linux-kernel - Re: [PATCH 20/21] binder: reverse locking order in shrinker callback

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Date:   Fri, 1 Dec 2023 07:42:11 +0000
From:   Carlos Llamas <cmllamas@...gle.com>
To:     Alice Ryhl <aliceryhl@...gle.com>
Cc:     Arve Hjønnevåg <arve@...roid.com>,
        Christian Brauner <brauner@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        kernel-team@...roid.com, linux-kernel@...r.kernel.org,
        Martijn Coenen <maco@...roid.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Todd Kjos <tkjos@...roid.com>
Subject: Re: [PATCH 20/21] binder: reverse locking order in shrinker callback

On Tue, Nov 07, 2023 at 09:08:46AM +0000, Alice Ryhl wrote:
> Carlos Llamas <cmllamas@...gle.com> writes:
> This reverses the order so that __free_page is called before
> zap_page_range_single. Is that okay?

It is not OK. Userspace would still have access to the page between
__free_page() and zap_page() calls. This is bad, so thanks for catching
this.

> Another option would be to do something along these lines:
> 
>     page_to_free = page->page_ptr;
>     page->page_ptr = NULL;
> 
>     [snip]
> 
>     mmap_read_unlock(mm);
>     mmput_async(mm);
>     __free_page(page_to_free);
> 
> This way you wouldn't reverse the order. Also, you reduce the amount of
> time you spend holding the mmap read lock, since the page is freed
> without holding the lock.
> 

Thanks, I've added this approach to v2.

--
Carlos Llamas