Re: [PATCH 2/6] x86/mm/pageattr: Do not strip pte flags from cpa->pfn

Matt Fleming
2015-11-12T19:28:56Z

On Thu, 12 Nov, at 07:47:30PM, Borislav Petkov wrote:
> On Thu, Nov 12, 2015 at 03:40:19PM +0000, Matt Fleming wrote:
> > Removing the PAGE_NX bit from cpa->pfn will corrupt the page frame
> > number address rather than removing PAGE_NX as the code intends. This
> > is unlikley to be a problem in practice because _PAGE_BIT_NX is bit 63
> > and most machines do not have page frame numbers that reach that high.
> >
> > Still, pte flags are never stored in cpa->pfn so we can safely delete
> > the code.
> >
> > Cc: Borislav Petkov <bp@alien8.de>
> > Cc: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
> > Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
> > ---
> > arch/x86/mm/pageattr.c | 5 -----
> > 1 file changed, 5 deletions(-)
> >
> > diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
> > index 893921b12272..d5240be55915 100644
> > --- a/arch/x86/mm/pageattr.c
> > +++ b/arch/x86/mm/pageattr.c
> > @@ -885,11 +885,6 @@ static void populate_pte(struct cpa_data *cpa,
> > pte = pte_offset_kernel(pmd, start);
> >
> > while (num_pages-- && start < end) {
> > -
> > - /* deal with the NX bit */
> > - if (!(pgprot_val(pgprot) & _PAGE_NX))
> > - cpa->pfn &= ~_PAGE_NX;
> > -
> > set_pte(pte, pfn_pte(cpa->pfn, pgprot));
> >
> > start += PAGE_SIZE;
> > --
>
> I think this should be part of the 1st patch because there you're
> correcting ->pfn to actually be a pfn.

OK, that's fine by me. I split this out into a separate patch so that
this single logical change could be debated independently of the other
->pfn change, and because I was less certain on this one.

I fold it into the first patch.