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Date:   Thu, 16 Jul 2020 17:17:56 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Arve Hjonnevag <arve@android.com>,
        Todd Kjos <tkjos@android.com>,
        Martijn Coenen <maco@android.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Christian Brauner <christian@brauner.io>,
        syzbot <syzbot+e5344baa319c9a96edec@syzkaller.appspotmail.com>,
        acme@kernel.org, alexander.shishkin@linux.intel.com,
        jolsa@redhat.com, linux-kernel@vger.kernel.org,
        mark.rutland@arm.com, mingo@redhat.com, namhyung@kernel.org,
        peterz@infradead.org, syzkaller-bugs@googlegroups.com,
        "open list:ANDROID DRIVERS" <devel@driverdev.osuosl.org>,
        linux-mm <linux-mm@kvack.org>
Subject: Re: [PATCH v2] binder: Don't use mmput() from shrinker function.
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On Fri 17-07-20 00:12:15, 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.
> 
> Commit a1b2289cef92ef0e ("android: binder: drop lru lock in isolate
> callback") replaced mmput() with mmput_async() in order to avoid sleeping
> with spinlock held. But this patch replaces mmput() with mmput_async() in
> order not to start __mmput() from shrinker context.
> 
> [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>

Reviewed-by: Michal Hocko <mhocko@suse.com>

Thanks!

> ---
>  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