arm64: Take into account ID_AA64PFR0_EL1.CSV3

For non-KASLR kernels where the KPTI behaviour has not been overridden
on the command line we can use ID_AA64PFR0_EL1.CSV3 to determine whether
or not we should unmap the kernel whilst running at EL0.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

2 files changed, 8 insertions, 1 deletions

diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
index ab637886a6f9..262ae18f0e05 100644
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h
@@ -438,6 +438,7 @@
 #define ID_AA64ISAR1_DPB_SHIFT          0
 
 /* id_aa64pfr0 */
+#define ID_AA64PFR0_CSV3_SHIFT          60
 #define ID_AA64PFR0_SVE_SHIFT           32
 #define ID_AA64PFR0_GIC_SHIFT           24
 #define ID_AA64PFR0_ASIMD_SHIFT         20
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 1f024049eb30..0f3c96172f0f 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -146,6 +146,7 @@ static const struct arm64_ftr_bits ftr_id_aa64isar1[] = {
 };
 
 static const struct arm64_ftr_bits ftr_id_aa64pfr0[] = {
+        ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_CSV3_SHIFT, 4, 0),
         ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_SVE_SHIFT, 4, 0),
         ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_GIC_SHIFT, 4, 0),
         S_ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_ASIMD_SHIFT, 4, ID_AA64PFR0_ASIMD_NI),
@@ -852,6 +853,8 @@ 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)
 {
+        u64 pfr0 = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
+
         /* Forced on command line? */
         if (__kpti_forced) {
                 pr_info_once("kernel page table isolation forced %s by command line option\n",
@@ -863,7 +866,9 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
         if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
                 return true;
 
-        return false;
+        /* Defer to CPU feature registers */
+        return !cpuid_feature_extract_unsigned_field(pfr0,
+                                                     ID_AA64PFR0_CSV3_SHIFT);
 }
 
 static int __init parse_kpti(char *str)
@@ -968,6 +973,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
         },
 #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
         {
+                .desc = "Kernel page table isolation (KPTI)",
                 .capability = ARM64_UNMAP_KERNEL_AT_EL0,
                 .def_scope = SCOPE_SYSTEM,
                 .matches = unmap_kernel_at_el0,