[v2,04/23] kvm: x86: Exclude unpermitted xfeatures at KVM_GET_SUPPORTED_CPUID

Commit Message

Jing Liu Dec. 17, 2021, 3:29 p.m. UTC

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 <[e-mail]>
Signed-off-by: Jing Liu <[e-mail]>
---
 Documentation/virt/kvm/api.rst | 4 ++++
 arch/x86/kvm/cpuid.c           | 9 ++++++---
 2 files changed, 10 insertions(+), 3 deletions(-)

Comments

Paolo Bonzini Dec. 20, 2021, 1:20 p.m. UTC | #1

On 12/17/21 16:29, Jing Liu wrote:
>   
> +Permissions must be set via prctl() for dynamically-enabled XSAVE
> +features before calling this ioctl. Otherwise those feature bits are
> +excluded.
> +

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.

Thanks,

Paolo

Patch

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index aeeb071c7688..eb5671ca2dba 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -1684,6 +1684,10 @@  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).
 
+Permissions must be set via prctl() for dynamically-enabled XSAVE
+features before calling this ioctl. Otherwise those feature bits are
+excluded.
+
 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 148003e26cbb..4855344091b8 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -812,11 +812,13 @@  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;
 
@@ -863,6 +865,7 @@  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)) {