linux-kernel - Re: [RFC PATCH v2 15/27] mm/mprotect: Prevent mprotect from changing shadow stack

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Date:   Wed, 11 Jul 2018 11:12:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omiun.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>
Subject: Re: [RFC PATCH v2 15/27] mm/mprotect: Prevent mprotect from changing
 shadow stack

On Tue, Jul 10, 2018 at 04:10:08PM -0700, Dave Hansen wrote:
> On 07/10/2018 03:26 PM, Yu-cheng Yu wrote:
> > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> 
> This still needs a changelog, even if you think it's simple.
> > --- a/mm/mprotect.c
> > +++ b/mm/mprotect.c
> > @@ -446,6 +446,15 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
> >  	error = -ENOMEM;
> >  	if (!vma)
> >  		goto out;
> > +
> > +	/*
> > +	 * Do not allow changing shadow stack memory.
> > +	 */
> > +	if (vma->vm_flags & VM_SHSTK) {
> > +		error = -EINVAL;
> > +		goto out;
> > +	}
> > +
> 
> I think this is a _bit_ draconian.  Why shouldn't we be able to use
> protection keys with a shadow stack?  Or, set it to PROT_NONE?

Right, and then there's also madvise() and some of the other accessors.

Why do we need to disallow this? AFAICT the worst that can happen is
that a process wrecks itself, so what?