Re: [PATCH 1/4] KVM: Always flush async #PF workqueue when vCPU is being destroyed

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Sean Christopherson <seanjc@google.com> writes:

> On Fri, Jan 26, 2024, Vitaly Kuznetsov wrote:
>> > +static void kvm_flush_and_free_async_pf_work(struct kvm_async_pf *work)
>> > +{
>> > +	/*
>> > +	 * The async #PF is "done", but KVM must wait for the work item itself,
>> > +	 * i.e. async_pf_execute(), to run to completion.  If KVM is a module,
>> > +	 * KVM must ensure *no* code owned by the KVM (the module) can be run
>> > +	 * after the last call to module_put(), i.e. after the last reference
>> > +	 * to the last vCPU's file is put.
>> > +	 *
>> 
>> Do I understand correctly that the problem is also present on the
>> "normal" path, i.e.:
>> 
>>   KVM_REQ_APF_READY
>>      kvm_check_async_pf_completion()
>>          kmem_cache_free(,work)
>> 
>> on one CPU can actually finish _before_ work is fully flushed on the
>> other (async_pf_execute() has already added an item to 'done' list but
>> hasn't completed)? Is it just the fact that the window of opportunity
>> to get the freed item re-purposed is so short that no real issue was
>> ever noticed?
>
> Sort of?  It's not a problem with accessing a freed obect, the issue is that
> async_pf_execute() can still be executing while KVM-the-module is unloaded.
>
> The reason the "normal" path is problematic is because it removes the
> work entry from async_pf.done and from async_pf.queue, i.e. makes it invisible
> to kvm_arch_destroy_vm().  So to hit the bug where the "normal" path processes
> the completed work, userspace would need to terminate the VM (i.e. exit() or
> close all fds), and KVM would need to completely tear down the VM, all before
> async_pf_execute() finished it's last few instructions.
>
> Which is possible, just comically unlikely :-)
>
>> In that case I'd suggest we emphasize that in the comment as currently it
>> sounds like kvm_arch_destroy_vm() is the only probemmatic path.
>
> How's this?
>
> 	/*
> 	 * The async #PF is "done", but KVM must wait for the work item itself,
> 	 * i.e. async_pf_execute(), to run to completion.  If KVM is a module,
> 	 * KVM must ensure *no* code owned by the KVM (the module) can be run
> 	 * after the last call to module_put().  Note, flushing the work item
> 	 * is always required when the item is taken off the completion queue.
> 	 * E.g. even if the vCPU handles the item in the "normal" path, the VM
> 	 * could be terminated before async_pf_execute() completes.
> 	 *
> 	 * Wake all events skip the queue and go straight done, i.e. don't
> 	 * need to be flushed (but sanity check that the work wasn't queued).
> 	 */
>

Sounds good to me, thanks!

>> > +	 * Wake all events skip the queue and go straight done, i.e. don't
>> > +	 * need to be flushed (but sanity check that the work wasn't queued).
>> > +	 */
>> > +	if (work->wakeup_all)
>> > +		WARN_ON_ONCE(work->work.func);
>> > +	else
>> > +		flush_work(&work->work);
>> > +	kmem_cache_free(async_pf_cache, work);
>> >  }
>> >  
>> >  void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu)
>> > @@ -114,7 +132,6 @@ void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu)
>> >  #else
>> >  		if (cancel_work_sync(&work->work)) {
>> >  			mmput(work->mm);
>> > -			kvm_put_kvm(vcpu->kvm); /* == work->vcpu->kvm */
>> >  			kmem_cache_free(async_pf_cache, work);
>> >  		}
>> >  #endif
>> > @@ -126,7 +143,18 @@ void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu)
>> >  			list_first_entry(&vcpu->async_pf.done,
>> >  					 typeof(*work), link);
>> >  		list_del(&work->link);
>> > -		kmem_cache_free(async_pf_cache, work);
>> > +
>> > +		spin_unlock(&vcpu->async_pf.lock);
>> > +
>> > +		/*
>> > +		 * The async #PF is "done", but KVM must wait for the work item
>> > +		 * itself, i.e. async_pf_execute(), to run to completion.  If
>> > +		 * KVM is a module, KVM must ensure *no* code owned by the KVM
>> > +		 * (the module) can be run after the last call to module_put(),
>> > +		 * i.e. after the last reference to the last vCPU's file is put.
>> > +		 */
>
> Doh, this is a duplicate comment, I'll delete it.
>  
>> > +		kvm_flush_and_free_async_pf_work(work);
>> > +		spin_lock(&vcpu->async_pf.lock);
>> >  	}
>> >  	spin_unlock(&vcpu->async_pf.lock);
>> >  
>> > @@ -151,7 +179,7 @@ void kvm_check_async_pf_completion(struct kvm_vcpu *vcpu)
>> >  
>> >  		list_del(&work->queue);
>> >  		vcpu->async_pf.queued--;
>> > -		kmem_cache_free(async_pf_cache, work);
>> > +		kvm_flush_and_free_async_pf_work(work);
>> >  	}
>> >  }
>> >  
>> > @@ -186,7 +214,6 @@ bool kvm_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>> >  	work->arch = *arch;
>> >  	work->mm = current->mm;
>> >  	mmget(work->mm);
>> > -	kvm_get_kvm(work->vcpu->kvm);
>> >  
>> >  	INIT_WORK(&work->work, async_pf_execute);
>> 
>> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> 
>> -- 
>> Vitaly
>> 
>

-- 
Vitaly