Re: [PATCH 02/13] mm: Revalidate anon_vma in page_lock_anon_vma()

[KAMEZAWA Hiroyuki]

On Fri,  9 Apr 2010 15:34:33 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:

> > On Fri, 9 Apr 2010 13:16:41 +1000
> > Nick Piggin <npiggin@xxxxxxx> wrote:
> > 
> > > On Thu, Apr 08, 2010 at 09:17:39PM +0200, Peter Zijlstra wrote:
> > > > There is nothing preventing the anon_vma from being detached while we
> > > > are spinning to acquire the lock. Most (all?) current users end up
> > > > calling something like vma_address(page, vma) on it, which has a
> > > > fairly good chance of weeding out wonky vmas.
> > > > 
> > > > However suppose the anon_vma got freed and re-used while we were
> > > > waiting to acquire the lock, and the new anon_vma fits with the
> > > > page->index (because that is the only thing vma_address() uses to
> > > > determine if the page fits in a particular vma, we could end up
> > > > traversing faulty anon_vma chains.
> > > > 
> > > > Close this hole for good by re-validating that page->mapping still
> > > > holds the very same anon_vma pointer after we acquire the lock, if not
> > > > be utterly paranoid and retry the whole operation (which will very
> > > > likely bail, because it's unlikely the page got attached to a different
> > > > anon_vma in the meantime).
> > > 
> > > Hm, looks like a bugfix? How was this supposed to be safe?
> > > 
> > IIUC.
> > 
> > Before Rik's change to anon_vma, once page->mapping is set as anon_vma | 0x1,
> > it's not modified until the page is freed.
> > After the patch, do_wp_page() overwrite page->mapping when it reuse existing
> > page.
> 
> Why?
> IIUC. page->mapping dereference in page_lock_anon_vma() makes four story.
> 
> 1. the anon_vma is valid
> 	-> do page_referenced_one(). 
> 2. the anon_vma is invalid and freed to buddy
> 	-> bail out by page_mapped(), no touch anon_vma
> 3. the anon_vma is kfreed, and not reused
> 	-> bail out by page_mapped()
> 4. the anon_vma is kfreed, but reused as another anon_vma
> 	-> bail out by page_check_address()
> 
> Now we have to consider 5th story.
> 
> 5. the anon_vma is exchanged another anon_vma by do_wp_page.
> 	-> bail out by above bailing out stuff.
> 
> 
> I agree peter's patch makes sense. but I don't think Rik's patch change
> locking rule.
> 

Hmm, I think following.

Assume a page is ANON and SwapCache, and it has only one reference.
Consider it's read-only mapped and cause do_wp_page().
page_mapcount(page) == 1 here.

    CPU0                          CPU1

1. do_wp_page()
2. .....
3. replace anon_vma.     anon_vma = lock_page_anon_vma()

So, lock_page_anon_vma() may have lock on wrong anon_vma, here.(mapcount=1)

4. modify pte to writable.        do something...

After lock, in CPU1, a pte of estimated address by vma_address(vma, page)
containes pfn of the page and page_check_address() will success.

I'm not sure how this is dangerouns.
But it's possible that CPU1 cannot notice there was anon_vma replacement.
And modifies pte withoug holding anon vma's lock which the code believes
it's holded.

Thanks,
-Kame




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