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SubjectRe: [PATCH 05/14] arm64: Add 'ssbd' command-line option
On Tue, May 22, 2018 at 04:06:39PM +0100, Marc Zyngier wrote:
> 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.
>
> 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 | 102 ++++++++++++++++++++----
> 3 files changed, 109 insertions(+), 16 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index f2040d46f095..646e112c6f63 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: Unconditionnaly enable mitigation for
> + for both kernel and userspace
> + force-off: Unconditionnaly 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..9bc548e22784 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_EL1_ENTRY 1

The EL1_ENTRY part of the name is a bit misleading, since this doesn't
apply to EL1->EL1 exceptions (and as with many other bits of the arm64
code, it's arguably misleading in the VHE case).

Perhaps ARM64_SSBD_KERNEL, which would align with the parameter name?

Not a big deal either way, and otherwise this looks good to me.
Regardless:

Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> +#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 7fd6d5b001f5..f1d4e75b0ddd 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_EL1_ENTRY;
> +
> +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_EL1_ENTRY, },
> +};
> +
> +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)
> @@ -272,44 +304,82 @@ 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:
> + 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);
> + do_ssbd(false);
> + required = false;
> + break;
> +
> + case ARM64_SSBD_EL1_ENTRY:
> + if (required) {
> + __this_cpu_write(arm64_ssbd_callback_required, 1);
> + do_ssbd(true);
> + }
> + break;
> +
> + case ARM64_SSBD_FORCE_ENABLE:
> + pr_info_once("%s forced from command-line\n", entry->desc);
> do_ssbd(true);
> + required = true;
> + break;
> +
> + default:
> + WARN_ON(1);
> + break;
> }
>
> - return supported;
> + return required;
> }
> #endif /* CONFIG_ARM64_SSBD */
>
> --
> 2.14.2
>

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