      uprobe_write_opcode			      __mmput
	update_ref_ctr				        uprobe_clear_state
    	  mutex_lock(&delayed_uprobe_lock)	          mutex_lock(&delayed_uprobe_lock);
	    allocation -> reclaim

But in order for this to happen the shrinker would have to do the last
put on the mm. But mm cannot go away from under uprobe_mmap so those two
paths cannot race with each other.

Unless I am missing something this is a false positive. I do not mind
using mmput_async from the shrinker as a workaround but the changelog
should be explicit about the fact.

> [1] https://syzkaller.appspot.com/bug?id=bc9e7303f537c41b2b0cc2dfcea3fc42964c2d45
> 
> Reported-by: syzbot <syzbot+1068f09c44d151250c33@syzkaller.appspotmail.com>
> Reported-by: syzbot <syzbot+e5344baa319c9a96edec@syzkaller.appspotmail.com>
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
>  drivers/android/binder_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
> index 42c672f1584e..cbe6aa77d50d 100644
> --- a/drivers/android/binder_alloc.c
> +++ b/drivers/android/binder_alloc.c
> @@ -947,7 +947,7 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
>  		trace_binder_unmap_user_end(alloc, index);
>  	}
>  	mmap_read_unlock(mm);
> -	mmput(mm);
> +	mmput_async(mm);
>  
>  	trace_binder_unmap_kernel_start(alloc, index);
>  
> -- 
> 2.18.4
> 

-- 
Michal Hocko
SUSE Labs