mm: avoid setting up anonymous pages into file mapping

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().

Let's change page fault handler to use do_anonymous_page() only on
anonymous VMA (->vm_ops == NULL) and make sure that the VMA is not
shared.

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

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Willy Tarreau <w@1wt.eu>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 7 deletions

diff --git a/mm/memory.c b/mm/memory.c
index a84fbb772034..388dcf9aa283 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2670,6 +2670,10 @@ static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
 
         pte_unmap(page_table);
 
+        /* File mapping without ->vm_ops ? */
+        if (vma->vm_flags & VM_SHARED)
+                return VM_FAULT_SIGBUS;
+
         /* Check if we need to add a guard page to the stack */
         if (check_stack_guard_page(vma, address) < 0)
                 return VM_FAULT_SIGSEGV;
@@ -3099,6 +3103,9 @@ 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);
+        /* The VMA was not fully populated on mmap() or missing VM_DONTEXPAND */
+        if (!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);
@@ -3244,13 +3251,12 @@ 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);
-                        }
-                        return do_anonymous_page(mm, vma, address,
-                                                 pte, pmd, flags);
+                        if (vma->vm_ops)
+                                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);