x86/kvm/vmx: Add MDS protection when L1D Flush is not active

CPUs which are affected by L1TF and MDS mitigate MDS with the L1D Flush on
VMENTER when updated microcode is installed.

If a CPU is not affected by L1TF or if the L1D Flush is not in use, then
MDS mitigation needs to be invoked explicit.

For these cases, follow the host mitigation state and invoke the MDS
mitigation before VMENTER.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

CVE-2018-12126
CVE-2018-12127
CVE-2018-12130

(backported from commit e3f1fbbd5ffb7ca71bf13d447983c03cf4e27030)
[juergh: Adjusted path arch/x86/kvm/vmx/vmx.c -> arch/x86/kvm/vmx.c.]
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

arch/x86/kernel/cpu/bugs.c

@@ -76,6 +76,7 @@ u64 __ro_after_init x86_amd_ls_cfg_ssbd_mask;
 
 /* Control MDS CPU buffer clear before returning to user space */
 DEFINE_STATIC_KEY_FALSE(mds_user_clear);
+EXPORT_SYMBOL_GPL(mds_user_clear);
 
 void __init check_bugs(void)
 {

arch/x86/kvm/vmx.c

@@ -8968,8 +8968,11 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
 	 */
 	x86_spec_ctrl_set_guest(vmx->spec_ctrl, 0);
 
+	/* L1D Flush includes CPU buffer clear to mitigate MDS */
 	if (static_branch_unlikely(&vmx_l1d_should_flush))
 		vmx_l1d_flush(vcpu);
+	else if (static_branch_unlikely(&mds_user_clear))
+		mds_clear_cpu_buffers();
 
 	vmx->__launched = vmx->loaded_vmcs->launched;
 	asm(