linux-kernel - Re: [PATCH v2 03/28] binder: fix race between mmput() and do

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Date: Fri, 19 Jan 2024 06:48:43 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Carlos Llamas <cmllamas@...gle.com>
Cc: Arve Hjønnevåg <arve@...roid.com>,
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Christian Brauner <brauner@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Brian Swetland <swetland@...gle.com>, linux-kernel@...r.kernel.org,
	kernel-team@...roid.com, Alice Ryhl <aliceryhl@...gle.com>,
	Greg Kroah-Hartman <gregkh@...e.de>, stable@...r.kernel.org
Subject: Re: [PATCH v2 03/28] binder: fix race between mmput() and do_exit()

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@...gle.com>
> > Signed-off-by: Carlos Llamas <cmllamas@...gle.com>
> > ---
> 
> Sorry, I forgot to Cc: stable@...r.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>