arm64: Rewrite Spectre-v4 mitigation code

Rewrite the Spectre-v4 mitigation handling code to follow the same
approach as that taken by Spectre-v2.

For now, report to KVM that the system is vulnerable (by forcing
'ssbd_state' to ARM64_SSBD_UNKNOWN), as this will be cleared up in
subsequent steps.
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/processor.h

@@ -198,25 +198,12 @@ static inline void start_thread_common(struct pt_regs *regs, unsigned long pc)
 		regs->pmr_save = GIC_PRIO_IRQON;
 }
 
-static inline void set_ssbs_bit(struct pt_regs *regs)
-{
-	regs->pstate |= PSR_SSBS_BIT;
-}
-
-static inline void set_compat_ssbs_bit(struct pt_regs *regs)
-{
-	regs->pstate |= PSR_AA32_SSBS_BIT;
-}
-
 static inline void start_thread(struct pt_regs *regs, unsigned long pc,
 				unsigned long sp)
 {
 	start_thread_common(regs, pc);
 	regs->pstate = PSR_MODE_EL0t;
-
-	if (arm64_get_ssbd_state() != ARM64_SSBD_FORCE_ENABLE)
-		set_ssbs_bit(regs);
-
+	spectre_v4_enable_task_mitigation(current);
 	regs->sp = sp;
 }
 
@@ -233,9 +220,7 @@ static inline void compat_start_thread(struct pt_regs *regs, unsigned long pc,
 	regs->pstate |= PSR_AA32_E_BIT;
 #endif
 
-	if (arm64_get_ssbd_state() != ARM64_SSBD_FORCE_ENABLE)
-		set_compat_ssbs_bit(regs);
-
+	spectre_v4_enable_task_mitigation(current);
 	regs->compat_sp = sp;
 }
 #endif

arch/arm64/include/asm/spectre.h

@@ -24,4 +24,9 @@ enum mitigation_state arm64_get_spectre_v2_state(void);
 bool has_spectre_v2(const struct arm64_cpu_capabilities *cap, int scope);
 void spectre_v2_enable_mitigation(const struct arm64_cpu_capabilities *__unused);
 
+enum mitigation_state arm64_get_spectre_v4_state(void);
+bool has_spectre_v4(const struct arm64_cpu_capabilities *cap, int scope);
+void spectre_v4_enable_mitigation(const struct arm64_cpu_capabilities *__unused);
+void spectre_v4_enable_task_mitigation(struct task_struct *tsk);
+
 #endif	/* __ASM_SPECTRE_H */

arch/arm64/kernel/cpu_errata.c

@@ -106,62 +106,7 @@ cpu_enable_trap_ctr_access(const struct arm64_cpu_capabilities *cap)
 		sysreg_clear_set(sctlr_el1, SCTLR_EL1_UCT, 0);
 }
 
-DEFINE_PER_CPU_READ_MOSTLY(u64, arm64_ssbd_callback_required);
-
-int ssbd_state __read_mostly = ARM64_SSBD_KERNEL;
-static bool __ssb_safe = true;
-
-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)
-{
-	u32 insn;
-
-	BUG_ON(nr_inst != 1);
-
-	switch (arm_smccc_1_1_get_conduit()) {
-	case SMCCC_CONDUIT_HVC:
-		insn = aarch64_insn_get_hvc_value();
-		break;
-	case SMCCC_CONDUIT_SMC:
-		insn = aarch64_insn_get_smc_value();
-		break;
-	default:
-		return;
-	}
-
-	*updptr = cpu_to_le32(insn);
-}
+int ssbd_state __read_mostly = ARM64_SSBD_UNKNOWN;
 
 void __init arm64_enable_wa2_handling(struct alt_instr *alt,
 				      __le32 *origptr, __le32 *updptr,
@@ -177,144 +122,6 @@ void __init arm64_enable_wa2_handling(struct alt_instr *alt,
 		*updptr = cpu_to_le32(aarch64_insn_gen_nop());
 }
 
