arm64: Use a variable to store non-global mappings decision

Refactor the code which checks to see if we need to use non-global
mappings to use a variable instead of checking with the CPU capabilities
each time, doing the initial check for KPTI early in boot before we
start allocating memory so we still avoid transitioning to non-global
mappings in common cases.

Since this variable always matches our decision about non-global
mappings this means we can also combine arm64_kernel_use_ng_mappings()
and arm64_unmap_kernel_at_el0() into a single function, the variable
simply stores the result and the decision code is elsewhere. We could
just have the users check the variable directly but having a function
makes it clear that these uses are read-only.

The result is that we simplify the code a bit and reduces the amount of
code executed at runtime.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

2 files changed, 59 insertions, 3 deletions

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 88aa5ab02926..4a031111ceb5 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -47,6 +47,9 @@ static struct arm64_cpu_capabilities const __ro_after_init *cpu_hwcaps_ptrs[ARM6
 /* Need also bit for ARM64_CB_PATCH */
 DECLARE_BITMAP(boot_capabilities, ARM64_NPATCHABLE);
 
+bool arm64_use_ng_mappings = false;
+EXPORT_SYMBOL(arm64_use_ng_mappings);
+
 /*
  * Flag to indicate if we have computed the system wide
  * capabilities based on the boot time active CPUs. This
@@ -966,6 +969,53 @@ has_useable_cnp(const struct arm64_cpu_capabilities *entry, int scope)
 	return has_cpuid_feature(entry, scope);
 }
 
+/*
+ * This check is triggered during the early boot before the cpufeature
+ * is initialised. Checking the status on the local CPU allows the boot
+ * CPU to detect the need for non-global mappings and thus avoiding a
+ * pagetable re-write after all the CPUs are booted. This check will be
+ * anyway run on individual CPUs, allowing us to get the consistent
+ * state once the SMP CPUs are up and thus make the switch to non-global
+ * mappings if required.
+ */
+bool kaslr_requires_kpti(void)
+{
+	bool tx1_bug;
+	u64 ftr;
+
+	if (!IS_ENABLED(CONFIG_RANDOMIZE_BASE))
+		return false;
+
+	/*
+	 * E0PD does a similar job to KPTI so can be used instead
+	 * where available.
+	 */
+	if (IS_ENABLED(CONFIG_ARM64_E0PD)) {
+		ftr = read_sysreg_s(SYS_ID_AA64MMFR2_EL1);
+		if ((ftr >> ID_AA64MMFR2_E0PD_SHIFT) & 0xf)
+			return false;
+	}
+
+	/*
+	 * Systems affected by Cavium erratum 24756 are incompatible
+	 * with KPTI.
+	 */
+	if (!IS_ENABLED(CONFIG_CAVIUM_ERRATUM_27456)) {
+		tx1_bug = false;
+	} else if (!static_branch_likely(&arm64_const_caps_ready)) {
+		extern const struct midr_range cavium_erratum_27456_cpus[];
+
+		tx1_bug = is_midr_in_range_list(read_cpuid_id(),
+						cavium_erratum_27456_cpus);
+	} else {
+		tx1_bug = __cpus_have_const_cap(ARM64_WORKAROUND_CAVIUM_27456);
+	}
+	if (tx1_bug)
+		return false;
+
+	return kaslr_offset() > 0;
+}
+
 static bool __meltdown_safe = true;
 static int __kpti_forced; /* 0: not forced, >0: forced on, <0: forced off */
 
@@ -1044,7 +1094,6 @@ kpti_install_ng_mappings(const struct arm64_cpu_capabilities *__unused)
 	extern kpti_remap_fn idmap_kpti_install_ng_mappings;
 	kpti_remap_fn *remap_fn;
 
-	static bool kpti_applied = false;
 	int cpu = smp_processor_id();
 
 	/*
@@ -1052,7 +1101,7 @@ kpti_install_ng_mappings(const struct arm64_cpu_capabilities *__unused)
 	 * it already or we have KASLR enabled and therefore have not
 	 * created any global mappings at all.
 	 */
-	if (kpti_applied || kaslr_offset() > 0)
+	if (arm64_use_ng_mappings)
 		return;
 
 	remap_fn = (void *)__pa_symbol(idmap_kpti_install_ng_mappings);
@@ -1062,7 +1111,7 @@ kpti_install_ng_mappings(const struct arm64_cpu_capabilities *__unused)
 	cpu_uninstall_idmap();
 
 	if (!cpu)
-		kpti_applied = true;
+		arm64_use_ng_mappings = true;
 
 	return;
 }
diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
index 56f664561754..b6f9455d7ca3 100644
--- a/arch/arm64/kernel/setup.c
+++ b/arch/arm64/kernel/setup.c
@@ -285,6 +285,13 @@ void __init setup_arch(char **cmdline_p)
 
 	*cmdline_p = boot_command_line;
 
+	/*
+	 * If know now we are going to need KPTI then use non-global
+	 * mappings from the start, avoiding the cost of rewriting
+	 * everything later.
+	 */
+	arm64_use_ng_mappings = kaslr_requires_kpti();
+
 	early_fixmap_init();
 	early_ioremap_init();