[PATCH v5] mm: huge-vmap: fail gracefully on unexpected huge vmap mappings

[mark.rutland@arm.com (Mark Rutland)]

On Fri, Jun 09, 2017 at 08:22:26AM +0000, Ard Biesheuvel wrote:
> Existing code that uses vmalloc_to_page() may assume that any
> address for which is_vmalloc_addr() returns true may be passed
> into vmalloc_to_page() to retrieve the associated struct page.
> 
> This is not un unreasonable assumption to make, but on architectures
> that have CONFIG_HAVE_ARCH_HUGE_VMAP=y, it no longer holds, and we
> need to ensure that vmalloc_to_page() does not go off into the weeds
> trying to dereference huge PUDs or PMDs as table entries.
> 
> Given that vmalloc() and vmap() themselves never create huge
> mappings or deal with compound pages at all, there is no correct
> answer in this case, so return NULL instead, and issue a warning.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
> v5: - fix typo
> 
> v4: - use pud_bad/pmd_bad instead of pud_huge/pmd_huge, which don't require
>       changes to hugetlb.h, and give us what we need on all architectures
>     - move WARN_ON_ONCE() calls out of conditionals
>     - add explanatory comment
> 
>  mm/vmalloc.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 34a1c3e46ed7..0fcd371266a4 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -287,10 +287,21 @@ struct page *vmalloc_to_page(const void *vmalloc_addr)
>  	if (p4d_none(*p4d))
>  		return NULL;
>  	pud = pud_offset(p4d, addr);
> -	if (pud_none(*pud))
> +
> +	/*
> +	 * Don't dereference bad PUD or PMD (below) entries. This will also
> +	 * identify huge mappings, which we may encounter on architectures
> +	 * that define CONFIG_HAVE_ARCH_HUGE_VMAP=y. Such regions will be
> +	 * identified as vmalloc addresses by is_vmalloc_addr(), but are
> +	 * not [unambiguously] associated with a struct page, so there is
> +	 * no correct value to return for them.
> +	 */
> +	WARN_ON_ONCE(pud_bad(*pud));
> +	if (pud_none(*pud) || pud_bad(*pud))
>  		return NULL;

Nit: the WARN_ON_ONCE() can be folded into the conditional:

	if (pud_none(*pud) || WARN_ON_ONCE(pud_bad(*pud)))
		reutrn NULL;

>  	pmd = pmd_offset(pud, addr);
> -	if (pmd_none(*pmd))
> +	WARN_ON_ONCE(pmd_bad(*pmd));
> +	if (pmd_none(*pmd) || pmd_bad(*pmd))
>  		return NULL;

Likewise here.

Otherwise, looks good to me. FWIW:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Thanks,
Mark.

>  
>  	ptep = pte_offset_map(pmd, addr);
> -- 
> 2.9.3
>