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

HTH
-Greg


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