[PATCH] mm: avoid setting up anonymous pages into file mapping

[Kirill A. Shutemov]

Reading page fault handler code I've noticed that under right
circumstances kernel would map anonymous pages into file mappings:
if the VMA doesn't have vm_ops->fault() and the VMA wasn't fully
populated on ->mmap(), kernel would handle page fault to not populated
pte with do_anonymous_page().

There's chance that it was done intentionally, but I don't see good
justification for this. We just hide bugs in broken drivers.

Let's change page fault handler to use do_anonymous_page() only on
anonymous VMA (->vm_ops == NULL).

For file mappings without vm_ops->fault() page fault on pte_none() entry
would lead to SIGBUS.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 mm/memory.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 8a2fc9945b46..f3ee782059e3 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3115,6 +3115,9 @@ static int do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 			- vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
 
 	pte_unmap(page_table);
+
+	if (unlikely(!vma->vm_ops->fault))
+		return VM_FAULT_SIGBUS;
 	if (!(flags & FAULT_FLAG_WRITE))
 		return do_read_fault(mm, vma, address, pmd, pgoff, flags,
 				orig_pte);
@@ -3260,13 +3263,13 @@ static int handle_pte_fault(struct mm_struct *mm,
 	barrier();
 	if (!pte_present(entry)) {
 		if (pte_none(entry)) {
-			if (vma->vm_ops) {
-				if (likely(vma->vm_ops->fault))
-					return do_fault(mm, vma, address, pte,
-							pmd, flags, entry);
+			if (!vma->vm_ops) {
+				return do_anonymous_page(mm, vma, address, pte,
+						pmd, flags);
+			} else {
+				return do_fault(mm, vma, address, pte, pmd,
+						flags, entry);
 			}
-			return do_anonymous_page(mm, vma, address,
-						 pte, pmd, flags);
 		}
 		return do_swap_page(mm, vma, address,
 					pte, pmd, flags, entry);
-- 
2.1.4

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