[PATCH] mm: hugetlb_vmemmap: fix a race between vmemmap pmd split

[Muchun Song]

The local variable @page in __split_vmemmap_huge_pmd() to obtain a pmd
page without holding page_table_lock may possiblely get the page table
page instead of a huge pmd page.  The effect may be in set_pte_at()
since we may pass an invalid page struct, if set_pte_at() wants to
access the page struct (e.g. CONFIG_PAGE_TABLE_CHECK is enabled), it
may crash the kernel.  So fix it. And inline __split_vmemmap_huge_pmd()
since it only has one user.

Fixes: d8d55f5616cf ("mm: sparsemem: use page table lock to protect kernel pmd operations")
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/hugetlb_vmemmap.c | 34 ++++++++++++++--------------------
 1 file changed, 14 insertions(+), 20 deletions(-)

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index c2007ef5e9b0..4b9734777f69 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -36,14 +36,22 @@ struct vmemmap_remap_walk {
 	struct list_head	*vmemmap_pages;
 };
 
-static int __split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
+static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
 {
 	pmd_t __pmd;
 	int i;
 	unsigned long addr = start;
-	struct page *page = pmd_page(*pmd);
-	pte_t *pgtable = pte_alloc_one_kernel(&init_mm);
+	struct page *head;
+	pte_t *pgtable;
+
+	spin_lock(&init_mm.page_table_lock);
+	head = pmd_leaf(*pmd) ? pmd_page(*pmd) : NULL;
+	spin_unlock(&init_mm.page_table_lock);
 
+	if (!head)
+		return 0;
+
+	pgtable = pte_alloc_one_kernel(&init_mm);
 	if (!pgtable)
 		return -ENOMEM;
 
@@ -53,7 +61,7 @@ static int __split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
 		pte_t entry, *pte;
 		pgprot_t pgprot = PAGE_KERNEL;
 
-		entry = mk_pte(page + i, pgprot);
+		entry = mk_pte(head + i, pgprot);
 		pte = pte_offset_kernel(&__pmd, addr);
 		set_pte_at(&init_mm, addr, pte, entry);
 	}
@@ -65,8 +73,8 @@ static int __split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
 		 * be treated as indepdenent small pages (as they can be freed
 		 * individually).
 		 */
-		if (!PageReserved(page))
-			split_page(page, get_order(PMD_SIZE));
+		if (!PageReserved(head))
+			split_page(head, get_order(PMD_SIZE));
 
 		/* Make pte visible before pmd. See comment in pmd_install(). */
 		smp_wmb();
@@ -80,20 +88,6 @@ static int __split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
 	return 0;
 }
 
-static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
-{
-	int leaf;
-
-	spin_lock(&init_mm.page_table_lock);
-	leaf = pmd_leaf(*pmd);
-	spin_unlock(&init_mm.page_table_lock);
-
-	if (!leaf)
-		return 0;
-
-	return __split_vmemmap_huge_pmd(pmd, start);
-}
-
 static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
 			      unsigned long end,
 			      struct vmemmap_remap_walk *walk)
-- 
2.11.0