linux-kernel - Re: [PATCH 09/10] KVM: x86: Suppress failures on userspace access to advertised, unsupported MSRs

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Message-ID: <4c0cd892-8b7c-451b-9c04-2e83f33bef0f@intel.com>
Date: Fri, 26 Apr 2024 20:36:21 +0800
From: "Yang, Weijiang" <weijiang.yang@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH 09/10] KVM: x86: Suppress failures on userspace access to
 advertised, unsupported MSRs

On 4/26/2024 2:14 AM, Sean Christopherson wrote:
> Extend KVM's suppression of failures due to a userspace access to an
> unsupported, but advertised as a "to save" MSR to all MSRs, not just those
> that happen to reach the default case statements in kvm_get_msr_common()
> and kvm_set_msr_common().  KVM's soon-to-be-established ABI is that if an
> MSR is advertised to userspace, then userspace is allowed to read the MSR,
> and write back the value that was read, i.e. why an MSR is unsupported
> doesn't change KVM's ABI.
>
> Practically speaking, this is very nearly a nop, as the only other paths
> that return KVM_MSR_RET_UNSUPPORTED are {svm,vmx}_get_feature_msr(), and
> it's unlikely, though not impossible, that userspace is using KVM_GET_MSRS
> on unsupported MSRs.
>
> The primary goal of moving the suppression to common code is to allow
> returning KVM_MSR_RET_UNSUPPORTED as appropriate throughout KVM, without
> having to manually handle the "is userspace accessing an advertised"
> waiver.  I.e. this will allow formalizing KVM's ABI without incurring a
> high maintenance cost.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/x86.c | 27 +++++++++------------------
>   1 file changed, 9 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 04a5ae853774..4c91189342ff 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -527,6 +527,15 @@ static __always_inline int kvm_do_msr_access(struct kvm_vcpu *vcpu, u32 msr,
>   	if (ret != KVM_MSR_RET_UNSUPPORTED)
>   		return ret;
>   
> +	/*
> +	 * Userspace is allowed to read MSRs, and write '0' to MSRs, that KVM
> +	 * reports as to-be-saved, even if an MSR isn't fully supported.
> +	 * Simply check that @data is '0', which covers both the write '0' case
> +	 * and all reads (in which case @data is zeroed on failure; see above).
> +	 */
> +	if (host_initiated && !*data && kvm_is_msr_to_save(msr))
> +		return 0;
> +

IMHO,  it's worth to document above phrase into virt/kvm/api.rst KVM_{GET, SET}_MSRS
sections as a note because when users space reads/writes MSRs successfully, it doesn't
necessarily mean the operation really took effect. Maybe it's  just due to the fact they're
exposed in "to-be-saved" list.

>   	if (!ignore_msrs) {
>   		kvm_debug_ratelimited("unhandled %s: 0x%x data 0x%llx\n",
>   				      op, msr, *data);
> @@ -4163,14 +4172,6 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   		if (kvm_pmu_is_valid_msr(vcpu, msr))
>   			return kvm_pmu_set_msr(vcpu, msr_info);
>   
> -		/*
> -		 * Userspace is allowed to write '0' to MSRs that KVM reports
> -		 * as to-be-saved, even if an MSRs isn't fully supported.
> -		 */
> -		if (msr_info->host_initiated && !data &&
> -		    kvm_is_msr_to_save(msr))
> -			break;
> -
>   		return KVM_MSR_RET_UNSUPPORTED;
>   	}
>   	return 0;
> @@ -4522,16 +4523,6 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   		if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
>   			return kvm_pmu_get_msr(vcpu, msr_info);
>   
> -		/*
> -		 * Userspace is allowed to read MSRs that KVM reports as
> -		 * to-be-saved, even if an MSR isn't fully supported.
> -		 */
> -		if (msr_info->host_initiated &&
> -		    kvm_is_msr_to_save(msr_info->index)) {
> -			msr_info->data = 0;
> -			break;
> -		}
> -
>   		return KVM_MSR_RET_UNSUPPORTED;
>   	}
>   	return 0;