mm: avoid setting up anonymous pages into file mapping

commit 6b7339f4c31ad69c8e9c0b2859276e22cf72176d upstream.

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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/mm/memory.c b/mm/memory.c
index 452b8ba..7762b1d 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3153,6 +3153,10 @@ 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;
@@ -3412,6 +3416,9 @@ static int do_linear_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;
        return __do_fault(mm, vma, address, pmd, pgoff, flags, orig_pte);
 }
 
@@ -3470,11 +3477,9 @@ int handle_pte_fault(struct mm_struct *mm,
        entry = *pte;
        if (!pte_present(entry)) {
                if (pte_none(entry)) {
-                       if (vma->vm_ops) {
-                               if (likely(vma->vm_ops->fault))
-                                       return do_linear_fault(mm, vma, address,
+                       if (vma->vm_ops)
+                               return do_linear_fault(mm, vma, address,
                                                pte, pmd, flags, entry);
-                       }
                        return do_anonymous_page(mm, vma, address,
                                                 pte, pmd, flags);
                }