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

[ard.biesheuvel@linaro.org (Ard Biesheuvel)]

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;
 	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);
-- 
2.9.3