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Date: Thu, 18 Jan 2024 19:29:07 +0000
From: Carlos Llamas <cmllamas@google.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Arve =?iso-8859-1?B?SGr4bm5lduVn?= <arve@android.com>,
	Todd Kjos <tkjos@android.com>, Martijn Coenen <maco@android.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Christian Brauner <brauner@kernel.org>,
	Suren Baghdasaryan <surenb@google.com>,
	Brian Swetland <swetland@google.com>
Cc: linux-kernel@vger.kernel.org, kernel-team@android.com,
	Alice Ryhl <aliceryhl@google.com>,
	Greg Kroah-Hartman <gregkh@suse.de>, stable@vger.kernel.org
Subject: Re: [PATCH v2 03/28] binder: fix race between mmput() and do_exit()
Message-ID: <Zal8A95q3jVl4nu5@google.com>
References: <20231201172212.1813387-1-cmllamas@google.com>
 <20231201172212.1813387-4-cmllamas@google.com>
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In-Reply-To: <20231201172212.1813387-4-cmllamas@google.com>

On Fri, Dec 01, 2023 at 05:21:32PM +0000, Carlos Llamas wrote:
> Task A calls binder_update_page_range() to allocate and insert pages on
> a remote address space from Task B. For this, Task A pins the remote mm
> via mmget_not_zero() first. This can race with Task B do_exit() and the
> final mmput() refcount decrement will come from Task A.
> 
>   Task A            | Task B
>   ------------------+------------------
>   mmget_not_zero()  |
>                     |  do_exit()
>                     |    exit_mm()
>                     |      mmput()
>   mmput()           |
>     exit_mmap()     |
>       remove_vma()  |
>         fput()      |
> 
> In this case, the work of ____fput() from Task B is queued up in Task A
> as TWA_RESUME. So in theory, Task A returns to userspace and the cleanup
> work gets executed. However, Task A instead sleep, waiting for a reply
> from Task B that never comes (it's dead).
> 
> This means the binder_deferred_release() is blocked until an unrelated
> binder event forces Task A to go back to userspace. All the associated
> death notifications will also be delayed until then.
> 
> In order to fix this use mmput_async() that will schedule the work in
> the corresponding mm->async_put_work WQ instead of Task A.
> 
> Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Carlos Llamas <cmllamas@google.com>
> ---

Sorry, I forgot to Cc: stable@vger.kernel.org.

--
Carlos Llamas