Re: [PATCH 4.19 093/105] KVM: x86: Skip EFER vs. guest CPUID checks for host-initiated writes

[Pavel Machek]

Hi!

> From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> 
> commit 11988499e62b310f3bf6f6d0a807a06d3f9ccc96 upstream.
> 
> KVM allows userspace to violate consistency checks related to the
> guest's CPUID model to some degree.  Generally speaking, userspace has
> carte blanche when it comes to guest state so long as jamming invalid
> state won't negatively affect the host.
> 
> Currently this is seems to be a non-issue as most of the interesting
> EFER checks are missing, e.g. NX and LME, but those will be added
> shortly.  Proactively exempt userspace from the CPUID checks so as not
> to break userspace.
> 
> Note, the efer_reserved_bits check still applies to userspace writes as
> that mask reflects the host's capabilities, e.g. KVM shouldn't allow a
> guest to run with NX=1 if it has been disabled in the host.

>  arch/x86/kvm/x86.c |   37 ++++++++++++++++++++++++-------------
>  1 file changed, 24 insertions(+), 13 deletions(-)
> 
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
>  
> -static int set_efer(struct kvm_vcpu *vcpu, u64 efer)
> +static int set_efer(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  {
>  	u64 old_efer = vcpu->arch.efer;
> +	u64 efer = msr_info->data;
>  
> -	if (!kvm_valid_efer(vcpu, efer))
> -		return 1;
> +	if (efer & efer_reserved_bits)
> +		return false;
>  
> -	if (is_paging(vcpu)
> -	    && (vcpu->arch.efer & EFER_LME) != (efer & EFER_LME))
> -		return 1;
> +	if (!msr_info->host_initiated) {
> +		if (!__kvm_valid_efer(vcpu, efer))
> +			return 1;

We have "return false" in function returning int. Plus calling
convention here seems to be "nonzero on error" so it should be
returning 1?

> @@ -2356,7 +2367,7 @@ int kvm_set_msr_common(struct kvm_vcpu *
>  		vcpu->arch.arch_capabilities = data;
>  		break;
>  	case MSR_EFER:
> -		return set_efer(vcpu, data);
> +		return set_efer(vcpu, msr_info);
>  	case MSR_K7_HWCR:
>  		data &= ~(u64)0x40;	/* ignore flush filter disable */
>  		data &= ~(u64)0x100;	/* ignore ignne emulation enable */
> 

-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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