linux-kernel - Re: [PATCH v2] binder: Don't use mmput() from shrinker function.

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Date:   Thu, 16 Jul 2020 16:53:31 -0700
From:   Todd Kjos <tkjos@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjonnevag <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <christian@...uner.io>,
        syzbot <syzbot+e5344baa319c9a96edec@...kaller.appspotmail.com>,
        acme@...nel.org, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Ingo Molnar <mingo@...hat.com>, namhyung@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH v2] binder: Don't use mmput() from shrinker function.

On Thu, Jul 16, 2020 at 8:18 AM Michal Hocko <mhocko@...nel.org> wrote:
>
> 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@...kaller.appspotmail.com>
> > Reported-by: syzbot <syzbot+e5344baa319c9a96edec@...kaller.appspotmail.com>
> > Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
>
> Reviewed-by: Michal Hocko <mhocko@...e.com>

Acked-by: Todd Kjos <tkjos@...gle.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