Re: Ugly rmap NULL ptr deref oopsie on hibernate (was Linux 2.6.34-rc3)

[Minchan Kim <minchan.kim@gmail.com>]

Hi, Linus. 

On Tue, 2010-04-06 at 08:55 -0700, Linus Torvalds wrote:
> 
> On Wed, 7 Apr 2010, Minchan Kim wrote:
> > 
> > Let's see the unlink_anon_vmas. 
> > 
> > 1. list_for_each_entry_safe(avc,next, vma->anon_vma_chain, same_vma)
> > 2. 	anon_vma_unlink
> > 3. 		spin_lock(anon_vma->lock) <-- HERE LOCK.
> > 4.		list_del(anon_vma_chain->same_anon_vma);
> > 
> > What if anon_vma is destroyed and reuse by SLAB_XXX_RCU for another
> > anon_vma object between 2 and 3?
> > I mean how to make sure 3) does lock valid anon_vma? 
> > 
> > I hope it is culprit.
> 
> I don't think so. That isn't the racy case. We're working with a 
> anon_vma_chain, so the anonvma is all there.
> 

But the anon_vma is using for another anon_vma. 
Nonetheless, anon_vma_unlink does list_del(anon_vma's same_anon_vma).
I doubt it. 

> The racy case is when we look up an anonvma by the page, and the page gets 
> unmapped at the same time because somebody else is travelling over the LRU 
> list of the page itself, isn't it?

Yes. but I thought page might travel with anon_vmas which have
same_anon_vma deleted by race.

> 
> I do wonder if "page_lock_anon_vma()" should check the whole 
> "page_mapped()" case _after_ taking the anon_vma lock. Because if the race 
> happens, we're following a anon_vma list that has nothing to do with that 
> page (it's stilla _valid_ list, since we locked the anon_vma, but will it 
> be ok?)

So we always use it with (vma_address and page_check_address) to make
sure validation of anon_vma.
But I think it's not good design. I want to hold lock ahead checking of
page_mapped but maybe performance issue? I am not sure. 

> 
> IOW, what is it that really keeps the anon_vma list reliable _and_ 
> relevant wrt the page? We know we may get a stale anon_vma, are we ok if 
> that anon_vma list doesn't actually have anything to do with the page any 
> more?
> I think the first check in "page_address_in_vma()" protects us, but 
> whatever.
> 
> However, that made me look at the PAGE_MIGRATION case. That seems to be 
> just broken. It's doing that page_anon_vma() + spin_lock without holding 
> any RCU locks, so there is no guarantee that anon_vma there is at all 
> valid.

FYI, recently there is a patch about migration case. 
http://lkml.org/lkml/2010/4/2/145


> 
> Is that function always called with rcu_read_lock()? 
> 
> 			Linus


-- 
Kind regards,
Minchan Kim