Hi,

On 01/31/2019 03:28 AM, Julien Thierry wrote:

Hi Jeremy,

On 25/01/2019 18:07, Jeremy Linton wrote:

quoted

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 <redacted>
---
  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;

Do we need to check the cpuid registers if the CPU is in the known safe
list?

I don't believe so. In the previous patch where this was broken out 
these checks were just or'ed together. In this path it just seemed a 
little cleaner than adding the additional check/or'ing the results 
here/whatever as we only want to set it safe (never the other way 
around). AKA, i'm running out of horizontal space, and I want to keep 
the 'defer to registers' comment.

Otherwise:

Reviewed-by: Julien Thierry <redacted>

quoted

+
+	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

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