[PATCH RFC 5/5] arm64/speculation: Add support for 'cpu_spec_mitigations=' cmdline options

[Josh Poimboeuf]

Configure arm64 runtime CPU speculation bug mitigations in accordance
with the 'cpu_spec_mitigations=' cmdline options.  This affects
Meltdown and Speculative Store Bypass.

The default behavior is unchanged.

Signed-off-by: Josh Poimboeuf <jpoimboe at redhat.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 2 ++
 arch/arm64/kernel/cpu_errata.c                  | 4 ++++
 arch/arm64/kernel/cpufeature.c                  | 6 ++++++
 3 files changed, 12 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index e838af96daa4..0b54385ee7a8 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2553,11 +2553,13 @@
 			off
 				Disable all speculative CPU mitigations.
 				Equivalent to: nopti [x86, powerpc]
+					       kpti=0 [arm64]
 					       nospectre_v1 [powerpc]
 					       nospectre_v2 [x86, powerpc, s390]
 					       spectre_v2_user=off [x86]
 					       nobp=0 [s390]
 					       spec_store_bypass_disable=off [x86, powerpc]
+					       ssbd=force-off [arm64]
 					       l1tf=off [x86]
 
 			auto (default)
diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
index 9950bb0cbd52..db8d27e3fb1c 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/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>
@@ -385,6 +386,9 @@ static bool has_ssbd_mitigation(const struct arm64_cpu_capabilities *entry,
 		return false;
 	}
 
+	if (cpu_spec_mitigations == CPU_SPEC_MITIGATIONS_OFF)
+		ssbd_state = ARM64_SSBD_FORCE_DISABLE;
+
 	switch (psci_ops.conduit) {
 	case PSCI_CONDUIT_HVC:
 		arm_smccc_1_1_hvc(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 4061de10cea6..4512b582d50f 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/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>
@@ -978,6 +979,11 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
 		__kpti_forced = -1;
 	}
 
+	if (cpu_spec_mitigations == CPU_SPEC_MITIGATIONS_OFF) {
+		str = "cpu_spec_mitigations=off";
+		__kpti_forced = -1;
+	}
+
 	/* Forced? */
 	if (__kpti_forced) {
 		pr_info_once("kernel page table isolation forced %s by %s\n",
-- 
2.17.2