Re: Ugly rmap NULL ptr deref oopsie on hibernate (was Linux2.6.34-rc3)

[Peter Zijlstra]

On Wed, 2010-04-07 at 11:16 +0200, Johannes Weiner wrote:
> On Wed, Apr 07, 2010 at 10:36:43AM +0200, Peter Zijlstra wrote:
> > On Tue, 2010-04-06 at 11:28 -0700, Linus Torvalds wrote:
> > > Just as an example of the kind of code that makes me worry:
> > > 
> > >         void unlink_anon_vmas(struct vm_area_struct *vma)
> > >         {
> > >                 struct anon_vma_chain *avc, *next;
> > >                         
> > >                 /* Unlink each anon_vma chained to the VMA. */
> > >                 list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
> > >                         anon_vma_unlink(avc);
> > >                         list_del(&avc->same_vma);
> > >                         anon_vma_chain_free(avc);
> > >                 }
> > >         }
> > > 
> > > Now, think about what happens for the *last* entry in that avc chain. It 
> > > will call that "anon_vma_unlink()" thing, which will delete perhaps the 
> > > last entry in the "same_anon_vma" one, and then it does
> > > 
> > >         if (empty)
> > >                 anon_vma_free(anon_vma);
> > > 
> > > *before* unlink_anon_vma's has actually does that
> > > 
> > >         list_del(&avc->same_vma);
> > > 
> > > and what we essentially have is a stale anon_vma_chain entry that still 
> > > exists on that same_vma list, and points to an anon_vma that already got 
> > > deleted.
> > > 
> > > Does it matter? I really can't see that it does. 
> > 
> > I think it does, the anon_vma thing has an RCU destroyed slab, but that
> > doesn't mean the anon_vma object itself is rcu delayed. The moment we
> > free it it can be re-used. So the above use after free is a bug.
> 
> It frees avc->anon_vma, not avc.

Sure, freeing avc does not involve RCU in any way.

>   So the sequence is
> 
> 	free(avc->anon_vma) in anon_vma_unlink()
> 	list_del(&avc->same_vma) in unlink_anon_vmas()
> 
> It's not a use-after free.  A problem would be if somebody should find the
> avc through this list (it is the vma->anon_vma_chain list) when its anon_vma
> pointer is invalid.
> 
> I don't think this can happen, however.  Both the unlinking and the looking
> at the list happen under vma->vm_mm's mmap_sem held for writing.

What I was worried about was it freeing anon_vma and then still having
the avc on list. But I guess that cannot happen because it only frees if
its actually empty.


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