-void arm64_set_ssbd_mitigation(bool state)
-{
-	int conduit;
-
-	if (this_cpu_has_cap(ARM64_SSBS)) {
-		if (state)
-			asm volatile(SET_PSTATE_SSBS(0));
-		else
-			asm volatile(SET_PSTATE_SSBS(1));
-		return;
-	}
-
-	conduit = arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_WORKAROUND_2, state,
-				       NULL);
-
-	WARN_ON_ONCE(conduit == SMCCC_CONDUIT_NONE);
-}
-
-static bool has_ssbd_mitigation(const struct arm64_cpu_capabilities *entry,
-				    int scope)
-{
-	struct arm_smccc_res res;
-	bool required = true;
-	s32 val;
-	bool this_cpu_safe = false;
-	int conduit;
-
-	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;
-
-	if (this_cpu_has_cap(ARM64_SSBS)) {
-		if (!this_cpu_safe)
-			__ssb_safe = false;
-		required = false;
-		goto out_printmsg;
-	}
-
-	conduit = arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
-				       ARM_SMCCC_ARCH_WORKAROUND_2, &res);
-
-	if (conduit == SMCCC_CONDUIT_NONE) {
-		ssbd_state = ARM64_SSBD_UNKNOWN;
-		if (!this_cpu_safe)
-			__ssb_safe = false;
-		return false;
-	}
-
-	val = (s32)res.a0;
-
-	switch (val) {
-	case SMCCC_RET_NOT_SUPPORTED:
-		ssbd_state = ARM64_SSBD_UNKNOWN;
-		if (!this_cpu_safe)
-			__ssb_safe = false;
-		return false;
-
-	/* machines with mixed mitigation requirements must not return this */
-	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:
-		__ssb_safe = false;
-		required = true;
-		break;
-
-	case 1:	/* Mitigation not required on this CPU */
-		required = false;
-		break;
-
-	default:
-		WARN_ON(1);
-		if (!this_cpu_safe)
-			__ssb_safe = false;
-		return false;
-	}
-
-	switch (ssbd_state) {
-	case ARM64_SSBD_FORCE_DISABLE:
-		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:
-		arm64_set_ssbd_mitigation(true);
-		required = true;
-		break;
-
-	default:
-		WARN_ON(1);
-		break;
-	}
-
-out_printmsg:
-	switch (ssbd_state) {
-	case ARM64_SSBD_FORCE_DISABLE:
-		pr_info_once("%s disabled from command-line\n", entry->desc);
-		break;
-
-	case ARM64_SSBD_FORCE_ENABLE:
-		pr_info_once("%s forced from command-line\n", entry->desc);
-		break;
-	}
-
-	return required;
-}
-
-static void cpu_enable_ssbd_mitigation(const struct arm64_cpu_capabilities *cap)
-{
-	if (ssbd_state != ARM64_SSBD_FORCE_DISABLE)
-		cap->matches(cap, SCOPE_LOCAL_CPU);
-}
-
-/* known invulnerable cores */
-static const struct midr_range arm64_ssb_cpus[] = {
-	MIDR_ALL_VERSIONS(MIDR_CORTEX_A35),
-	MIDR_ALL_VERSIONS(MIDR_CORTEX_A53),
-	MIDR_ALL_VERSIONS(MIDR_CORTEX_A55),
-	MIDR_ALL_VERSIONS(MIDR_BRAHMA_B53),
-	MIDR_ALL_VERSIONS(MIDR_QCOM_KRYO_3XX_SILVER),
-	MIDR_ALL_VERSIONS(MIDR_QCOM_KRYO_4XX_SILVER),
-	{},
-};
-
 #ifdef CONFIG_ARM64_ERRATUM_1463225
 DEFINE_PER_CPU(int, __in_cortex_a76_erratum_1463225_wa);
 
