Messages in this thread | | | Date | Wed, 7 Apr 2010 08:46:16 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: Ugly rmap NULL ptr deref oopsie on hibernate (was Linux 2.6.34-rc3) |
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On Wed, 7 Apr 2010, 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.
Well, it's not really a "use after free" - it's just that a stale pointer still exists in a live data structure that is linked into the list. I don't think there is a real bug there, simply because I don't think anybody will be accessing that list (we should hopefully have all the sufficient mutual exclusion in place).
So I just think it is bad form to potentially free something before we get rid of all pointers to it. So to me it's a cleanliness issue: good code shouldn't do things like that, and it would be much cleaner to remove the AVC entry that has a pointer to the anon_vma _before_ we might be freeing the anon_vma.
Maybe I'm just anal.
Linus
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