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

[KAMEZAWA Hiroyuki]

On Fri,  9 Apr 2010 16:29:59 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:

> > 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.
> 
> 
> Hehe, page_referenced() already can take unstable VM_LOCKED value. So,
> In worst case we make false positive pageout, but it's not disaster.
> I think. Anyway "use after free" don't happen by this blutal code.
> 
> However, I think you pointed one good thing. before Rik patch, we don't have
> page->mapping reassignment. then, we didn't need rcu_dereference().
> but now it can happen. so, I think rcu_dereference() is better.
> 
> Perhaps, I'm missing something.
> 

Hmm. I wonder we can check "whether we lock valid anon_vma or not" only under
pte_lock or lock_page().
==
	anon_vma = page_anon_vma();
	lock(anon_vma->lock);
	....
	page_check_address(page)
		....
		pte_lock();
		if (page_anon_vma(page) == anon_vma)
			# anon_vma replacement happens!
	unlock(anon_vma->lock);
==
So, rather than page_lock_anon_vma(),  page_check_address() may have to check anon_vma
replacement....But I cannot think of dangerous case which can cause panic for now.
I may miss something...


Thanks,
-Kame

> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 8b088f0..b4a0b5b 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -295,7 +295,7 @@ struct anon_vma *page_lock_anon_vma(struct page *page)
>  	unsigned long anon_mapping;
>  
>  	rcu_read_lock();
> -	anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);
> +	anon_mapping = (unsigned long) rcu_dereference(page->mapping);
>  	if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
>  		goto out;
>  	if (!page_mapped(page))
> 
> 
> 
> 
> 

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