Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

Andy Lutomirski
2020-09-16T21:00:00Z

On Mon, Sep 14, 2020 at 2:14 PM Dave Hansen <dave.hansen@intel.com> wrote:
>
> On 9/14/20 11:31 AM, Andy Lutomirski wrote:
> > No matter what we do, the effects of calling vfork() are going to be a
> > bit odd with SHSTK enabled. I suppose we could disallow this, but
> > that seems likely to cause its own issues.
>
> What's odd about it? If you're a vfork()'d child, you can't touch the
> stack at all, right? If you do, you or your parent will probably die a
> horrible death.
>

An evil program could vfork(), have the child do a bunch of returns
and a bunch of calls, and exit. The net effect would be to change the
parent's shadow stack contents. In a sufficiently strict model, this
is potentially problematic.

The question is: how much do we want to protect userspace from itself?

--Andy