[PATCH v2 10/20] arm64: capabilities: Restrict KPTI detection to boot-time CPUs

[Suzuki K Poulose]

KPTI is treated as a system wide feature, where we enable the feature
when all the CPUs on the system suffers from the security vulnerability,
unless it is forced via kernel command line. Also, if a late CPU needs
KPTI but KPTI was not enabled at boot time, the CPU is currently allowed
to boot, which is a potential security vulnerability.  This patch ensures
that late CPUs are rejected as appropriate if they need KPTI but it wasn't
enabled.

Cc: Will Deacon <will.deacon at arm.com>
Cc: Dave Martin <dave.martin at arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose at arm.com>
---
 arch/arm64/include/asm/cpufeature.h |  9 +++++++++
 arch/arm64/kernel/cpufeature.c      | 11 ++++++-----
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 7bb3fdec827e..71993dd4afae 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -223,6 +223,15 @@ 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 ecc87aa74c64..4a55492784b7 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -862,9 +862,8 @@ static bool has_no_fpsimd(const struct arm64_cpu_capabilities *entry, int __unus
 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)
 {
-	u64 pfr0 = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
 
 	/* Forced on command line? */
 	if (__kpti_forced) {
@@ -885,8 +884,7 @@ 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 int __init parse_kpti(char *str)
@@ -1008,7 +1006,10 @@ 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,
+		.sys_reg = SYS_ID_AA64PFR0_EL1,
+		.field_pos = ID_AA64PFR0_CSV3_SHIFT,
+		.min_field_value = 1,
 		.matches = unmap_kernel_at_el0,
 	},
 #endif
-- 
2.14.3