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From:   Andy Lutomirski <luto@kernel.org>
Date:   Wed, 16 Sep 2020 06:52:31 -0700
Message-ID: <CALCETrXGSXzqdAOK7tnDDYMJ5Uj4=u=AEvgdroS3BxRoyO3r+g@mail.gmail.com>
Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Dave Martin <Dave.Martin@arm.com>,
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        Balbir Singh <bsingharora@gmail.com>,
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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