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 10:50:45AM -0400, Gregory Haskins wrote:
>> 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 */.
> The entire entry is cached not a pointer to an entry! kfree().

<light bulb goes off>

Ah, I see.  I missed that detail that it was a structure copy, not a
pointer copy.

My bad.  You are right, and I am wrong.  I retract the 1/3 patch.

Kind Regards,
-Greg


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