Subject: [PATCH v3] mm: Fix mremap not considering huge pmd devmap

[Fan Yang]

Hi,

The original code in mm/mremap.c checks huge pmd by:

		if (is_swap_pmd(*old_pmd) || pmd_trans_huge(*old_pmd)) {

However, a DAX mapped nvdimm is mapped as huge page (by default) but
it is not transparent huge page (_PAGE_PSE | PAGE_DEVMAP).  This
commit changes the condition to include the case.

This addresses CVE-2020-10757.

Fixes: 5c7fb56e5e3f ("mm, dax: dax-pmd vs thp-pmd vs hugetlbfs-pmd")
Cc: <stable@xxxxxxxxxxxxxxx>
Reported-by: Fan Yang <Fan_Yang@xxxxxxxxxxx>
Signed-off-by: Fan Yang <Fan_Yang@xxxxxxxxxxx>
Tested-by: Fan Yang <Fan_Yang@xxxxxxxxxxx>
Tested-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

---

Changelog v2->v3:
- Added "Acked-by: Kirill..."

Changelog v1->v2:
- Removed some paragraph in commit msg, removed the comment in
  mm/mremap.c, and added a NOTE in where pmd_trans_huge is defined.
- Added "Reviewed-by: Dan..."
- Added "Fixes: 5c7fb56e5e3f..."
- Added "Cc: <stable@xxxxxxxxxxxxxxx>"
---
 arch/x86/include/asm/pgtable.h | 1 +
 mm/mremap.c                    | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 4d02e64af1b3..19cdeebfbde6 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -257,6 +257,7 @@ static inline int pmd_large(pmd_t pte)
 }
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
+/* NOTE: when predicate huge page, consider also pmd_devmap, or use pmd_large */
 static inline int pmd_trans_huge(pmd_t pmd)
 {
 	return (pmd_val(pmd) & (_PAGE_PSE|_PAGE_DEVMAP)) == _PAGE_PSE;
diff --git a/mm/mremap.c b/mm/mremap.c
index 6aa6ea605068..57b1f999f789 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -266,7 +266,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 		new_pmd = alloc_new_pmd(vma->vm_mm, vma, new_addr);
 		if (!new_pmd)
 			break;
-		if (is_swap_pmd(*old_pmd) || pmd_trans_huge(*old_pmd)) {
+		if (is_swap_pmd(*old_pmd) || pmd_trans_huge(*old_pmd) || pmd_devmap(*old_pmd)) {
 			if (extent == HPAGE_PMD_SIZE) {
 				bool moved;
 				/* See comment in move_ptes() */
-- 
2.25.4