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

[Peter Zijlstra <peterz@infradead.org>]

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.