[v7 09/10] arm64: add sysfs vulnerability show for speculative store bypass

[Jeremy Linton]

Return status based on ssbd_state and the arm64 SSBS feature. If
the mitigation is disabled, or the firmware isn't responding then
return the expected machine state based on a whitelist of known
good cores.

Given a heterogeneous machine, the overall machine vulnerability
must be a tristate to assure any vulnerable cores transition to
vulnerable and stay there. Further, we delay transitioning to
vulnerable until we know the firmware isn't responding to avoid a
case where we miss the whitelist, but the firmware goes ahead and
reports the core is not vulnerable.

Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
---
 arch/arm64/kernel/cpu_errata.c | 62 ++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
index 6958dcdabf7d..a1f3188c7be0 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -278,6 +278,13 @@ static int detect_harden_bp_fw(void)
 DEFINE_PER_CPU_READ_MOSTLY(u64, arm64_ssbd_callback_required);
 
 int ssbd_state __read_mostly = ARM64_SSBD_KERNEL;
+static enum {SSB_UNSET, SSB_SAFE, SSB_UNSAFE} __ssb_safe = SSB_UNSET;
+
+static inline void ssb_safe(void)
+{
+	if (__ssb_safe == SSB_UNSET)
+		__ssb_safe = SSB_SAFE;
+}
 
 static const struct ssbd_options {
 	const char	*str;
@@ -383,16 +390,25 @@ static bool has_ssbd_mitigation(const struct arm64_cpu_capabilities *entry,
 	struct arm_smccc_res res;
 	bool required = true;
 	s32 val;
+	bool this_cpu_safe = false;
 
 	WARN_ON(scope != SCOPE_LOCAL_CPU || preemptible());
 
 	if (this_cpu_has_cap(ARM64_SSBS)) {
 		required = false;
+		ssb_safe();
 		goto out_printmsg;
 	}
 
+	if (is_midr_in_range_list(read_cpuid_id(), entry->midr_range_list)) {
+		ssb_safe();
+		this_cpu_safe = true;
+	}
+
 	if (psci_ops.smccc_version == SMCCC_VERSION_1_0) {
 		ssbd_state = ARM64_SSBD_UNKNOWN;
+		if (!this_cpu_safe)
+			__ssb_safe = SSB_UNSAFE;
 		return false;
 	}
 
@@ -409,6 +425,8 @@ static bool has_ssbd_mitigation(const struct arm64_cpu_capabilities *entry,
 
 	default:
 		ssbd_state = ARM64_SSBD_UNKNOWN;
+		if (!this_cpu_safe)
+			__ssb_safe = SSB_UNSAFE;
 		return false;
 	}
 
@@ -417,23 +435,31 @@ static bool has_ssbd_mitigation(const struct arm64_cpu_capabilities *entry,
 	switch (val) {
 	case SMCCC_RET_NOT_SUPPORTED:
 		ssbd_state = ARM64_SSBD_UNKNOWN;
+		if (!this_cpu_safe)
+			__ssb_safe = SSB_UNSAFE;
 		return false;
 
+	/* machines with mixed mitigation requirements must not return this */
 	case SMCCC_RET_NOT_REQUIRED:
 		pr_info_once("%s mitigation not required\n", entry->desc);
 		ssbd_state = ARM64_SSBD_MITIGATED;
+		ssb_safe();
 		return false;
 
 	case SMCCC_RET_SUCCESS:
+		__ssb_safe = SSB_UNSAFE;
 		required = true;
 		break;
 
 	case 1:	/* Mitigation not required on this CPU */
 		required = false;
+		ssb_safe();
 		break;
 
 	default:
 		WARN_ON(1);
+		if (!this_cpu_safe)
+			__ssb_safe = SSB_UNSAFE;
 		return false;
 	}
 
@@ -474,6 +500,14 @@ static bool has_ssbd_mitigation(const struct arm64_cpu_capabilities *entry,
 	return required;
 }
 
+/* known invulnerable cores */
+static const struct midr_range arm64_ssb_cpus[] = {
+	MIDR_ALL_VERSIONS(MIDR_CORTEX_A35),
+	MIDR_ALL_VERSIONS(MIDR_CORTEX_A53),
+	MIDR_ALL_VERSIONS(MIDR_CORTEX_A55),
+	{},
+};
+
 static void __maybe_unused
 cpu_enable_cache_maint_trap(const struct arm64_cpu_capabilities *__unused)
 {
@@ -769,6 +803,7 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
 		.capability = ARM64_SSBD,
 		.type = ARM64_CPUCAP_LOCAL_CPU_ERRATUM,
 		.matches = has_ssbd_mitigation,
+		.midr_range_list = arm64_ssb_cpus,
 	},
 #ifdef CONFIG_ARM64_ERRATUM_1188873
 	{
@@ -807,3 +842,30 @@ ssize_t cpu_show_spectre_v2(struct device *dev, struct device_attribute *attr,
 
 	return sprintf(buf, "Vulnerable\n");
 }
+
+ssize_t cpu_show_spec_store_bypass(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	/*
+	 *  Two assumptions: First, ssbd_state reflects the worse case
+	 *  for heterogeneous machines, and that if SSBS is supported its
+	 *  supported by all cores.
+	 */
+	switch (ssbd_state) {
+	case ARM64_SSBD_MITIGATED:
+		return sprintf(buf, "Not affected\n");
+
+	case ARM64_SSBD_KERNEL:
+	case ARM64_SSBD_FORCE_ENABLE:
+		if (cpus_have_cap(ARM64_SSBS))
+			return sprintf(buf, "Not affected\n");
+		if (IS_ENABLED(CONFIG_ARM64_SSBD))
+			return sprintf(buf,
+			    "Mitigation: Speculative Store Bypass disabled\n");
+	}
+
+	if (__ssb_safe == SSB_SAFE)
+		return sprintf(buf, "Not affected\n");
+
+	return sprintf(buf, "Vulnerable\n");
+}
-- 
2.20.1