Re: [PATCH] KVM: nVMX: Eliminate vmcs02 pool

[Paolo Bonzini <pbonzini@redhat.com>]

On 27/11/2017 18:43, Mark Kanda wrote:
> On 11/23/2017 5:46 PM, Paolo Bonzini wrote:
>> On 21/11/2017 18:22, Mark Kanda wrote:
>>> -    nested_free_all_saved_vmcss(vmx);
>>> +    free_loaded_vmcs(&vmx->nested.vmcs02);
>>
>> Please add to free_loaded_vmcs a WARN that the passed vmcs is not
>> vmx->loaded_vmcs.
> 
> Sure.
> 
> However, I don't see a way to access vmx from the passed in vmcs, which
> would necessitate passing in vmx for the WARN (multiple callers) - I may
> be missing something..

free_loaded_vmcs is only ever used on VMCS02's, so you can change it to

static void vmx_nested_free_vmcs02(struct vcpu_vmx *vmx)
{
	struct loaded_vmcs *loaded_vmcs = &vmx->nested.vmcs02;

	/*
	 * Just leak the VMCS02 if the WARN triggers.  Better than
	 * a use-after-free.
	 */
	if (WARN_ON(vmx->loaded_vmcs == loaded_vmcs))
		return;
	...
}

> I'm happy to do this, but it seems possibly excessive for the sole
> purpose of adding the WARN. Thoughts?

We've had this kind of bug in the past, so I prefer to err on the side
of safety.

Paolo