arm64: kernel: Add support for Privileged Access Never

'Privileged Access Never' is a new arm8.1 feature which prevents
privileged code from accessing any virtual address where read or write
access is also permitted at EL0.

This patch enables the PAN feature on all CPUs, and modifies {get,put}_user
helpers temporarily to permit access.

This will catch kernel bugs where user memory is accessed directly.
'Unprivileged loads and stores' using ldtrb et al are unaffected by PAN.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
[will: use ALTERNATIVE in asm and tidy up pan_enable check]
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/cpufeature.c

@@ -21,6 +21,7 @@
 #include <linux/types.h>
 #include <asm/cpu.h>
 #include <asm/cpufeature.h>
+#include <asm/processor.h>
 
 static bool
 feature_matches(u64 reg, const struct arm64_cpu_capabilities *entry)
@@ -39,6 +40,15 @@ has_id_aa64pfr0_feature(const struct arm64_cpu_capabilities *entry)
 	return feature_matches(val, entry);
 }
 
+static bool __maybe_unused
+has_id_aa64mmfr1_feature(const struct arm64_cpu_capabilities *entry)
+{
+	u64 val;
+
+	val = read_cpuid(id_aa64mmfr1_el1);
+	return feature_matches(val, entry);
+}
+
 static const struct arm64_cpu_capabilities arm64_features[] = {
 	{
 		.desc = "GIC system register CPU interface",
@@ -47,6 +57,16 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.field_pos = 24,
 		.min_field_value = 1,
 	},
+#ifdef CONFIG_ARM64_PAN
+	{
+		.desc = "Privileged Access Never",
+		.capability = ARM64_HAS_PAN,
+		.matches = has_id_aa64mmfr1_feature,
+		.field_pos = 20,
+		.min_field_value = 1,
+		.enable = cpu_enable_pan,
+	},
+#endif /* CONFIG_ARM64_PAN */
 	{},
 };