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

[Yu-cheng Yu <yu-cheng.yu@intel.com>]

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@intel.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@intel.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.