Re: [PATCH v3] mm: huge-vmap: fail gracefully on unexpected huge vmap mappings

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Jun 08, 2017 at 02:51:08PM +0000, Ard Biesheuvel wrote:
> On 8 June 2017 at 13:28, Mark Rutland <mark.rutland@arm.com> wrote:
> > On Thu, Jun 08, 2017 at 01:59:46PM +0100, Mark Rutland wrote:
> >> On Thu, Jun 08, 2017 at 11:35:48AM +0000, Ard Biesheuvel wrote:
> >> > @@ -287,10 +288,10 @@ struct page *vmalloc_to_page(const void *vmalloc_addr)
> >> >     if (p4d_none(*p4d))
> >> >             return NULL;
> >> >     pud = pud_offset(p4d, addr);
> >> > -   if (pud_none(*pud))
> >> > +   if (pud_none(*pud) || WARN_ON_ONCE(pud_huge(*pud)))
> >> >             return NULL;
> >> >     pmd = pmd_offset(pud, addr);
> >> > -   if (pmd_none(*pmd))
> >> > +   if (pmd_none(*pmd) || WARN_ON_ONCE(pmd_huge(*pmd)))
> >> >             return NULL;
> >>
> >> I think it might be better to use p*d_bad() here, since that doesn't
> >> depend on CONFIG_HUGETLB_PAGE.
> >>
> >> While the cross-arch semantics are a little fuzzy, my understanding is
> >> those should return true if an entry is not a pointer to a next level of
> >> table (so pXd_huge(p) implies pXd_bad(p)).
> >
> > Ugh; it turns out this isn't universally true.
> >
> > I see that at least arch/hexagon's pmd_bad() always returns 0, and they
> > support CONFIG_HUGETLB_PAGE.
> >
> 
> Well, the comment in arch/hexagon/include/asm/pgtable.h suggests otherwise:
> 
> /*  HUGETLB not working currently  */

Ah; I missed that.

> > So I guess there isn't an arch-neutral, always-available way of checking
> > this. Sorry for having mislead you.
> >
> > For arm64, p*d_bad() would still be preferable, so maybe we should check
> > both?
> 
> I am primarily interested in hardening architectures that define
> CONFIG_HAVE_ARCH_HUGE_VMAP, given that they intentionally create huge
> mappings in the VMALLOC area which this code may choke on. So whether
> pmd_bad() always returns 0 on an arch that does not define
> CONFIG_HAVE_ARCH_HUGE_VMAP does not really matter, because it simply
> nullifies this change for that particular architecture.
> 
> So as long as x86 and arm64 [which are the only ones to define
> CONFIG_HAVE_ARCH_HUGE_VMAP atm] work correctly with pXd_bad(), I think
> we should use it instead of pXd_huge(),

Sure; that sounds good to me.

Thanks,
Mark.

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