Re: [KVM PATCH v3 1/3] KVM: fix race in irq_routing logic

[Gleb Natapov <gleb@redhat.com>]

On Tue, Oct 27, 2009 at 10:00:15AM -0400, Gregory Haskins wrote:
> Gregory Haskins wrote:
> > Gleb Natapov wrote:
> >> On Mon, Oct 26, 2009 at 12:21:57PM -0400, Gregory Haskins wrote:
> >>> The current code suffers from the following race condition:
> >>>
> >>> thread-1                                    thread-2
> >>> -----------------------------------------------------------
> >>>
> >>> kvm_set_irq() {
> >>>    rcu_read_lock()
> >>>    irq_rt = rcu_dereference(table);
> >>>    rcu_read_unlock();
> >>>
> >>>                                        kvm_set_irq_routing() {
> >>>                                           mutex_lock();
> >>>                                           irq_rt = table;
> >>>                                           rcu_assign_pointer();
> >>>                                           mutex_unlock();
> >>>                                           synchronize_rcu();
> >>>
> >>>                                           kfree(irq_rt);
> >>>
> >>>    irq_rt->entry->set(); /* bad */
> >>>
> >> This is not what happens. irq_rt is never accessed outside read-side
> >> critical section.
> > 
> > Sorry, I was generalizing to keep the comments short.  I figured it
> > would be clear what I was actually saying, but realize in retrospect
> > that I was a little ambiguous.
> 
> Here is a revised problem statement
> 
>     thread-1                                    thread-2
>     -----------------------------------------------------------
> 
>     kvm_set_irq() {
>        rcu_read_lock()
>        irq_rt = rcu_dereference(table);
>        entry_cache = get_entries(irq_rt);
>        rcu_read_unlock();
> 
>                                             invalidate_entries(irq_rt);
> 
>        for_each_entry(entry_cache)
> 	  entry->set(); /* bad */
> 
>     -------------------------------------------------------------
> 
> 
> "invalidate_entries()" may be any operation that deletes an entry at
> run-time (doesn't exist today), or as the guest is shutting down.  As
> far as I can tell, the current code does not protect us from either
> condition, and my proposed patch protects us from both.  Did I miss
> anything?
> 
Yes. What happened to irq_rt is completely irrelevant at the point you
marked /* bad */.

--
			Gleb.