linux-kernel - Re: [PATCH 02/21] binder: fix use-after-free in shinker's callback

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Date:   Tue,  7 Nov 2023 09:07:56 +0000
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     Carlos Llamas <cmllamas@...gle.com>
Cc:     "Liam R . Howlett" <liam.howlett@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Arve Hjønnevåg" <arve@...roid.com>,
        Christian Brauner <brauner@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Martijn Coenen <maco@...roid.com>,
        Michal Hocko <mhocko@...nel.org>,
        Minchan Kim <minchan@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Todd Kjos <tkjos@...roid.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        stable@...r.kernel.org
Subject: Re: [PATCH 02/21] binder: fix use-after-free in shinker's callback

Carlos Llamas <cmllamas@...gle.com> writes:
> The mmap read lock is used during the shrinker's callback, which means
> that using alloc->vma pointer isn't safe as it can race with munmap().
> As of commit dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in
> munmap") the mmap lock is downgraded after the vma has been isolated.
> 
> I was able to reproduce this issue by manually adding some delays and
> triggering page reclaiming through the shrinker's debug sysfs. The
> following KASAN report confirms the UAF:
> 
>   [...snip...]
> 
> Fix this issue by performing instead a vma_lookup() which will fail to
> find the vma that was isolated before the mmap lock downgrade. Note that
> this option has better performance than upgrading to a mmap write lock
> which would increase contention. Plus, mmap_write_trylock() has been
> recently removed anyway.
> 
> Fixes: dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap")
> Cc: stable@...r.kernel.org
> Cc: Liam Howlett <liam.howlett@...cle.com>
> Cc: Minchan Kim <minchan@...nel.org>
> Signed-off-by: Carlos Llamas <cmllamas@...gle.com>

This change makes sense to me, and I agree that the code still needs to
run when the vma is null.

Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>