Re: [PATCH] KVM: nVMX: Fix setting of CR0 and CR4 in guest mode

[Jan Kiszka <jan.kiszka@web.de>]

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On 2013-03-04 20:33, Gleb Natapov wrote:
> On Mon, Mar 04, 2013 at 08:23:52PM +0100, Jan Kiszka wrote:
>> On 2013-03-04 19:39, Gleb Natapov wrote:
>>> On Mon, Mar 04, 2013 at 07:08:08PM +0100, Jan Kiszka wrote:
>>>> On 2013-03-04 18:56, Gleb Natapov wrote:
>>>>> On Mon, Mar 04, 2013 at 03:25:47PM +0100, Jan Kiszka wrote:
>>>>>> On 2013-03-04 15:15, Gleb Natapov wrote:
>>>>>>> On Mon, Mar 04, 2013 at 03:09:51PM +0100, Jan Kiszka wrote:
>>>>>>>> On 2013-03-04 14:22, Gleb Natapov wrote:
>>>>>>>>> On Thu, Feb 28, 2013 at 10:44:47AM +0100, Jan Kiszka wrote:
>>>>>>>>>> The logic for calculating the value with which we call kvm_set_cr0/4 was
>>>>>>>>>> broken (will definitely be visible with nested unrestricted guest mode
>>>>>>>>>> support). Also, we performed the check regarding CR0_ALWAYSON too early
>>>>>>>>>> when in guest mode.
>>>>>>>>>>
>>>>>>>>>> What really needs to be done on both CR0 and CR4 is to mask out L1-owned
>>>>>>>>>> bits and merge them in from GUEST_CR0/4. In contrast, arch.cr0/4 and
>>>>>>>>>> arch.cr0/4_guest_owned_bits contain the mangled L0+L1 state and, thus,
>>>>>>>>>> are not suited as input.
>>>>>>>>>>
>>>>>>>>>> For both CRs, we can then apply the check against VMXON_CRx_ALWAYSON and
>>>>>>>>>> refuse the update if it fails. To be fully consistent, we implement this
>>>>>>>>>> check now also for CR4.
>>>>>>>>>>
>>>>>>>>>> Finally, we have to set the shadow to the value L2 wanted to write
>>>>>>>>>> originally.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>>>>>>>>> ---
>>>>>>>>>>
>>>>>>>>>> Found while making unrestricted guest mode working. Not sure what impact
>>>>>>>>>> the bugs had on current feature level, if any.
>>>>>>>>>>
>>>>>>>>>> For interested folks, I've pushed my nEPT environment here:
>>>>>>>>>>
>>>>>>>>>>     git://git.kiszka.org/linux-kvm.git nept-hacking
>>>>>>>>>>
>>>>>>>>>>  arch/x86/kvm/vmx.c |   49 ++++++++++++++++++++++++++++++-------------------
>>>>>>>>>>  1 files changed, 30 insertions(+), 19 deletions(-)
>>>>>>>>>>
>>>>>>>>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>>>>>>>>> index 7cc566b..d1dac08 100644
>>>>>>>>>> --- a/arch/x86/kvm/vmx.c
>>>>>>>>>> +++ b/arch/x86/kvm/vmx.c
>>>>>>>>>> @@ -4605,37 +4605,48 @@ vmx_patch_hypercall(struct kvm_vcpu *vcpu, unsigned char *hypercall)
>>>>>>>>>>  /* called to set cr0 as appropriate for a mov-to-cr0 exit. */
>>>>>>>>>>  static int handle_set_cr0(struct kvm_vcpu *vcpu, unsigned long val)
>>>>>>>>>>  {
>>>>>>>>>> -	if (to_vmx(vcpu)->nested.vmxon &&
>>>>>>>>>> -	    ((val & VMXON_CR0_ALWAYSON) != VMXON_CR0_ALWAYSON))
>>>>>>>>>> -		return 1;
>>>>>>>>>> -
>>>>>>>>>>  	if (is_guest_mode(vcpu)) {
>>>>>>>>>> -		/*
>>>>>>>>>> -		 * We get here when L2 changed cr0 in a way that did not change
>>>>>>>>>> -		 * any of L1's shadowed bits (see nested_vmx_exit_handled_cr),
>>>>>>>>>> -		 * but did change L0 shadowed bits. This can currently happen
>>>>>>>>>> -		 * with the TS bit: L0 may want to leave TS on (for lazy fpu
>>>>>>>>>> -		 * loading) while pretending to allow the guest to change it.
>>>>>>>>>> -		 */
>>>>>>>>> Can't say I understand this patch yet, but it looks like the comment is
>>>>>>>>> still valid. Why have you removed it?
>>>>>>>>
>>>>>>>> L0 allows L1 or L2 at most to own TS, the rest is host-owned. I think
>>>>>>>> the comment was always misleading.
>>>>>>>>
>>>>>>> I do not see how it is misleading. For everything but TS we will not get
>>>>>>> here (if L1 is kvm). For TS we will get here if L1 allows L2 to change
>>>>>>> it, but L0 does not.
>>>>>>
>>>>>> For everything *but guest-owned* we will get here, thus for most CR0
>>>>>> accesses (bit-wise, not regarding frequency).
>>>>>>
>>>>> I do not see how. If bit is trapped by L1 we will not get here. We will
>>>>> do vmexit to L1 instead. nested_vmx_exit_handled_cr() check this condition.
>>>>> I am not arguing about you code (didn't grok it yet), but the comment
>>>>> still make sense to me.
>>>>
>>>> "We get here when L2 changed cr0 in a way that did not change any of
>>>> L1's shadowed bits (see nested_vmx_exit_handled_cr), but did change L0
>>>> shadowed bits." That I can sign. But the rest about TS is just
>>>> misleading as we trap _every_ change in L0 - except for TS under certain
>>>> conditions. The old code was tested against TS only, that's what the
>>>> comment witness.
>>>>
>>> TS is just an example of how we can get here with KVM on KVM. Obviously
>>> other hypervisors may have different configuration. L2 may allow full
>>> guest access to CR0 and then each CR0 write by L2 will be handled here.
>>> Under what other condition "we trap _every_ change in L0 - except for
>>> TS" here?
>>
>> On FPU activation:
>>
>>     cr0_guest_owned_bits = X86_CR0_TS;
>>
>> And on FPU deactivation:
>>
>>     cr0_guest_owned_bits = 0;
>>
> That's exactly TS case that comment explains. Note that
> CR0_GUEST_HOST_MASK = ~cr0_guest_owned_bits.

Again, it's the inverse of what the comment suggest: we enter
handle_set_cr0 for every change on CR0 that doesn't match the shadow -
except TS was given to the guest by both L1 and L0 (or TS isn't changed
as well).

> 
>>>
>>>> If you prefer, I'll leave part one in.
>>>>
>>> Please do so. Without the comment it is not obvious why exit condition
>>> is not checked here. Still do not see why you object to TS part.
>>
>> It describes a corner case in a way that suggests this is the only
>> reason why we get here.
>>
> For KVM on KVM it is.

Which is, sorry, irrelevant.

Jan



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