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From: Rik van Riel on 6 Apr 2010 21:30
On 04/06/2010 08:10 PM, Linus Torvalds wrote:

> That said, the code that _really_ confuses me is the stuff that uses
> "anon_vma_clone()". Could you please also explain the code flow of
> vma_adjust() to mere mortals, please?

That's easier said than done. I spent 3 days with pen and paper,
going over that code before I made the anon_vma changes, first
verifying that the code is indeed correct and then figuring out
how I could make the anon_vma changes safely.

I am not happy with the complexity of the code around vma_adjust,
but could not find a way to simplify it and still keep merging
VMAs the way we do.

My largest change to vma_adjust was moving some code closer to
the beginning of the function, so I could bail out if the
allocation failed, without making change to the vma...

> I suspect Borislav is sleeping. But at least we have a patch for him to
> test when he wakes up ;)

I am looking forward to the test results.

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From: Johannes Weiner on 7 Apr 2010 05:20
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.
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From: Rik van Riel on 7 Apr 2010 10:20
On 04/07/2010 04:36 AM, Peter Zijlstra wrote:
> On Tue, 2010-04-06 at 11:28 -0700, Linus Torvalds wrote:

>> 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.

Peter, the avc is an anon_vma_chain, which is a different
object than the anon_vma itself. There is no use after free
of an anon_vma object in unlink_anon_vmas + anon_vma_unlink.
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