@@ -674,12 +481,11 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
 	},
 #endif
 	{
-		.desc = "Speculative Store Bypass Disable",
+		.desc = "Spectre-v4",
 		.capability = ARM64_SPECTRE_V4,
 		.type = ARM64_CPUCAP_LOCAL_CPU_ERRATUM,
-		.matches = has_ssbd_mitigation,
-		.cpu_enable = cpu_enable_ssbd_mitigation,
-		.midr_range_list = arm64_ssb_cpus,
+		.matches = has_spectre_v4,
+		.cpu_enable = spectre_v4_enable_mitigation,
 	},
 #ifdef CONFIG_ARM64_ERRATUM_1418040
 	{
@@ -732,18 +538,3 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
 	{
 	}
 };
-
-ssize_t cpu_show_spec_store_bypass(struct device *dev,
-		struct device_attribute *attr, char *buf)
-{
-	if (__ssb_safe)
-		return sprintf(buf, "Not affected\n");
-
-	switch (ssbd_state) {
-	case ARM64_SSBD_KERNEL:
-	case ARM64_SSBD_FORCE_ENABLE:
-		return sprintf(buf, "Mitigation: Speculative Store Bypass disabled via prctl\n");
-	}
-
-	return sprintf(buf, "Vulnerable\n");
-}

arch/arm64/kernel/cpufeature.c

@@ -1583,46 +1583,6 @@ static void cpu_has_fwb(const struct arm64_cpu_capabilities *__unused)
 	WARN_ON(val & (7 << 27 | 7 << 21));
 }
 
-static int ssbs_emulation_handler(struct pt_regs *regs, u32 instr)
-{
-	if (user_mode(regs))
-		return 1;
-
-	if (instr & BIT(PSTATE_Imm_shift))
-		regs->pstate |= PSR_SSBS_BIT;
-	else
-		regs->pstate &= ~PSR_SSBS_BIT;
-
-	arm64_skip_faulting_instruction(regs, 4);
-	return 0;
-}
-
-static struct undef_hook ssbs_emulation_hook = {
-	.instr_mask	= ~(1U << PSTATE_Imm_shift),
-	.instr_val	= 0xd500401f | PSTATE_SSBS,
-	.fn		= ssbs_emulation_handler,
-};
-
-static void cpu_enable_ssbs(const struct arm64_cpu_capabilities *__unused)
-{
-	static bool undef_hook_registered = false;
-	static DEFINE_RAW_SPINLOCK(hook_lock);
-
-	raw_spin_lock(&hook_lock);
-	if (!undef_hook_registered) {
-		register_undef_hook(&ssbs_emulation_hook);
-		undef_hook_registered = true;
-	}
-	raw_spin_unlock(&hook_lock);
-
-	if (arm64_get_ssbd_state() == ARM64_SSBD_FORCE_DISABLE) {
-		sysreg_clear_set(sctlr_el1, 0, SCTLR_ELx_DSSBS);
-		arm64_set_ssbd_mitigation(false);
-	} else {
-		arm64_set_ssbd_mitigation(true);
-	}
-}
-
 #ifdef CONFIG_ARM64_PAN
 static void cpu_enable_pan(const struct arm64_cpu_capabilities *__unused)
 {
@@ -1983,7 +1943,6 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.field_pos = ID_AA64PFR1_SSBS_SHIFT,
 		.sign = FTR_UNSIGNED,
 		.min_field_value = ID_AA64PFR1_SSBS_PSTATE_ONLY,
-		.cpu_enable = cpu_enable_ssbs,
 	},
 #ifdef CONFIG_ARM64_CNP
 	{

