[PATCH 5.15 052/148] KVM: x86: Inject #UD on emulated XSETBV if XSAVES isnt enabled

Greg Kroah-Hartman
2022-09-26T11:59:22Z

From: Sean Christopherson <seanjc@google.com>

commit 50b2d49bafa16e6311ab2da82f5aafc5f9ada99b upstream.

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>
Message-Id: <20220824033057.3576315-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kvm/emulate.c | 3 +++
arch/x86/kvm/x86.c | 1 +
2 files changed, 4 insertions(+)

--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -4122,6 +4122,9 @@ static int em_xsetbv(struct x86_emulate_
{
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);
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1021,6 +1021,7 @@ static int __kvm_set_xcr(struct kvm_vcpu

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);