KVM: x86: Harden KVM against imbalanced load/put of guest FPU state

Assert, via KVM_BUG_ON(), that guest FPU state isn't/is in use when
loading/putting the FPU to help detect KVM bugs without needing an assist
from KASAN.  If an imbalanced load/put is detected, skip the redundant
load/put to avoid clobbering guest state and/or crashing the host.

Note, kvm_access_xstate_msr() already provides a similar assertion.

Reviewed-by: Yao Yuan <yaoyuan@linux.alibaba.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Link: https://patch.msgid.link/20251030185802.3375059-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/x86.c

@@ -11807,6 +11807,9 @@ static int complete_emulated_mmio(struct kvm_vcpu *vcpu)
 /* Swap (qemu) user FPU context for the guest FPU context. */
 static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
 {
+	if (KVM_BUG_ON(vcpu->arch.guest_fpu.fpstate->in_use, vcpu->kvm))
+		return;
+
 	/* Exclude PKRU, it's restored separately immediately after VM-Exit. */
 	fpu_swap_kvm_fpstate(&vcpu->arch.guest_fpu, true);
 	trace_kvm_fpu(1);
@@ -11815,6 +11818,9 @@ static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
 /* When vcpu_run ends, restore user space FPU context. */
 static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
 {
+	if (KVM_BUG_ON(!vcpu->arch.guest_fpu.fpstate->in_use, vcpu->kvm))
+		return;
+
 	fpu_swap_kvm_fpstate(&vcpu->arch.guest_fpu, false);
 	++vcpu->stat.fpu_reload;
 	trace_kvm_fpu(0);