linux-kernel - Re: maple tree change made it possible for VMA iteration to see same VMA twice due to late vma

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Date:   Fri, 22 Sep 2023 20:02:57 +0200
From:   Jann Horn <jannh@...gle.com>
To:     "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: maple tree change made it possible for VMA iteration to see same
 VMA twice due to late vma_merge() failure

On Fri, Sep 22, 2023 at 7:52 PM Liam R. Howlett <Liam.Howlett@...cle.com> wrote:
>
> ...
> >
> > Looking at this, I think it's best to make a label and undo the
> > vma_prev() with a vma_next() - at least for now.
> >
> > I'm also reading this for the error path on dup_anon_vma() failure, and
> > it appears to also have an issue which I'd like to point out here before
> > I send the fix for the first issue.
> >
> > -----------
> >                 vma_start_write(next);
> >                 remove = next;                          /* case 1 */
> >                 vma_end = next->vm_end;
> >                 err = dup_anon_vma(prev, next);
> >                 if (curr) {                             /* case 6 */
> >                         vma_start_write(curr);
> >                         remove = curr;
> >                         remove2 = next;
> >                         if (!next->anon_vma)
> >                                 err = dup_anon_vma(prev, curr);
> > -----------
> >
> > Since dup_anon_vma() can fail, I think here in case 6 we could overwrite
> > the failure.
> >
> > That is, we will fail to clone the anon vma and mask the failure if we
> > are running case 6 with an anon in next.  Once the first dup_anon_vma()
> > returns error, the next call to clone curr vma may return 0 if there is
> > no anon vma (this, I think _must_ be the case). Then we are in a
> > situation where we will be removing next and expanding prev over curr
> > and next, but have not dup'ed the anon vma from next.
> >
>
> I think I am incorrect in the error being overwritten because we won't
> call dup_anon_vma(prev, curr) if the source of the previous call (next)
> has an anon vma.

Hm, yeah. It looks pretty dodgy and I guess it could use a comment,
but as you said, it seems to actually not be a problem...

We could do "err |= dup_anon_vma(...)" there for clarity instead, as
long as the only thing we care about is whether we have a nonzero
error...