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

[Gregory Haskins]

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.

Yes, irq_rt is not accessed outside the RSCS.  However, the entry
pointers stored in the irq_rt->map are, and this is equally problematic
afaict.

In this particular case we seem to never delete entries at run-time once
they are established.  Therefore, while perhaps sloppy, its technically
safe to leave them unprotected from this perspective.  The issue is more
related to shutdown since a kvm_set_irq() caller could be within the
aforementioned race-region and call entry->set() after the guest is
gone.  Or did I miss something?

> Data is copied from irq_rt onto the stack and this copy is accessed
> outside critical section.

As mentioned above, I do not believe this really protect us.  And even
if it did, the copy is just a work-around to avoid sleeping within the
standard RCU RSCS, which is what SRCU is designed for.  So rather than
inventing an awkward two-phased stack based solution, it's better to
reuse the provided tools, IMO.

To flip it around:  Is there any reason why an SRCU would not work here,
and thus we were forced to use something like the stack-copy approach?

Kind Regards,
-Greg

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