arch/arm64/kernel/entry.S

@@ -132,8 +132,8 @@ alternative_else_nop_endif
 	 * them if required.
 	 */
 	.macro	apply_ssbd, state, tmp1, tmp2
-alternative_cb	arm64_enable_wa2_handling
-	b	.L__asm_ssbd_skip\@
+alternative_cb	spectre_v4_patch_fw_mitigation_enable
+	b	.L__asm_ssbd_skip\@		// Patched to NOP
 alternative_cb_end
 	ldr_this_cpu	\tmp2, arm64_ssbd_callback_required, \tmp1
 	cbz	\tmp2,	.L__asm_ssbd_skip\@
@@ -141,7 +141,7 @@ alternative_cb_end
 	tbnz	\tmp2, #TIF_SSBD, .L__asm_ssbd_skip\@
 	mov	w0, #ARM_SMCCC_ARCH_WORKAROUND_2
 	mov	w1, #\state
-alternative_cb	arm64_update_smccc_conduit
+alternative_cb	spectre_v4_patch_fw_mitigation_conduit
 	nop					// Patched to SMC/HVC #0
 alternative_cb_end
 .L__asm_ssbd_skip\@:

arch/arm64/kernel/hibernate.c

@@ -332,11 +332,7 @@ int swsusp_arch_suspend(void)
 		 * mitigation off behind our back, let's set the state
 		 * to what we expect it to be.
 		 */
-		switch (arm64_get_ssbd_state()) {
-		case ARM64_SSBD_FORCE_ENABLE:
-		case ARM64_SSBD_KERNEL:
-			arm64_set_ssbd_mitigation(true);
-		}
+		spectre_v4_enable_mitigation(NULL);
 	}
 
 	local_daif_restore(flags);

arch/arm64/kernel/process.c

@@ -421,8 +421,7 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
 		    cpus_have_const_cap(ARM64_HAS_UAO))
 			childregs->pstate |= PSR_UAO_BIT;
 
-		if (arm64_get_ssbd_state() == ARM64_SSBD_FORCE_DISABLE)
-			set_ssbs_bit(childregs);
+		spectre_v4_enable_task_mitigation(p);
 
 		if (system_uses_irq_prio_masking())
 			childregs->pmr_save = GIC_PRIO_IRQON;
@@ -472,8 +471,6 @@ void uao_thread_switch(struct task_struct *next)
  */
 static void ssbs_thread_switch(struct task_struct *next)
 {
-	struct pt_regs *regs = task_pt_regs(next);
-
 	/*
 	 * Nothing to do for kernel threads, but 'regs' may be junk
 	 * (e.g. idle task) so check the flags and bail early.
@@ -485,18 +482,10 @@ static void ssbs_thread_switch(struct task_struct *next)
 	 * If all CPUs implement the SSBS extension, then we just need to
 	 * context-switch the PSTATE field.
 	 */
-	if (cpu_have_feature(cpu_feature(SSBS)))
-		return;
-
-	/* If the mitigation is enabled, then we leave SSBS clear. */
-	if ((arm64_get_ssbd_state() == ARM64_SSBD_FORCE_ENABLE) ||
-	    test_tsk_thread_flag(next, TIF_SSBD))
+	if (cpus_have_const_cap(ARM64_SSBS))
 		return;
 
-	if (compat_user_mode(regs))
-		set_compat_ssbs_bit(regs);
-	else if (user_mode(regs))
-		set_ssbs_bit(regs);
+	spectre_v4_enable_task_mitigation(next);
 }
 
 /*

arch/arm64/kernel/proton-pack.c

@@ -320,79 +320,394 @@ void spectre_v2_enable_mitigation(const struct arm64_cpu_capabilities *__unused)
 	update_mitigation_state(&spectre_v2_state, state);
 }
 
