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    Subject[PATCH 4.14 46/64] binder: Dont use mmput() from shrinker function.
    Date
    From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>

    commit f867c771f98891841c217fa8459244ed0dd28921 upstream.

    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>
    Acked-by: Todd Kjos <tkjos@google.com>
    Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
    Cc: stable <stable@vger.kernel.org>
    Link: https://lore.kernel.org/r/4ba9adb2-43f5-2de0-22de-f6075c1fab50@i-love.sakura.ne.jp
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/android/binder_alloc.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/drivers/android/binder_alloc.c
    +++ b/drivers/android/binder_alloc.c
    @@ -968,7 +968,7 @@ enum lru_status binder_alloc_free_page(s
    trace_binder_unmap_user_end(alloc, index);
    }
    up_read(&mm->mmap_sem);
    - mmput(mm);
    + mmput_async(mm);

    trace_binder_unmap_kernel_start(alloc, index);


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