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SubjectRe: [PATCH] mm: fix lazy vmap purging (use-after-free error)
On Fri, Feb 20, 2009 at 04:18:12PM +0200, Pekka Enberg wrote:
> On Fri, 2009-02-20 at 15:01 +0100, Ingo Molnar wrote:
> > * Ingo Molnar <mingo@elte.hu> wrote:
> >
> > > ah, indeed:
> > >
> > > list_del_rcu(&va->list);
> > >
> > > i suspect it could be hit big time in a workload that opens
> > > more than 512 files, as expand_files() uses a
> > > vmalloc()+vfree() pair in that case.
> >
> > hm, perhaps it's not a problem after all. The freeing is done
> > via rcu, and list_del_rcu() leaves the forward pointer intact.
> >
> > So how did it happen that the entry got kfree()d before the loop
> > was done? We are in a spinlocked section so the CPU should not
> > have entered rcu processing.
>
> RCU. Lets CC Paul.
>
> Looking at it, Documentation/RCU/checklist.txt states that:
>
> 7. If the updater uses call_rcu(), then the corresponding readers
> must use rcu_read_lock() and rcu_read_unlock().
>
> which we don't do for this loop. I fail to see how it could be a
> kmemcheck false positive, so it's probably a real bug.

And checklist.txt rule #7 is even more important in preemptable kernels,
especially if CONFIG_PREEMPT_RT.

Thanx, Paul


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