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