KVM: x86: Inject #UD on emulated XSETBV if XSAVES isn't enabled

Inject #UD when emulating XSETBV if CR4.OSXSAVE is not set.  This also
covers the "XSAVE not supported" check, as setting CR4.OSXSAVE=1 #GPs if
XSAVE is not supported (and userspace gets to keep the pieces if it
forces incoherent vCPU state).

Add a comment to kvm_emulate_xsetbv() to call out that the CPU checks
CR4.OSXSAVE before checking for intercepts.  AMD'S APM implies that #UD
has priority (says that intercepts are checked before #GP exceptions),
while Intel's SDM says nothing about interception priority.  However,
testing on hardware shows that both AMD and Intel CPUs prioritize the #UD
over interception.

Fixes: 02d4160f ("x86: KVM: add xsetbv to the emulator")
Cc: stable@vger.kernel.org
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220824033057.3576315-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -4132,6 +4132,9 @@ static int em_xsetbv(struct x86_emulate_ctxt *ctxt)
 {
 	u32 eax, ecx, edx;
 
+	if (!(ctxt->ops->get_cr(ctxt, 4) & X86_CR4_OSXSAVE))
+		return emulate_ud(ctxt);
+
 	eax = reg_read(ctxt, VCPU_REGS_RAX);
 	edx = reg_read(ctxt, VCPU_REGS_RDX);
 	ecx = reg_read(ctxt, VCPU_REGS_RCX);

arch/x86/kvm/x86.c

@@ -1065,6 +1065,7 @@ static int __kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr)
 
 int kvm_emulate_xsetbv(struct kvm_vcpu *vcpu)
 {
+	/* Note, #UD due to CR4.OSXSAVE=0 has priority over the intercept. */
 	if (static_call(kvm_x86_get_cpl)(vcpu) != 0 ||
 	    __kvm_set_xcr(vcpu, kvm_rcx_read(vcpu), kvm_read_edx_eax(vcpu))) {
 		kvm_inject_gp(vcpu, 0);