[PATCH v4 07/12] arm64: add sysfs vulnerability show for meltdown

[Jeremy Linton <jeremy.linton@arm.com>]

Display the mitigation status if active, otherwise
assume the cpu is safe unless it doesn't have CSV3
and isn't in our whitelist.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
---
 arch/arm64/kernel/cpufeature.c | 33 +++++++++++++++++++++++++++------
 1 file changed, 27 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index a9e18b9cdc1e..624dfe0b5cdd 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -944,6 +944,8 @@ has_useable_cnp(const struct arm64_cpu_capabilities *entry, int scope)
 	return has_cpuid_feature(entry, scope);
 }
 
+/* default value is invalid until unmap_kernel_at_el0() runs */
+static bool __meltdown_safe = true;
 static int __kpti_forced; /* 0: not forced, >0: forced on, <0: forced off */
 
 static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
@@ -962,6 +964,16 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
 		{ /* sentinel */ }
 	};
 	char const *str = "command line option";
+	bool meltdown_safe;
+
+	meltdown_safe = is_midr_in_range_list(read_cpuid_id(), kpti_safe_list);
+
+	/* Defer to CPU feature registers */
+	if (has_cpuid_feature(entry, scope))
+		meltdown_safe = true;
+
+	if (!meltdown_safe)
+		__meltdown_safe = false;
 
 	/*
 	 * For reasons that aren't entirely clear, enabling KPTI on Cavium
@@ -984,12 +996,7 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
 	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
 		return kaslr_offset() > 0;
 
-	/* Don't force KPTI for CPUs that are not vulnerable */
-	if (is_midr_in_range_list(read_cpuid_id(), kpti_safe_list))
-		return false;
-
-	/* Defer to CPU feature registers */
-	return !has_cpuid_feature(entry, scope);
+	return !meltdown_safe;
 }
 
 static void
@@ -2055,3 +2062,17 @@ static int __init enable_mrs_emulation(void)
 }
 
 core_initcall(enable_mrs_emulation);
+
+#ifdef CONFIG_GENERIC_CPU_VULNERABILITIES
+ssize_t cpu_show_meltdown(struct device *dev, struct device_attribute *attr,
+		char *buf)
+{
+	if (arm64_kernel_unmapped_at_el0())
+		return sprintf(buf, "Mitigation: KPTI\n");
+
+	if (__meltdown_safe)
+		return sprintf(buf, "Not affected\n");
+
+	return sprintf(buf, "Vulnerable\n");
+}
+#endif
-- 
2.17.2