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

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

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
value to return in this case, so return NULL instead, and issue a
warning.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
This is a followup to '[PATCH v2] mm: vmalloc: make vmalloc_to_page()
deal with PMD/PUD mappings', hence the v3. The root issue with /proc/kcore
on arm64 is now handled by '[PATCH] mm: vmalloc: simplify vread/vwrite to
use existing mappings' [1], and this patch now only complements it by
taking care of other vmalloc_to_page() users.

[0] http://marc.info/?l=linux-mm&m=149641886821855&w=2
[1] http://marc.info/?l=linux-mm&m=149685966530180&w=2

 include/linux/hugetlb.h | 13 +++++++++----
 mm/vmalloc.c            |  5 +++--
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index b857fc8cc2ec..b7166e5426b6 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -121,8 +121,6 @@ struct page *follow_huge_pmd(struct mm_struct *mm, unsigned long address,
 				pmd_t *pmd, int flags);
 struct page *follow_huge_pud(struct mm_struct *mm, unsigned long address,
 				pud_t *pud, int flags);
-int pmd_huge(pmd_t pmd);
-int pud_huge(pud_t pud);
 unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
 		unsigned long address, unsigned long end, pgprot_t newprot);
 
@@ -150,8 +148,6 @@ static inline void hugetlb_show_meminfo(void)
 #define follow_huge_pmd(mm, addr, pmd, flags)	NULL
 #define follow_huge_pud(mm, addr, pud, flags)	NULL
 #define prepare_hugepage_range(file, addr, len)	(-EINVAL)
-#define pmd_huge(x)	0
-#define pud_huge(x)	0
 #define is_hugepage_only_range(mm, addr, len)	0
 #define hugetlb_free_pgd_range(tlb, addr, end, floor, ceiling) ({BUG(); 0; })
 #define hugetlb_fault(mm, vma, addr, flags)	({ BUG(); 0; })
@@ -190,6 +186,15 @@ static inline void __unmap_hugepage_range(struct mmu_gather *tlb,
 }
 
 #endif /* !CONFIG_HUGETLB_PAGE */
+
+#if defined(CONFIG_HUGETLB_PAGE) || defined(CONFIG_HAVE_ARCH_HUGE_VMAP)
+int pmd_huge(pmd_t pmd);
+int pud_huge(pud_t pud);
+#else
+#define pmd_huge(x)	0
+#define pud_huge(x)	0
+#endif
+
 /*
  * hugepages at page global directory. If arch support
  * hugepages at pgd level, they need to define this.
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 982d29511f92..67e1a304c467 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -12,6 +12,7 @@
 #include <linux/mm.h>
 #include <linux/module.h>
 #include <linux/highmem.h>
+#include <linux/hugetlb.h>
 #include <linux/sched/signal.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
@@ -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;
 
 	ptep = pte_offset_map(pmd, addr);
-- 
2.9.3

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