linux-kernel - Re: [PATCH 7/9] x86/mm: Shadow stack page fault error checking

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Date:   Thu, 07 Jun 2018 09:46:09 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. J. Lu" <hjl.tools@...il.com>,
        "Shanbhogue, Vedvyas" <vedvyas.shanbhogue@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Oleg Nesterov <oleg@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        mike.kravetz@...cle.com
Subject: Re: [PATCH 7/9] x86/mm: Shadow stack page fault error checking

On Thu, 2018-06-07 at 09:26 -0700, Andy Lutomirski wrote:
> On Thu, Jun 7, 2018 at 7:40 AM Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:
> >
> > If a page fault is triggered by a shadow stack access (e.g.
> > call/ret) or shadow stack management instructions (e.g.
> > wrussq), then bit[6] of the page fault error code is set.
> >
> > In access_error(), we check if a shadow stack page fault
> > is within a shadow stack memory area.
> >
> > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> 
> > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > index 73bd8c95ac71..2b3b9170109c 100644
> > --- a/arch/x86/mm/fault.c
> > +++ b/arch/x86/mm/fault.c
> > @@ -1166,6 +1166,17 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
> >                                        (error_code & X86_PF_INSTR), foreign))
> >                 return 1;
> >
> > +       /*
> > +        * Verify X86_PF_SHSTK is within a shadow stack VMA.
> > +        * It is always an error if there is a shadow stack
> > +        * fault outside a shadow stack VMA.
> > +        */
> > +       if (error_code & X86_PF_SHSTK) {
> > +               if (!(vma->vm_flags & VM_SHSTK))
> > +                       return 1;
> > +               return 0;
> > +       }
> > +
> 
> What, if anything, would go wrong without this change?  It seems like
> it might be purely an optimization.  If so, can you mention that in
> the comment?

Without this check, the page fault code could overlook the fact that the
application is trying to use non shadow stack area for shadow stack.
I will add this to the comments.