Re: [PATCH] mm: fix lazy vmap purging (use-after-free error)

[Ingo Molnar]

* Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

> On Fri, Feb 20, 2009 at 03:51:28PM +0100, Vegard Nossum wrote:
> > 2009/2/20 Ingo Molnar <mingo@xxxxxxx>:
> > >
> > > * Ingo Molnar <mingo@xxxxxxx> 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.
> > 
> > Well, it's not the particular line that you posted, in any case.
> > That's &va->list, but the traversed list is &va->purge_list.
> > 
> > I thought it would be the line:
> > 
> >         call_rcu(&va->rcu_head, rcu_free_va);
> > 
> > (which does kfree() in the callback) that was the problem.
> > 
> > > 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.
> > 
> > I added some printks to __free_vmap_area() and rcu_free_va(), and it
> > shows that the kfree() is being called immediately (inside the list
> > traversal). So the call_rcu() is happening immediately (or almost
> > immediately).
> > 
> > If I've understood correctly, the RCU processing can happen inside a
> > spinlock, as long as interrupts are enabled. (Won't the timer IRQ
> > trigger softirq processing, which triggers RCU callback processing,
> > for example?)
> > 
> > And interrupts are enabled when this happens: EFLAGS: 00000292
> > 
> > Please correct me if I am wrong!
> 
> If you are using preemptable RCU, and if the read side 
> accesses are not protected by rcu_read_lock(), this can 
> happen.  At least for values of "immediately" in the 
> millisecond range.
> 
> If you were using classic or hierarchical RCU, the fact that 
> the call_rcu() is within a spinlock (as opposed to mutex) 
> critical section should prevent the grace period from ending.
> 
> So, what flavor of RCU were you using?

well, even in preemptible RCU the grace period should be 
extended as long as we are non-preempt (which we are here), 
correct?

Preemptible RCU does make an rcu_read_lock() critical section 
preemptible, so if this were protected by rcu_read_lock() it 
would be a bug. But it does not make spin_lock() section 
preemptible, and this is a spin_lock(&vmap_area_lock) section:

                spin_lock(&vmap_area_lock);
-               list_for_each_entry(va, &valist, purge_list)
+               list_for_each_entry_safe(va, n_va, &valist, purge_list)
                       	__free_vmap_area(va);
                spin_unlock(&vmap_area_lock);

	Ingo
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