Commit 445ecdf7 authored by Jing Liu's avatar Jing Liu Committed by Paolo Bonzini
Browse files

kvm: x86: Exclude unpermitted xfeatures at KVM_GET_SUPPORTED_CPUID 



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: default avatarPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: default avatarJing Liu <jing2.liu@intel.com>
Signed-off-by: default avatarYang Zhong <yang.zhong@intel.com>
Message-Id: <20220105123532.12586-5-yang.zhong@intel.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent cc04b6a2

Documentation/virt/kvm/api.rst

+4 −0
Original line number Diff line number Diff line
@@ -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

arch/x86/kvm/cpuid.c

+6 −3
Original line number Diff line number Diff line
@@ -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);

		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)) {