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Date: Fri, 19 Jan 2024 17:06:13 +0000
From: Carlos Llamas <cmllamas@google.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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: <ZaqsBTQ1x5lQG9Rq@google.com>
References: <20231201172212.1813387-1-cmllamas@google.com>
 <20231201172212.1813387-4-cmllamas@google.com>
 <Zal8A95q3jVl4nu5@google.com>
 <2024011935-placidly-justifier-4338@gregkh>
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In-Reply-To: <2024011935-placidly-justifier-4338@gregkh>

On Fri, Jan 19, 2024 at 06:48:43AM +0100, Greg Kroah-Hartman wrote:
> 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>

Oops, here is the complete info:

Commit ID: 9a9ab0d963621d9d12199df9817e66982582d5a5
Subject:   "binder: fix race between mmput() and do_exit()"
Reason:    Fixes a race condition in binder.
Versions:  v4.19+

Note this will have a trivial conflict in v4.19 and v5.10 kernels as
commit d8ed45c5dcd4 is not there. Please let me know if I should send
those patches separately.

Thanks,
--
Carlos Llamas