arm64/speculation: Support 'mitigations=' cmdline option

Configure arm64 runtime CPU speculation bug mitigations in accordance
with the 'mitigations=' cmdline option.  This affects Meltdown, Spectre
v2, and Speculative Store Bypass.

The default behavior is unchanged.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
[will: reorder checks so KASLR implies KPTI and SSBS is affected by cmdline]
Signed-off-by: Will Deacon <will.deacon@arm.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -2545,8 +2545,8 @@
 			http://repo.or.cz/w/linux-2.6/mini2440.git
 
 	mitigations=
-			[X86,PPC,S390] Control optional mitigations for CPU
-			vulnerabilities.  This is a set of curated,
+			[X86,PPC,S390,ARM64] Control optional mitigations for
+			CPU vulnerabilities.  This is a set of curated,
 			arch-independent options, each of which is an
 			aggregation of existing arch-specific options.
 
@@ -2555,11 +2555,13 @@
 				improves system performance, but it may also
 				expose users to several CPU vulnerabilities.
 				Equivalent to: nopti [X86,PPC]
+					       kpti=0 [ARM64]
 					       nospectre_v1 [PPC]
 					       nobp=0 [S390]
-					       nospectre_v2 [X86,PPC,S390]
+					       nospectre_v2 [X86,PPC,S390,ARM64]
 					       spectre_v2_user=off [X86]
 					       spec_store_bypass_disable=off [X86,PPC]
+					       ssbd=force-off [ARM64]
 					       l1tf=off [X86]
 
 			auto (default)

arch/arm64/kernel/cpu_errata.c

@@ -19,6 +19,7 @@
 #include <linux/arm-smccc.h>
 #include <linux/psci.h>
 #include <linux/types.h>
+#include <linux/cpu.h>
 #include <asm/cpu.h>
 #include <asm/cputype.h>
 #include <asm/cpufeature.h>
@@ -386,6 +387,9 @@ static bool has_ssbd_mitigation(const struct arm64_cpu_capabilities *entry,
 
 	WARN_ON(scope != SCOPE_LOCAL_CPU || preemptible());
 
+	if (cpu_mitigations_off())
+		ssbd_state = ARM64_SSBD_FORCE_DISABLE;
+
 	/* delay setting __ssb_safe until we get a firmware response */
 	if (is_midr_in_range_list(read_cpuid_id(), entry->midr_range_list))
 		this_cpu_safe = true;
@@ -589,7 +593,7 @@ check_branch_predictor(const struct arm64_cpu_capabilities *entry, int scope)
 	}
 
 	/* forced off */
-	if (__nospectre_v2) {
+	if (__nospectre_v2 || cpu_mitigations_off()) {
 		pr_info_once("spectrev2 mitigation disabled by command line option\n");
 		__hardenbp_enab = false;
 		return false;

arch/arm64/kernel/cpufeature.c

@@ -25,6 +25,7 @@
 #include <linux/stop_machine.h>
 #include <linux/types.h>
 #include <linux/mm.h>
+#include <linux/cpu.h>
 #include <asm/cpu.h>
 #include <asm/cpufeature.h>
 #include <asm/cpu_ops.h>
@@ -966,7 +967,7 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
 		MIDR_ALL_VERSIONS(MIDR_HISI_TSV110),
 		{ /* sentinel */ }
 	};
-	char const *str = "command line option";
+	char const *str = "kpti command line option";
 	bool meltdown_safe;
 
 	meltdown_safe = is_midr_in_range_list(read_cpuid_id(), kpti_safe_list);
@@ -996,6 +997,11 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
 		}
 	}
 
+	if (cpu_mitigations_off() && !__kpti_forced) {
+		str = "mitigations=off";
+		__kpti_forced = -1;
+	}
+
 	if (!IS_ENABLED(CONFIG_UNMAP_KERNEL_AT_EL0)) {
 		pr_info_once("kernel page table isolation disabled by kernel configuration\n");
 		return false;