[PATCH v5] mm: huge-vmap: fail gracefully on unexpected huge vmap mappings
Laura Abbott
labbott at redhat.com
Fri Jun 9 11:13:12 PDT 2017
On 06/09/2017 01:22 AM, 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 at linaro.org>
Reviewed-by: Laura Abbott <labbott at redhat.com>
> ---
> 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;
> pmd = pmd_offset(pud, addr);
> - if (pmd_none(*pmd))
> + WARN_ON_ONCE(pmd_bad(*pmd));
> + if (pmd_none(*pmd) || pmd_bad(*pmd))
> return NULL;
>
> ptep = pte_offset_map(pmd, addr);
>
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