-/* Spectre v4 prctl */
-static void ssbd_ssbs_enable(struct task_struct *task)
+/*
+ * Spectre v4.
+ *
+ * If you thought Spectre v2 was nasty, wait until you see this mess. A CPU is
+ * either:
+ *
+ * - Mitigated in hardware and listed in our "safe list".
+ * - Mitigated in hardware via PSTATE.SSBS.
+ * - Mitigated in software by firmware (sometimes referred to as SSBD).
+ *
+ * Wait, that doesn't sound so bad, does it? Keep reading...
+ *
+ * A major source of headaches is that the software mitigation is enabled both
+ * on a per-task basis, but can also be forced on for the kernel, necessitating
+ * both context-switch *and* entry/exit hooks. To make it even worse, some CPUs
+ * allow EL0 to toggle SSBS directly, which can end up with the prctl() state
+ * being stale when re-entering the kernel. The usual big.LITTLE caveats apply,
+ * so you can have systems that have both firmware and SSBS mitigations. This
+ * means we actually have to reject late onlining of CPUs with mitigations if
+ * all of the currently onlined CPUs are safelisted, as the mitigation tends to
+ * be opt-in for userspace. Yes, really, the cure is worse than the disease.
+ *
+ * The only good part is that if the firmware mitigation is present, then it is
+ * present for all CPUs, meaning we don't have to worry about late onlining of a
+ * vulnerable CPU if one of the boot CPUs is using the firmware mitigation.
+ *
+ * Give me a VAX-11/780 any day of the week...
+ */
+static enum mitigation_state spectre_v4_state;
+
+/* This is the per-cpu state tracking whether we need to talk to firmware */
+DEFINE_PER_CPU_READ_MOSTLY(u64, arm64_ssbd_callback_required);
+
+enum spectre_v4_policy {
+	SPECTRE_V4_POLICY_MITIGATION_DYNAMIC,
+	SPECTRE_V4_POLICY_MITIGATION_ENABLED,
+	SPECTRE_V4_POLICY_MITIGATION_DISABLED,
+};
+
+static enum spectre_v4_policy __read_mostly __spectre_v4_policy;
+
+static const struct spectre_v4_param {
+	const char		*str;
+	enum spectre_v4_policy	policy;
+} spectre_v4_params[] = {
+	{ "force-on",	SPECTRE_V4_POLICY_MITIGATION_ENABLED, },
+	{ "force-off",	SPECTRE_V4_POLICY_MITIGATION_DISABLED, },
+	{ "kernel",	SPECTRE_V4_POLICY_MITIGATION_DYNAMIC, },
+};
+static int __init parse_spectre_v4_param(char *str)
+{
+	int i;
+
+	if (!str || !str[0])
+		return -EINVAL;
+
+	for (i = 0; i < ARRAY_SIZE(spectre_v4_params); i++) {
+		const struct spectre_v4_param *param = &spectre_v4_params[i];
+
+		if (strncmp(str, param->str, strlen(param->str)))
+			continue;
+
+		__spectre_v4_policy = param->policy;
+		return 0;
+	}
+
+	return -EINVAL;
+}
+early_param("ssbd", parse_spectre_v4_param);
+
+/*
+ * Because this was all written in a rush by people working in different silos,
+ * we've ended up with multiple command line options to control the same thing.
+ * Wrap these up in some helpers, which prefer disabling the mitigation if faced
+ * with contradictory parameters. The mitigation is always either "off",
+ * "dynamic" or "on".
+ */
+static bool spectre_v4_mitigations_off(void)
+{
+	bool ret = cpu_mitigations_off() ||
+		   __spectre_v4_policy == SPECTRE_V4_POLICY_MITIGATION_DISABLED;
+
+	if (ret)
+		pr_info_once("spectre-v4 mitigation disabled by command-line option\n");
+
+	return ret;
+}
+
+/* Do we need to toggle the mitigation state on entry to/exit from the kernel? */
+static bool spectre_v4_mitigations_dynamic(void)
 {
-	u64 val = is_compat_thread(task_thread_info(task)) ?
-		  PSR_AA32_SSBS_BIT : PSR_SSBS_BIT;
+	return !spectre_v4_mitigations_off() &&
+	       __spectre_v4_policy == SPECTRE_V4_POLICY_MITIGATION_DYNAMIC;
+}
 
