[PATCH 3/3] KVM: x86: Inject #UD on emulated XSETBV if XSAVES isn't enabled

[Sean Christopherson <seanjc@google.com>]

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: 02d4160fbd76 ("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>
---
 arch/x86/kvm/emulate.c | 3 +++
 arch/x86/kvm/x86.c     | 1 +
 2 files changed, 4 insertions(+)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index f092c54d1a2f..8ce5ae61fc41 100644
--- a/arch/x86/kvm/emulate.c
+++ b/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);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 97ab53046052..356d0475ab6d 100644
--- a/arch/x86/kvm/x86.c
+++ b/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);
-- 
2.37.1.595.g718a3a8f04-goog