Re: [PATCH v2] KVM: nVMX: Fix setting of CR0 and CR4 in guest mode

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

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On 2013-03-07 08:51, Gleb Natapov wrote:
> On Mon, Mar 04, 2013 at 08:40:29PM +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>
>> ---
>>
>> Changes in v2:
>>  - keep the non-misleading part of the comment in handle_set_cr0
>>
>>  arch/x86/kvm/vmx.c |   46 +++++++++++++++++++++++++++++++---------------
>>  1 files changed, 31 insertions(+), 15 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 7cc566b..832b7b4 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -4605,37 +4605,53 @@ 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)) {
>> +		struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
>> +		unsigned long orig_val = val;
>> +
>>  		/*
>>  		 * 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.
>> +		 * but did change L0 shadowed bits.
>>  		 */
>> -		if (kvm_set_cr0(vcpu, (val & vcpu->arch.cr0_guest_owned_bits) |
>> -			 (vcpu->arch.cr0 & ~vcpu->arch.cr0_guest_owned_bits)))
>> +		val = (val & ~vmcs12->cr0_guest_host_mask) |
>> +			(vmcs_read64(GUEST_CR0) & vmcs12->cr0_guest_host_mask);
> I think using GUEST_CR0 here is incorrect. It contains combination of bits
> set by L2, L1 and L0 and here we need to get only L2/L1 mix which is in
> vcpu->arch.cr0 (almost, but good enough for this case). Why vcpu->arch.cr0
> contains right L2/L1 mix?

L0/L1. E.g., kvm_set_cr0 unconditionally injects X86_CR0_ET and masks
out reserved bits. But you are right, GUEST_CR0 isn't much better. And
maybe that mangling in kvm_set_cr0 is a corner case we can ignore.

> Because it was set to vmcs12->guest_cr0 during
> L2 #vmentry. While L2 is running three things may happen to CR0:
> 
>  1. L2 writes to a bit that is not shadowed neither by L1 nor by L0. It
>     will go strait to GUEST_CR0.
>  2. L2 writes to a bit shadowed by L1. L1 #vmexit will be emulated. On the
>     next #vmetry vcpu->arch.cr0 will be set to whatever value L1 calculated.
>  3. L2 writes to a bit shadowed by L0, but not L1. This is the case we
>     are handling here. And if we will do it right vcpu->arch.cr0 will be
>     up-to-date at the end.
> 
> The only case when, while this code running, vcpu->arch.cr0 has not
> up-to-date value is if 1 happened, but since L2 guest overwriting cr0
> here anyway it does not matter what it previously set in GUEST_CR0. The
> correct bits are in a new cr0 value provided by val and accessible by
> (val & ~vmcs12->cr0_guest_host_mask).

I need to think about it again. Maybe vmcs12->guest_cr0 is best, but
that's a shot from the hips now.

Jan



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