x86/KVM: Warn user if KVM is loaded SMT and L1TF CPU bug being present

commit 26acfb666a473d960f0fd971fe68f3e3ad16c70b upstream

If the L1TF CPU bug is present we allow the KVM module to be loaded as the
major of users that use Linux and KVM have trusted guests and do not want a
broken setup.

Cloud vendors are the ones that are uncomfortable with CVE 2018-3620 and as
such they are the ones that should set nosmt to one.

Setting 'nosmt' means that the system administrator also needs to disable
SMT (Hyper-threading) in the BIOS, or via the 'nosmt' command line
parameter, or via the /sys/devices/system/cpu/smt/control. See commit
05736e4ac13c ("cpu/hotplug: Provide knobs to control SMT").

Other mitigations are to use task affinity, cpu sets, interrupt binding,
etc - anything to make sure that _only_ the same guests vCPUs are running
on sibling threads.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index e34e75cbd5574f9d0869654f641578017010497d..be17577f57d3206dccd8dff894c623762eb02c62 100644 (file)
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1989,6 +1989,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                        for all guests.
                        Default is 1 (enabled) if in 64-bit or 32-bit PAE mode.
 
+       kvm-intel.nosmt=[KVM,Intel] If the L1TF CPU bug is present (CVE-2018-3620)
+                       and the system has SMT (aka Hyper-Threading) enabled then
+                       don't allow guests to be created.
+
+                       Default is 0 (allow guests to be created).
+
        kvm-intel.ept=  [KVM,Intel] Disable extended page tables
                        (virtualized MMU) support on capable Intel chips.
                        Default is 1 (enabled)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 30b74b491909f51b110c18e55554f0455bf98c50..f43b6484ca2e20ba7f6dbb2c7dcdfc2065820a0e 100644 (file)
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -69,6 +69,9 @@ static const struct x86_cpu_id vmx_cpu_id[] = {
 };
 MODULE_DEVICE_TABLE(x86cpu, vmx_cpu_id);
 
+static bool __read_mostly nosmt;
+module_param(nosmt, bool, S_IRUGO);
+
 static bool __read_mostly enable_vpid = 1;
 module_param_named(vpid, enable_vpid, bool, 0444);
 
@@ -9298,6 +9301,20 @@ free_vcpu:
        return ERR_PTR(err);
 }
 
+#define L1TF_MSG "SMT enabled with L1TF CPU bug present. Refer to CVE-2018-3620 for details.\n"
+
+static int vmx_vm_init(struct kvm *kvm)
+{
+       if (boot_cpu_has(X86_BUG_L1TF) && cpu_smt_control == CPU_SMT_ENABLED) {
+               if (nosmt) {
+                       pr_err(L1TF_MSG);
+                       return -EOPNOTSUPP;
+               }
+               pr_warn(L1TF_MSG);
+       }
+       return 0;
+}
+
 static void __init vmx_check_processor_compat(void *rtn)
 {
        struct vmcs_config vmcs_conf;
@@ -11367,6 +11384,8 @@ static struct kvm_x86_ops vmx_x86_ops __ro_after_init = {
        .cpu_has_accelerated_tpr = report_flexpriority,
        .has_emulated_msr = vmx_has_emulated_msr,
 
+       .vm_init = vmx_vm_init,
+
        .vcpu_create = vmx_create_vcpu,
        .vcpu_free = vmx_free_vcpu,
        .vcpu_reset = vmx_vcpu_reset,

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 49acbd2fa81af9fb14d07a1962c54777a9f6bb9b..8be14464b43ca3bb7c975dfe82140f45bb86bbca 100644 (file)
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -358,6 +358,7 @@ EXPORT_SYMBOL_GPL(cpu_hotplug_enable);
 
 #ifdef CONFIG_HOTPLUG_SMT
 enum cpuhp_smt_control cpu_smt_control __read_mostly = CPU_SMT_ENABLED;
+EXPORT_SYMBOL_GPL(cpu_smt_control);
 
 static int __init smt_cmdline_disable(char *str)
 {