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

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

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  */


> 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(),

-- 
Ard.

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