[PATCH v16 06/51] KVM: x86: Check XSS validity against guest CPUIDs

Sean Christopherson
2025-09-19T22:33:13Z

From: Chao Gao <chao.gao@intel.com>

Maintain per-guest valid XSS bits and check XSS validity against them
rather than against KVM capabilities. This is to prevent bits that are
supported by KVM but not supported for a guest from being set.

Opportunistically return KVM_MSR_RET_UNSUPPORTED on IA32_XSS MSR accesses
if guest CPUID doesn't enumerate X86_FEATURE_XSAVES. Since
KVM_MSR_RET_UNSUPPORTED takes care of host_initiated cases, drop the
host_initiated check.

Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/include/asm/kvm_host.h | 3 ++-
arch/x86/kvm/cpuid.c | 12 ++++++++++++
arch/x86/kvm/x86.c | 7 +++----
3 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 8695967b7a31..7a7e6356a8dd 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -815,7 +815,6 @@ struct kvm_vcpu_arch {
bool at_instruction_boundary;
bool tpr_access_reporting;
bool xfd_no_write_intercept;
- u64 ia32_xss;
u64 microcode_version;
u64 arch_capabilities;
u64 perf_capabilities;
@@ -876,6 +875,8 @@ struct kvm_vcpu_arch {

u64 xcr0;
u64 guest_supported_xcr0;
+ u64 ia32_xss;
+ u64 guest_supported_xss;

struct kvm_pio_request pio;
void *pio_data;
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index efee08fad72e..6b8b5d8b13cc 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -263,6 +263,17 @@ static u64 cpuid_get_supported_xcr0(struct kvm_vcpu *vcpu)
return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
}

+static u64 cpuid_get_supported_xss(struct kvm_vcpu *vcpu)
+{
+ struct kvm_cpuid_entry2 *best;
+
+ best = kvm_find_cpuid_entry_index(vcpu, 0xd, 1);
+ if (!best)
+ return 0;
+
+ return (best->ecx | ((u64)best->edx << 32)) & kvm_caps.supported_xss;
+}
+
static __always_inline void kvm_update_feature_runtime(struct kvm_vcpu *vcpu,
struct kvm_cpuid_entry2 *entry,
unsigned int x86_feature,
@@ -424,6 +435,7 @@ void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
}

vcpu->arch.guest_supported_xcr0 = cpuid_get_supported_xcr0(vcpu);
+ vcpu->arch.guest_supported_xss = cpuid_get_supported_xss(vcpu);

vcpu->arch.pv_cpuid.features = kvm_apply_cpuid_pv_features_quirk(vcpu);

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d202d9532eb2..d4c192f4c06f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3984,15 +3984,14 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
}
break;
case MSR_IA32_XSS:
- if (!msr_info->host_initiated &&
- !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
- return 1;
+ if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
+ return KVM_MSR_RET_UNSUPPORTED;
/*
* KVM supports exposing PT to the guest, but does not support
* IA32_XSS[bit 8]. Guests have to use RDMSR/WRMSR rather than
* XSAVES/XRSTORS to save/restore PT MSRs.
*/
- if (data & ~kvm_caps.supported_xss)
+ if (data & ~vcpu->arch.guest_supported_xss)
return 1;
vcpu->arch.ia32_xss = data;
vcpu->arch.cpuid_dynamic_bits_dirty = true;
--
2.51.0.470.ga7dc726c21-goog