linux-kernel - Re: [PATCH 2/4] binder: fix OOB in binder_add_freeze

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Message-ID: <ZvRRJiRe7zwyPeY7@google.com>
Date: Wed, 25 Sep 2024 18:06:30 +0000
From: Carlos Llamas <cmllamas@...gle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	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>,
	Yu-Ting Tseng <yutingtseng@...gle.com>,
	linux-kernel@...r.kernel.org, kernel-team@...roid.com,
	stable@...r.kernel.org
Subject: Re: [PATCH 2/4] binder: fix OOB in binder_add_freeze_work()

On Wed, Sep 25, 2024 at 07:52:37PM +0200, Alice Ryhl wrote:
> > > I reviewed some other code paths to verify whether there are other
> > > problems with processes dying concurrently with operations on freeze
> > > notifications. I didn't notice any other memory safety issues, but I
> >
> > Yeah most other paths are protected with binder_procs_lock mutex.
> >
> > > noticed that binder_request_freeze_notification returns EINVAL if you
> > > try to use it with a node from a dead process. That seems problematic,
> > > as this means that there's no way to invoke that command without
> > > risking an EINVAL error if the remote process dies. We should not
> > > return EINVAL errors on correct usage of the driver.
> >
> > Agreed, this should probably be -ESRCH or something. I'll add it to v2,
> > thanks for the suggestion.
> 
> Well, maybe? I think it's best to not return errnos from these
> commands at all, as they obscure how many commands were processed.

This is problematic, particularly when it's a multi-command buffer.
Userspace doesn't really know which one failed and if any of them
succeeded. Agreed.

> 
> Since the node still exists even if the process dies, perhaps we can
> just let you create the freeze notification even if it's dead? We can
> make it end up in the same state as if you request a freeze
> notification and the process then dies afterwards.

It's a dead node, there is no process associated with it. It would be
incorrect to setup the notification as it doesn't have a frozen status
anymore. We can't determine the ref->node->proc->is_frozen?

We could silently fail and skip the notification, but I don't know if
userspace will attempt to release it later... and fail with EINVAL.

--
Carlos Llamas