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

BugLink: https://bugs.launchpad.net/bugs/1851876
[ Upstream commit d3aec8a28be3b88bf75442e7c24fd9da8d69a6df ]

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.deacon@arm.com>
Reviewed-by: Dave Martin <dave.martin@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 0252c73824baf387256af67e0b11ff50e6fc3102..ab9fe8053d023b69fe42101b1602876bfdf03d3f 100644 (file)
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -249,6 +249,15 @@ 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 ab7a97ee775238f0ee9d766a6bd3a05e59dd09c3..6de13744d1e8b35b33bc0186fb95d7ff0b893008 100644 (file)
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -879,10 +879,9 @@ 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)
 {
        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
@@ -918,8 +917,7 @@ 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
@@ -1066,7 +1064,15 @@ 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,
        },