x86/kvm: Serialize L1D flush parameter setter

Writes to the parameter files are not serialized at the sysfs core
level, so local serialization is required.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Jiri Kosina <jkosina@suse.cz>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20180713142322.873642605@linutronix.de

CVE-2018-3620
CVE-2018-3646
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

arch/x86/kvm/vmx.c

@@ -178,6 +178,7 @@ extern const ulong vmx_return;
 
 static DEFINE_STATIC_KEY_FALSE(vmx_l1d_should_flush);
 static DEFINE_STATIC_KEY_FALSE(vmx_l1d_flush_always);
+static DEFINE_MUTEX(vmx_l1d_flush_mutex);
 
 /* Storage for pre module init parameter parsing */
 static enum vmx_l1d_flush_state __read_mostly vmentry_l1d_flush_param = VMENTER_L1D_FLUSH_AUTO;
@@ -242,7 +243,7 @@ static int vmentry_l1d_flush_parse(const char *s)
 
 static int vmentry_l1d_flush_set(const char *s, const struct kernel_param *kp)
 {
-	int l1tf;
+	int l1tf, ret;
 
 	if (!boot_cpu_has(X86_BUG_L1TF))
 		return 0;
@@ -262,7 +263,10 @@ static int vmentry_l1d_flush_set(const char *s, const struct kernel_param *kp)
 		return 0;
 	}
 
-	return vmx_setup_l1d_flush(l1tf);
+	mutex_lock(&vmx_l1d_flush_mutex);
+	ret = vmx_setup_l1d_flush(l1tf);
+	mutex_unlock(&vmx_l1d_flush_mutex);
+	return ret;
 }
 
 static int vmentry_l1d_flush_get(char *s, const struct kernel_param *kp)