Re: mm: NULL ptr deref in balance_dirty_pages_ratelimited

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Feb 26, 2014 at 05:20:51PM +0200, Kirill A. Shutemov wrote:
> 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.

The compiler is most definitely within its rights to eliminate intermediate
variables if you don't use something like ACCESS_ONCE().  For more info,
see the LWN writeup:  http://lwn.net/Articles/508991/

							Thanx, Paul

> > >>
> > >> 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 */
> > >> --
> > >> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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> > >> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > >> Please read the FAQ at  http://www.tux.org/lkml/
> > >
> > > --
> > >  Kirill A. Shutemov
> > 
> > -- 
> > Regards,
> > --Bob
> 
> -- 
>  Kirill A. Shutemov
> 

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