Re: [PATCH 2/4] binder: fix OOB in binder_add_freeze_work()

[Carlos Llamas <cmllamas@google.com>]

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