From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Date: Wed, 20 Jun 2018 15:29:53 +0000 (-0400)
Subject: x86/KVM: Warn user if KVM is loaded SMT and L1TF CPU bug being present
X-Git-Tag: v4.17.15~50
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=8efd53e73ae385947d22c6d0c1fc6dc718048a63;p=thirdparty%2Fkernel%2Fstable.git

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index cd771f6b2bd86..56b9b2de00ab3 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1937,6 +1937,12 @@
 			[KVM,ARM] Allow use of GICv4 for direct injection of
 			LPIs.
 
+	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 7a28959f19857..b0558c1f6ca8f 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -71,6 +71,9 @@ 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);
 
@@ -10142,10 +10145,20 @@ 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 (!ple_gap)
 		kvm->arch.pause_in_guest = true;
+
+	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;
 }
 
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 60f26e9247af8..3de60b5d8ac1a 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -345,6 +345,7 @@ 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)
 {