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Subject[PATCH v2 05/17] arm64: Add 'ssbd' command-line option
Date
On a system where the firmware implements ARCH_WORKAROUND_2,
it may be useful to either permanently enable or disable the
workaround for cases where the user decides that they'd rather
not get a trap overhead, and keep the mitigation permanently
on or off instead of switching it on exception entry/exit.

In any case, default to the mitigation being enabled.

Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
---
Documentation/admin-guide/kernel-parameters.txt | 17 ++++
arch/arm64/include/asm/cpufeature.h | 6 ++
arch/arm64/kernel/cpu_errata.c | 103 ++++++++++++++++++++----
3 files changed, 110 insertions(+), 16 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f2040d46f095..ff4ba249a26f 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4092,6 +4092,23 @@
expediting. Set to zero to disable automatic
expediting.

+ ssbd= [ARM64,HW]
+ Speculative Store Bypass Disable control
+
+ On CPUs that are vulnerable to the Speculative
+ Store Bypass vulnerability and offer a
+ firmware based mitigation, this parameter
+ indicates how the mitigation should be used:
+
+ force-on: Unconditionally enable mitigation for
+ for both kernel and userspace
+ force-off: Unconditionally disable mitigation for
+ for both kernel and userspace
+ kernel: Always enable mitigation in the
+ kernel, and offer a prctl interface
+ to allow userspace to register its
+ interest in being mitigated too.
+
stack_guard_gap= [MM]
override the default stack gap protection. The value
is in page units and it defines how many pages prior
diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 09b0f2a80c8f..b50650f3e496 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -537,6 +537,12 @@ static inline u64 read_zcr_features(void)
return zcr;
}

+#define ARM64_SSBD_UNKNOWN -1
+#define ARM64_SSBD_FORCE_DISABLE 0
+#define ARM64_SSBD_KERNEL 1
+#define ARM64_SSBD_FORCE_ENABLE 2
+#define ARM64_SSBD_MITIGATED 3
+
#endif /* __ASSEMBLY__ */

#endif
diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
index 0c76c9f176bb..a838a3043d4e 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -235,6 +235,38 @@ enable_smccc_arch_workaround_1(const struct arm64_cpu_capabilities *entry)
#ifdef CONFIG_ARM64_SSBD
DEFINE_PER_CPU_READ_MOSTLY(u64, arm64_ssbd_callback_required);

+int ssbd_state __read_mostly = ARM64_SSBD_KERNEL;
+
+static const struct ssbd_options {
+ const char *str;
+ int state;
+} ssbd_options[] = {
+ { "force-on", ARM64_SSBD_FORCE_ENABLE, },
+ { "force-off", ARM64_SSBD_FORCE_DISABLE, },
+ { "kernel", ARM64_SSBD_KERNEL, },
+};
+
+static int __init ssbd_cfg(char *buf)
+{
+ int i;
+
+ if (!buf || !buf[0])
+ return -EINVAL;
+
+ for (i = 0; i < ARRAY_SIZE(ssbd_options); i++) {
+ int len = strlen(ssbd_options[i].str);
+
+ if (strncmp(buf, ssbd_options[i].str, len))
+ continue;
+
+ ssbd_state = ssbd_options[i].state;
+ return 0;
+ }
+
+ return -EINVAL;
+}
+early_param("ssbd", ssbd_cfg);
+
void __init arm64_update_smccc_conduit(struct alt_instr *alt,
__le32 *origptr, __le32 *updptr,
int nr_inst)
@@ -278,44 +310,83 @@ static bool has_ssbd_mitigation(const struct arm64_cpu_capabilities *entry,
int scope)
{
struct arm_smccc_res res;
- bool supported = true;
+ bool required = true;
+ s32 val;

WARN_ON(scope != SCOPE_LOCAL_CPU || preemptible());

- if (psci_ops.smccc_version == SMCCC_VERSION_1_0)
+ if (psci_ops.smccc_version == SMCCC_VERSION_1_0) {
+ ssbd_state = ARM64_SSBD_UNKNOWN;
return false;
+ }

- /*
- * The probe function return value is either negative
- * (unsupported or mitigated), positive (unaffected), or zero
- * (requires mitigation). We only need to do anything in the
- * last case.
- */
switch (psci_ops.conduit) {
case PSCI_CONDUIT_HVC:
arm_smccc_1_1_hvc(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
ARM_SMCCC_ARCH_WORKAROUND_2, &res);
- if ((int)res.a0 != 0)
- supported = false;
break;

case PSCI_CONDUIT_SMC:
arm_smccc_1_1_smc(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
ARM_SMCCC_ARCH_WORKAROUND_2, &res);
- if ((int)res.a0 != 0)
- supported = false;
break;

default:
- supported = false;
+ ssbd_state = ARM64_SSBD_UNKNOWN;
+ return false;
}

- if (supported) {
- __this_cpu_write(arm64_ssbd_callback_required, 1);
+ val = (s32)res.a0;
+
+ switch (val) {
+ case SMCCC_RET_NOT_SUPPORTED:
+ ssbd_state = ARM64_SSBD_UNKNOWN;
+ return false;
+
+ case SMCCC_RET_NOT_REQUIRED:
+ pr_info_once("%s mitigation not required\n", entry->desc);
+ ssbd_state = ARM64_SSBD_MITIGATED;
+ return false;
+
+ case SMCCC_RET_SUCCESS:
+ required = true;
+ break;
+
+ case 1: /* Mitigation not required on this CPU */
+ required = false;
+ break;
+
+ default:
+ WARN_ON(1);
+ return false;
+ }
+
+ switch (ssbd_state) {
+ case ARM64_SSBD_FORCE_DISABLE:
+ pr_info_once("%s disabled from command-line\n", entry->desc);
+ arm64_set_ssbd_mitigation(false);
+ required = false;
+ break;
+
+ case ARM64_SSBD_KERNEL:
+ if (required) {
+ __this_cpu_write(arm64_ssbd_callback_required, 1);
+ arm64_set_ssbd_mitigation(true);
+ }
+ break;
+
+ case ARM64_SSBD_FORCE_ENABLE:
+ pr_info_once("%s forced from command-line\n", entry->desc);
arm64_set_ssbd_mitigation(true);
+ required = true;
+ break;
+
+ default:
+ WARN_ON(1);
+ break;
}

- return supported;
+ return required;
}
#endif /* CONFIG_ARM64_SSBD */

--
2.14.2
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