[PATCH 2/6] arm64: add sysfs vulnerability show for meltdown

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

Add a simple state machine which will track whether
all the online cores in a machine are vulnerable.

Once that is done we have a fairly authoritative view
of the machine vulnerability, which allows us to make a
judgment about machine safety if it hasn't been mitigated.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
---
 arch/arm64/kernel/cpufeature.c | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 242898395f68..bea9adfef7fa 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -905,6 +905,8 @@ has_useable_cnp(const struct arm64_cpu_capabilities *entry, int scope)
 	return has_cpuid_feature(entry, scope);
 }
 
+static enum { A64_MELT_UNSET, A64_MELT_SAFE, A64_MELT_UNKN } __meltdown_safe = A64_MELT_UNSET;
+
 #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
 static int __kpti_forced; /* 0: not forced, >0: forced on, <0: forced off */
 
@@ -928,6 +930,15 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
 {
 	char const *str = "command line option";
 
+	bool meltdown_safe = is_cpu_meltdown_safe() ||
+		has_cpuid_feature(entry, scope);
+
+	/* Only safe if all booted cores are known safe */
+	if (meltdown_safe && __meltdown_safe == A64_MELT_UNSET)
+		__meltdown_safe = A64_MELT_SAFE;
+	else if (!meltdown_safe)
+		__meltdown_safe = A64_MELT_UNKN;
+
 	/*
 	 * For reasons that aren't entirely clear, enabling KPTI on Cavium
 	 * ThunderX leads to apparent I-cache corruption of kernel text, which
@@ -949,11 +960,7 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
 	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
 		return true;
 
-	if (is_cpu_meltdown_safe())
-		return false;
-
-	/* Defer to CPU feature registers */
-	return !has_cpuid_feature(entry, scope);
+	return !meltdown_safe;
 }
 
 static void
@@ -1920,3 +1927,17 @@ 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 == A64_MELT_SAFE)
+		return sprintf(buf, "Not affected\n");
+
+	return sprintf(buf, "Unknown\n");
+}
+#endif
-- 
2.17.2