Re: [PATCH v6 04/21] kvm: x86: Exclude unpermitted xfeatures at KVM_GET_SUPPORTED_CPUID

[Like Xu]

On 8/1/2022 2:54 am, Paolo Bonzini wrote:
> From: Jing Liu <jing2.liu@xxxxxxxxx>
>
> KVM_GET_SUPPORTED_CPUID should not include any dynamic xstates in
> CPUID[0xD] if they have not been requested with prctl. Otherwise
> a process which directly passes KVM_GET_SUPPORTED_CPUID to
> KVM_SET_CPUID2 would now fail even if it doesn't intend to use a
> dynamically enabled feature. Userspace must know that prctl is
> required and allocate >4K xstate buffer before setting any dynamic
> bit.
>
> Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Signed-off-by: Jing Liu <jing2.liu@xxxxxxxxx>
> Signed-off-by: Yang Zhong <yang.zhong@xxxxxxxxx>
> Message-Id: <20220105123532.12586-5-yang.zhong@xxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>   Documentation/virt/kvm/api.rst | 4 ++++
>   arch/x86/kvm/cpuid.c           | 9 ++++++---
>   2 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 6b683dfea8f2..f4ea5e41a4d0 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -1687,6 +1687,10 @@ userspace capabilities, and with user requirements (for example, the
>   user may wish to constrain cpuid to emulate older hardware, or for
>   feature consistency across a cluster).
>   
> +Dynamically-enabled feature bits need to be requested with
> +``arch_prctl()`` before calling this ioctl. Feature bits that have not
> +been requested are excluded from the result.
> +
>   Note that certain capabilities, such as KVM_CAP_X86_DISABLE_EXITS, may
>   expose cpuid features (e.g. MONITOR) which are not supported by kvm in
>   its default configuration. If userspace enables such capabilities, it
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index f3e6fda6b858..eb52dde5deec 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -815,11 +815,13 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>   				goto out;
>   		}
>   		break;
> -	case 0xd:
> -		entry->eax &= supported_xcr0;
> +	case 0xd: {
> +		u64 guest_perm = xstate_get_guest_group_perm();
> +
> +		entry->eax &= supported_xcr0 & guest_perm;
>   		entry->ebx = xstate_required_size(supported_xcr0, false);

If we choose to exclude unpermitted xfeatures in the entry->eax, why do
we choose to expose the size of unpermitted xfeatures in ebx and ecx?

This seems to be an inconsistency, how about:

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 1bd4d560cbdd..193cbf56a5fa 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -888,12 +888,12 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array 
*array, u32 function)
 		}
 		break;
 	case 0xd: {
-		u64 guest_perm = xstate_get_guest_group_perm();
+		u64 supported_xcr0 = supported_xcr0 & xstate_get_guest_group_perm();

-		entry->eax &= supported_xcr0 & guest_perm;
+		entry->eax &= supported_xcr0;
 		entry->ebx = xstate_required_size(supported_xcr0, false);
 		entry->ecx = entry->ebx;
-		entry->edx &= (supported_xcr0 & guest_perm) >> 32;
+		entry->edx &= supported_xcr0 >> 32;
 		if (!supported_xcr0)
 			break;

It also helps to fix the CPUID_D_1_EBX and later for (i = 2; i < 64; ++i);

Is there anything I've missed ?

>   		entry->ecx = entry->ebx;
> -		entry->edx &= supported_xcr0 >> 32;
> +		entry->edx &= (supported_xcr0 & guest_perm) >> 32;
>   		if (!supported_xcr0)
>   			break;
>   
> @@ -866,6 +868,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>   			entry->edx = 0;
>   		}
>   		break;
> +	}
>   	case 0x12:
>   		/* Intel SGX */
>   		if (!kvm_cpu_cap_has(X86_FEATURE_SGX)) {