Re: mm: NULL ptr deref in balance_dirty_pages_ratelimited

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Feb 26, 2014 at 10:48:30PM +0800, Bob Liu wrote:
> > Do you relay on unlock_page() to have a compiler barrier?
> >
> 
> Before your commit mapping is a local variable and be assigned before
> unlock_page():
> struct address_space *mapping = page->mapping;
> unlock_page(dirty_page);
> put_page(dirty_page);
> if ((dirtied || page_mkwrite) && mapping) {
> 
> 
> I'm afraid now "fault_page->mapping" might be changed to NULL after
> "if ((dirtied || vma->vm_ops->page_mkwrite) && fault_page->mapping) {"
> and then passed down to balance_dirty_pages_ratelimited(NULL).

I see what you try to fix. I wounder if we need to do

mapping = ACCESS_ONCE(fault_page->mapping);

instead.

The question is if compiler on its own can eliminate intermediate variable
and dereference fault_page->mapping twice, as code with my patch does.
I ask because smp_mb__after_clear_bit() in unlock_page() does nothing on
some architectures.

> >>
> >> diff --git a/mm/memory.c b/mm/memory.c
> >> index 548d97e..90cea22 100644
> >> --- a/mm/memory.c
> >> +++ b/mm/memory.c
> >> @@ -3419,6 +3419,7 @@ static int do_shared_fault(struct mm_struct *mm,
> >> struct vm_area_struct *vma,
> >>   pgoff_t pgoff, unsigned int flags, pte_t orig_pte)
> >>  {
> >>   struct page *fault_page;
> >> + struct address_space *mapping;
> >>   spinlock_t *ptl;
> >>   pte_t *pte;
> >>   int dirtied = 0;
> >> @@ -3454,13 +3455,14 @@ static int do_shared_fault(struct mm_struct
> >> *mm, struct vm_area_struct *vma,
> >>
> >>   if (set_page_dirty(fault_page))
> >>   dirtied = 1;
> >> + mapping = fault_page->mapping;
> >>   unlock_page(fault_page);
> >> - if ((dirtied || vma->vm_ops->page_mkwrite) && fault_page->mapping) {
> >> + if ((dirtied || vma->vm_ops->page_mkwrite) && mapping) {
> >>   /*
> >>   * Some device drivers do not set page.mapping but still
> >>   * dirty their pages
> >>   */
> >> - balance_dirty_pages_ratelimited(fault_page->mapping);
> >> + balance_dirty_pages_ratelimited(mapping);
> >>   }
> >>
> >>   /* file_update_time outside page_lock */
> >> --
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> >
> > --
> >  Kirill A. Shutemov
> 
> -- 
> Regards,
> --Bob

-- 
 Kirill A. Shutemov

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