Commit d3aec8a2 authored by Suzuki K Poulose's avatar Suzuki K Poulose Committed by Will Deacon

arm64: capabilities: Restrict KPTI detection to boot-time CPUs

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 <>
Reviewed-by: default avatarDave Martin <>
Signed-off-by: default avatarSuzuki K Poulose <>
Signed-off-by: default avatarWill Deacon <>
parent 5c137714
......@@ -248,6 +248,15 @@ extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
* 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.
struct arm64_cpu_capabilities {
const char *desc;
u16 capability;
......@@ -880,10 +880,9 @@ 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
......@@ -914,8 +913,7 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
/* Defer to CPU feature registers */
return !cpuid_feature_extract_unsigned_field(pfr0,
return !has_cpuid_feature(entry, scope);
static void
......@@ -1062,7 +1060,15 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
.desc = "Kernel page table isolation (KPTI)",
.capability = ARM64_UNMAP_KERNEL_AT_EL0,
* 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,
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