KPTI is treated as a system wide feature and is only detected if all
the CPUs in the sysetm needs the defense, unless it is forced via kernel
command line. This leaves a system with a mix of CPUs with and without
the defense vulnerable. Also, if a late CPU needs KPTI but KPTI was not
activated at boot time, the CPU is currently allowed to boot, which is a
potential security vulnerability.
This patch ensures that the KPTI is turned on if at least one CPU detects
the capability (i.e, change scope to SCOPE_LOCAL_CPU). Also rejetcs a late
CPU, if it requires the defense, when the system hasn't enabled it,

Cc: Will Deacon <will.dea...@arm.com>
Reviewed-by: Dave Martin <dave.mar...@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poul...@arm.com>
---
 arch/arm64/include/asm/cpufeature.h |  9 +++++++++
 arch/arm64/kernel/cpufeature.c      | 16 +++++++++++-----
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h 
b/arch/arm64/include/asm/cpufeature.h
index effb1c038221..4f050259dff3 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -248,6 +248,15 @@ extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
         ARM64_CPUCAP_OPTIONAL_FOR_LATE_CPU     |       \
         ARM64_CPUCAP_PERMITTED_FOR_LATE_CPU)
 
+/*
+ * CPU feature detected at boot time, on one or more CPUs. A late CPU
+ * is not allowed to have the capability when the system doesn't have it.
+ * It is Ok for a late CPU to miss the feature.
+ */
+#define ARM64_CPUCAP_BOOT_RESTRICTED_CPU_LOCAL_FEATURE \
+       (ARM64_CPUCAP_SCOPE_LOCAL_CPU           |       \
+        ARM64_CPUCAP_OPTIONAL_FOR_LATE_CPU)
+
 struct arm64_cpu_capabilities {
        const char *desc;
        u16 capability;
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 0955e02fa638..0bbab4bd7644 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -877,10 +877,9 @@ static bool has_cache_dic(const struct 
arm64_cpu_capabilities *entry,
 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,
-                               int __unused)
+                               int scope)
 {
        char const *str = "command line option";
-       u64 pfr0 = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
 
        /*
         * For reasons that aren't entirely clear, enabling KPTI on Cavium
@@ -911,8 +910,7 @@ static bool unmap_kernel_at_el0(const struct 
arm64_cpu_capabilities *entry,
        }
 
        /* Defer to CPU feature registers */
-       return !cpuid_feature_extract_unsigned_field(pfr0,
-                                                    ID_AA64PFR0_CSV3_SHIFT);
+       return !has_cpuid_feature(entry, scope);
 }
 
 static void
@@ -1059,7 +1057,15 @@ static const struct arm64_cpu_capabilities 
arm64_features[] = {
        {
                .desc = "Kernel page table isolation (KPTI)",
                .capability = ARM64_UNMAP_KERNEL_AT_EL0,
-               .type = ARM64_CPUCAP_SYSTEM_FEATURE,
+               .type = ARM64_CPUCAP_BOOT_RESTRICTED_CPU_LOCAL_FEATURE,
+               /*
+                * The ID feature fields below are used to indicate that
+                * the CPU doesn't need KPTI. See unmap_kernel_at_el0 for
+                * more details.
+                */
+               .sys_reg = SYS_ID_AA64PFR0_EL1,
+               .field_pos = ID_AA64PFR0_CSV3_SHIFT,
+               .min_field_value = 1,
                .matches = unmap_kernel_at_el0,
                .cpu_enable = kpti_install_ng_mappings,
        },
-- 
2.14.3

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