Re: [PATCH] KVM: nVMX: Replace kvm_set_cr0 with vmx_set_cr0 in load_vmcs12_host_state

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2013-04-30 13:46, Gleb Natapov wrote:
> On Sun, Apr 28, 2013 at 12:20:38PM +0200, Jan Kiszka wrote:
>> On 2013-02-23 22:35, Jan Kiszka wrote:
>>> From: Jan Kiszka <jan.kiszka@siemens.com>
>>>
>>> Likely a typo, but a fatal one as kvm_set_cr0 performs checks on the
>>> state transition that may prevent loading L1's cr0.
>>>
>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>> ---
>>>  arch/x86/kvm/vmx.c |    2 +-
>>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>> index 26d47e9..94f3b66 100644
>>> --- a/arch/x86/kvm/vmx.c
>>> +++ b/arch/x86/kvm/vmx.c
>>> @@ -7429,7 +7429,7 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
>>>  	 * fpu_active (which may have changed).
>>>  	 * Note that vmx_set_cr0 refers to efer set above.
>>>  	 */
>>> -	kvm_set_cr0(vcpu, vmcs12->host_cr0);
>>> +	vmx_set_cr0(vcpu, vmcs12->host_cr0);
>>>  	/*
>>>  	 * If we did fpu_activate()/fpu_deactivate() during L2's run, we need
>>>  	 * to apply the same changes to L1's vmcs. We just set cr0 correctly,
>>>
>>
>> This one still applies, is necessary for nested unrestricted guest mode,
>> and I'm still convinced it's an appropriate way to fix the bug. How to
>> proceed?
>>
> What check that is done by kvm_set_cr0() fails?

Would have to reproduce the bug to confirm, but from the top of my head
and from looking at the code again:

if (!is_paging(vcpu) && (cr0 & X86_CR0_PG)) {
	if ((vcpu->arch.efer & EFER_LME)) {
		int cs_db, cs_l;

		if (!is_pae(vcpu))
			return 1;
		kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
		if (cs_l)
			return 1;

I think to remember this last check triggered. When we come from the
guest with paging off, we may run through this check an incorrectly bail
out here when the host state fulfills the conditions (PG, EFER_LME, and
L bit set).

Jan

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