[PATCH v16 09/51] KVM: x86: Load guest FPU state when access XSAVE-managed MSRs

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

Load the guest's FPU state if userspace is accessing MSRs whose values
are managed by XSAVES. Introduce two helpers, kvm_{get,set}_xstate_msr(),
to facilitate access to such kind of MSRs.

If MSRs supported in kvm_caps.supported_xss are passed through to guest,
the guest MSRs are swapped with host's before vCPU exits to userspace and
after it reenters kernel before next VM-entry.

Because the modified code is also used for the KVM_GET_MSRS device ioctl(),
explicitly check @vcpu is non-null before attempting to load guest state.
The XSAVE-managed MSRs cannot be retrieved via the device ioctl() without
loading guest FPU state (which doesn't exist).

Note that guest_cpuid_has() is not queried as host userspace is allowed to
access MSRs that have not been exposed to the guest, e.g. it might do
KVM_SET_MSRS prior to KVM_SET_CPUID2.

The two helpers are put here in order to manifest accessing xsave-managed
MSRs requires special check and handling to guarantee the correctness of
read/write to the MSRs.

Co-developed-by: Yang Weijiang <weijiang.yang@intel.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
[sean: drop S_CET, add big comment, move accessors to x86.c]
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Xin Li (Intel) <xin@zytor.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/x86.c | 87 +++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 86 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 3e66d8c5000a..ae402463f991 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -136,6 +136,9 @@ static int __set_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
static void __get_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);

static DEFINE_MUTEX(vendor_module_lock);
+static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu);
+static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu);
+
struct kvm_x86_ops kvm_x86_ops __read_mostly;

#define KVM_X86_OP(func) \
@@ -3801,6 +3804,67 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
mark_page_dirty_in_slot(vcpu->kvm, ghc->memslot, gpa_to_gfn(ghc->gpa));
}

+/*
+ * Returns true if the MSR in question is managed via XSTATE, i.e. is context
+ * switched with the rest of guest FPU state. Note! S_CET is _not_ context
+ * switched via XSTATE even though it _is_ saved/restored via XSAVES/XRSTORS.
+ * Because S_CET is loaded on VM-Enter and VM-Exit via dedicated VMCS fields,
+ * the value saved/restored via XSTATE is always the host's value. That detail
+ * is _extremely_ important, as the guest's S_CET must _never_ be resident in
+ * hardware while executing in the host. Loading guest values for U_CET and
+ * PL[0-3]_SSP while executing in the kernel is safe, as U_CET is specific to
+ * userspace, and PL[0-3]_SSP are only consumed when transitioning to lower
+ * privilegel levels, i.e. are effectively only consumed by userspace as well.
+ */
+static bool is_xstate_managed_msr(struct kvm_vcpu *vcpu, u32 msr)
+{
+ if (!vcpu)
+ return false;
+
+ switch (msr) {
+ case MSR_IA32_U_CET:
+ return guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) ||
+ guest_cpu_cap_has(vcpu, X86_FEATURE_IBT);
+ case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
+ return guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK);
+ default:
+ return false;
+ }
+}
+
+/*
+ * Lock (and if necessary, re-load) the guest FPU, i.e. XSTATE, and access an
+ * MSR that is managed via XSTATE. Note, the caller is responsible for doing
+ * the initial FPU load, this helper only ensures that guest state is resident
+ * in hardware (the kernel can load its FPU state in IRQ context).
+ */
+static __always_inline void kvm_access_xstate_msr(struct kvm_vcpu *vcpu,
+ struct msr_data *msr_info,
+ int access)
+{
+ BUILD_BUG_ON(access != MSR_TYPE_R && access != MSR_TYPE_W);
+
+ KVM_BUG_ON(!is_xstate_managed_msr(vcpu, msr_info->index), vcpu->kvm);
+ KVM_BUG_ON(!vcpu->arch.guest_fpu.fpstate->in_use, vcpu->kvm);
+
+ kvm_fpu_get();
+ if (access == MSR_TYPE_R)
+ rdmsrq(msr_info->index, msr_info->data);
+ else
+ wrmsrq(msr_info->index, msr_info->data);
+ kvm_fpu_put();
+}
+
+static __maybe_unused void kvm_set_xstate_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
+{
+ kvm_access_xstate_msr(vcpu, msr_info, MSR_TYPE_W);
+}
+
+static __maybe_unused void kvm_get_xstate_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
+{
+ kvm_access_xstate_msr(vcpu, msr_info, MSR_TYPE_R);
+}
+
int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
{
u32 msr = msr_info->index;
@@ -4551,11 +4615,25 @@ static int __msr_io(struct kvm_vcpu *vcpu, struct kvm_msrs *msrs,
int (*do_msr)(struct kvm_vcpu *vcpu,
unsigned index, u64 *data))
{
+ bool fpu_loaded = false;
int i;

- for (i = 0; i < msrs->nmsrs; ++i)
+ for (i = 0; i < msrs->nmsrs; ++i) {
+ /*
+ * If userspace is accessing one or more XSTATE-managed MSRs,
+ * temporarily load the guest's FPU state so that the guest's
+ * MSR value(s) is resident in hardware and thus can be accessed
+ * via RDMSR/WRMSR.
+ */
+ if (!fpu_loaded && is_xstate_managed_msr(vcpu, entries[i].index)) {
+ kvm_load_guest_fpu(vcpu);
+ fpu_loaded = true;
+ }
if (do_msr(vcpu, entries[i].index, &entries[i].data))
break;
+ }
+ if (fpu_loaded)
+ kvm_put_guest_fpu(vcpu);

return i;
}
@@ -5965,6 +6043,7 @@ static int kvm_get_set_one_reg(struct kvm_vcpu *vcpu, unsigned int ioctl,
struct kvm_one_reg one_reg;
struct kvm_x86_reg_id *reg;
u64 __user *user_val;
+ bool load_fpu;
int r;

if (copy_from_user(&one_reg, argp, sizeof(one_reg)))
@@ -5991,12 +6070,18 @@ static int kvm_get_set_one_reg(struct kvm_vcpu *vcpu, unsigned int ioctl,

guard(srcu)(&vcpu->kvm->srcu);

+ load_fpu = is_xstate_managed_msr(vcpu, reg->index);
+ if (load_fpu)
+ kvm_load_guest_fpu(vcpu);
+
user_val = u64_to_user_ptr(one_reg.addr);
if (ioctl == KVM_GET_ONE_REG)
r = kvm_get_one_msr(vcpu, reg->index, user_val);
else
r = kvm_set_one_msr(vcpu, reg->index, user_val);

+ if (load_fpu)
+ kvm_put_guest_fpu(vcpu);
return r;
}

--
2.51.0.470.ga7dc726c21-goog