Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

[KAMEZAWA Hiroyuki]

On Mon, 15 Mar 2010 15:44:59 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:

> On Mon, 15 Mar 2010 15:28:15 +0900
> Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
> 
> > On Mon, Mar 15, 2010 at 2:34 PM, KAMEZAWA Hiroyuki
> > <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> > > On Mon, 15 Mar 2010 09:28:08 +0900
> > > Minchan Kim <minchan.kim@xxxxxxxxx> wrote:

> > I think above scenario make error "use-after-free", again.
> > What prevent above scenario?
> > 
> I think this patch is not complete. 
> I guess this patch in [1/11] is trigger for the race.
> ==
> +
> +	/* Drop an anon_vma reference if we took one */
> +	if (anon_vma && atomic_dec_and_lock(&anon_vma->migrate_refcount, &anon_vma->lock)) {
> +		int empty = list_empty(&anon_vma->head);
> +		spin_unlock(&anon_vma->lock);
> +		if (empty)
> +			anon_vma_free(anon_vma);
> +	}
> ==
> If my understainding in above is correct, this "modify" freed anon_vma.
> Then, use-after-free happens. (In old implementation, there are no refcnt,
> so, there is no use-after-free ops.)
> 
Sorry, about above, my understanding was wrong. anon_vma->lock is modifed even
in old code. Sorry for noise.

-Kame








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