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

Like Xu
2022-01-23T06:22:41Z

On 8/1/2022 2:54 am, Paolo Bonzini wrote:
> From: Jing Liu <jing2.liu@intel.com>
>
> 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@redhat.com>
> Signed-off-by: Jing Liu <jing2.liu@intel.com>
> Signed-off-by: Yang Zhong <yang.zhong@intel.com>
> Message-Id: <20220105123532.12586-5-yang.zhong@intel.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> 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)) {