Received-SPF: pass (google.com: domain of linux-kernel+bounces-30785-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Date: Fri, 19 Jan 2024 06:48:43 +0100
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Carlos Llamas <cmllamas@google.com>
Cc: 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>, 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: <2024011935-placidly-justifier-4338@gregkh>
References: <20231201172212.1813387-1-cmllamas@google.com>
 <20231201172212.1813387-4-cmllamas@google.com>
 <Zal8A95q3jVl4nu5@google.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Zal8A95q3jVl4nu5@google.com>

On Thu, Jan 18, 2024 at 07:29:07PM +0000, Carlos Llamas wrote:
> 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.

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>