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

[Gleb Natapov <gleb@redhat.com>]

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?

> -		if (kvm_set_cr0(vcpu, (val & vcpu->arch.cr0_guest_owned_bits) |
> -			 (vcpu->arch.cr0 & ~vcpu->arch.cr0_guest_owned_bits)))
> +		struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> +		unsigned long orig_val = val;
> +
> +		val = (val & ~vmcs12->cr0_guest_host_mask) |
> +			(vmcs_read64(GUEST_CR0) & vmcs12->cr0_guest_host_mask);
> +		if ((val & VMXON_CR0_ALWAYSON) != VMXON_CR0_ALWAYSON)
> +			return 1;
> +
> +		if (kvm_set_cr0(vcpu, val))
>  			return 1;
> -		vmcs_writel(CR0_READ_SHADOW, val);
> +		vmcs_writel(CR0_READ_SHADOW, orig_val);
>  		return 0;
> -	} else
> +	} else {
> +		if (to_vmx(vcpu)->nested.vmxon &&
> +		    ((val & VMXON_CR0_ALWAYSON) != VMXON_CR0_ALWAYSON))
> +			return 1;
>  		return kvm_set_cr0(vcpu, val);
> +	}
>  }
>  
>  static int handle_set_cr4(struct kvm_vcpu *vcpu, unsigned long val)
>  {
>  	if (is_guest_mode(vcpu)) {
> -		if (kvm_set_cr4(vcpu, (val & vcpu->arch.cr4_guest_owned_bits) |
> -			 (vcpu->arch.cr4 & ~vcpu->arch.cr4_guest_owned_bits)))
> +		struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> +		unsigned long orig_val = val;
> +
> +		val = (val & ~vmcs12->cr4_guest_host_mask) |
> +			(vmcs_readl(GUEST_CR4) & vmcs12->cr4_guest_host_mask);
> +		if ((val & VMXON_CR4_ALWAYSON) != VMXON_CR4_ALWAYSON)
> +			return 1;
> +
> +		if (kvm_set_cr4(vcpu, val))
>  			return 1;
> -		vmcs_writel(CR4_READ_SHADOW, val);
> +		vmcs_writel(CR4_READ_SHADOW, orig_val);
>  		return 0;
> -	} else
> +	} else {
> +		if (to_vmx(vcpu)->nested.vmxon &&
> +		    ((val & VMXON_CR4_ALWAYSON) != VMXON_CR4_ALWAYSON))
> +			return 1;
>  		return kvm_set_cr4(vcpu, val);
> +	}
>  }
>  
>  /* called to set cr0 as approriate for clts instruction exit. */
> -- 
> 1.7.3.4

--
			Gleb.