Define 5 new attack vectors that are used for controlling CPU
speculation mitigations and associated command line options.  Each
attack vector may be enabled or disabled, which affects the CPU
mitigations enabled.

The default settings for these attack vectors are consistent with
existing kernel defaults, other than the automatic disabling of VM-based
attack vectors if KVM support is not present.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
---
 include/linux/cpu.h | 11 +++++++++
 kernel/cpu.c        | 58 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+)

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index bdcec1732445..b25566e1fb04 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -189,6 +189,17 @@ void cpuhp_report_idle_dead(void);
 static inline void cpuhp_report_idle_dead(void) { }
 #endif /* #ifdef CONFIG_HOTPLUG_CPU */
 
+enum cpu_attack_vectors {
+	CPU_MITIGATE_USER_KERNEL,
+	CPU_MITIGATE_USER_USER,
+	CPU_MITIGATE_GUEST_HOST,
+	CPU_MITIGATE_GUEST_GUEST,
+	CPU_MITIGATE_CROSS_THREAD,
+	NR_CPU_ATTACK_VECTORS,
+};
+
+bool cpu_mitigate_attack_vector(enum cpu_attack_vectors v);
+
 #ifdef CONFIG_CPU_MITIGATIONS
 extern bool cpu_mitigations_off(void);
 extern bool cpu_mitigations_auto_nosmt(void);
diff --git a/kernel/cpu.c b/kernel/cpu.c
index d293d52a3e00..980653a55d9c 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -3201,6 +3201,22 @@ enum cpu_mitigations {
 
 static enum cpu_mitigations cpu_mitigations __ro_after_init = CPU_MITIGATIONS_AUTO;
 
+/*
+ * All except the cross-thread attack vector are mitigated by default.
+ * Cross-thread mitigation often requires disabling SMT which is too expensive
+ * to be enabled by default.
+ *
+ * Guest-to-Host and Guest-to-Guest vectors are only needed if KVM support is
+ * present.
+ */
+static bool cpu_mitigate_attack_vectors[NR_CPU_ATTACK_VECTORS] __ro_after_init = {
+	[CPU_MITIGATE_USER_KERNEL] = true,
+	[CPU_MITIGATE_USER_USER] = true,
+	[CPU_MITIGATE_GUEST_HOST] = IS_ENABLED(CONFIG_KVM),
+	[CPU_MITIGATE_GUEST_GUEST] = IS_ENABLED(CONFIG_KVM),
+	[CPU_MITIGATE_CROSS_THREAD] = false
+};
+
 static int __init mitigations_parse_cmdline(char *arg)
 {
 	if (!strcmp(arg, "off"))
@@ -3229,11 +3245,53 @@ bool cpu_mitigations_auto_nosmt(void)
 	return cpu_mitigations == CPU_MITIGATIONS_AUTO_NOSMT;
 }
 EXPORT_SYMBOL_GPL(cpu_mitigations_auto_nosmt);
+
+#define DEFINE_ATTACK_VECTOR(opt, v) \
+static int __init v##_parse_cmdline(char *arg) \
+{ \
+	if (!strcmp(arg, "off")) \
+		cpu_mitigate_attack_vectors[v] = false; \
+	else if (!strcmp(arg, "on")) \
+		cpu_mitigate_attack_vectors[v] = true; \
+	else \
+		pr_warn("Unsupported " opt "=%s\n", arg); \
+	return 0; \
+} \
+early_param(opt, v##_parse_cmdline)
+
+bool cpu_mitigate_attack_vector(enum cpu_attack_vectors v)
+{
+	BUG_ON(v >= NR_CPU_ATTACK_VECTORS);
+	return cpu_mitigate_attack_vectors[v];
+}
+EXPORT_SYMBOL_GPL(cpu_mitigate_attack_vector);
+
 #else
 static int __init mitigations_parse_cmdline(char *arg)
 {
 	pr_crit("Kernel compiled without mitigations, ignoring 'mitigations'; system may still be vulnerable\n");
 	return 0;
 }
+
+#define DEFINE_ATTACK_VECTOR(opt, v) \
+static int __init v##_parse_cmdline(char *arg) \
+{ \
+	pr_crit("Kernel compiled without mitigations, ignoring %s; system may still be vulnerable\n", opt); \
+	return 0; \
+} \
+early_param(opt, v##_parse_cmdline)
+
+bool cpu_mitigate_attack_vector(enum cpu_attack_vectors v)
+{
+	return false;
+}
+EXPORT_SYMBOL_GPL(cpu_mitigate_attack_vector);
+
 #endif
 early_param("mitigations", mitigations_parse_cmdline);
+
+DEFINE_ATTACK_VECTOR("mitigate_user_kernel", CPU_MITIGATE_USER_KERNEL);
+DEFINE_ATTACK_VECTOR("mitigate_user_user", CPU_MITIGATE_USER_USER);
+DEFINE_ATTACK_VECTOR("mitigate_guest_host", CPU_MITIGATE_GUEST_HOST);
+DEFINE_ATTACK_VECTOR("mitigate_guest_guest", CPU_MITIGATE_GUEST_GUEST);
+DEFINE_ATTACK_VECTOR("mitigate_cross_thread", CPU_MITIGATE_CROSS_THREAD);
-- 
2.34.1