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

[Gregory Haskins <gregory.haskins@gmail.com>]

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Gleb Natapov wrote:
> 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 */.

kfree() happened to irq_rt, and thus to the objects behind the pointers
in entry_cache at the point I marked /* bad */.

That certainly isn't /* good */ ;)

-Greg


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