Messages in this thread | | | Date | Wed, 7 Apr 2010 11:16:11 +0200 | From | Johannes Weiner <> | Subject | Re: Ugly rmap NULL ptr deref oopsie on hibernate (was Linux 2.6.34-rc3) |
| |
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. 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.
| |