-	task_pt_regs(task)->pstate |= val;
+static bool spectre_v4_mitigations_on(void)
+{
+	return !spectre_v4_mitigations_off() &&
+	       __spectre_v4_policy == SPECTRE_V4_POLICY_MITIGATION_ENABLED;
 }
 
-static void ssbd_ssbs_disable(struct task_struct *task)
+ssize_t cpu_show_spec_store_bypass(struct device *dev,
+				   struct device_attribute *attr, char *buf)
 {
-	u64 val = is_compat_thread(task_thread_info(task)) ?
-		  PSR_AA32_SSBS_BIT : PSR_SSBS_BIT;
+	switch (spectre_v4_state) {
+	case SPECTRE_UNAFFECTED:
+		return sprintf(buf, "Not affected\n");
+	case SPECTRE_MITIGATED:
+		return sprintf(buf, "Mitigation: Speculative Store Bypass disabled via prctl\n");
+	case SPECTRE_VULNERABLE:
+		fallthrough;
+	default:
+		return sprintf(buf, "Vulnerable\n");
+	}
+}
 
-	task_pt_regs(task)->pstate &= ~val;
+enum mitigation_state arm64_get_spectre_v4_state(void)
+{
+	return spectre_v4_state;
+}
+
+static enum mitigation_state spectre_v4_get_cpu_hw_mitigation_state(void)
+{
+	static const struct midr_range spectre_v4_safe_list[] = {
+		MIDR_ALL_VERSIONS(MIDR_CORTEX_A35),
+		MIDR_ALL_VERSIONS(MIDR_CORTEX_A53),
+		MIDR_ALL_VERSIONS(MIDR_CORTEX_A55),
+		MIDR_ALL_VERSIONS(MIDR_BRAHMA_B53),
+		MIDR_ALL_VERSIONS(MIDR_QCOM_KRYO_3XX_SILVER),
+		MIDR_ALL_VERSIONS(MIDR_QCOM_KRYO_4XX_SILVER),
+		{ /* sentinel */ },
+	};
+
+	if (is_midr_in_range_list(read_cpuid_id(), spectre_v4_safe_list))
+		return SPECTRE_UNAFFECTED;
+
+	/* CPU features are detected first */
+	if (this_cpu_has_cap(ARM64_SSBS))
+		return SPECTRE_MITIGATED;
+
+	return SPECTRE_VULNERABLE;
+}
+
+static enum mitigation_state spectre_v4_get_cpu_fw_mitigation_state(void)
+{
+	int ret;
+	struct arm_smccc_res res;
+
+	arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
+			     ARM_SMCCC_ARCH_WORKAROUND_2, &res);
+
+	ret = res.a0;
+	switch (ret) {
+	case SMCCC_RET_SUCCESS:
+		return SPECTRE_MITIGATED;
+	case SMCCC_ARCH_WORKAROUND_RET_UNAFFECTED:
+		fallthrough;
+	case SMCCC_RET_NOT_REQUIRED:
+		return SPECTRE_UNAFFECTED;
+	default:
+		fallthrough;
+	case SMCCC_RET_NOT_SUPPORTED:
+		return SPECTRE_VULNERABLE;
+	}
+}
+
+bool has_spectre_v4(const struct arm64_cpu_capabilities *cap, int scope)
+{
+	enum mitigation_state state;
+
+	WARN_ON(scope != SCOPE_LOCAL_CPU || preemptible());
+
+	state = spectre_v4_get_cpu_hw_mitigation_state();
+	if (state == SPECTRE_VULNERABLE)
+		state = spectre_v4_get_cpu_fw_mitigation_state();
+
+	return state != SPECTRE_UNAFFECTED;
+}
+
+static int ssbs_emulation_handler(struct pt_regs *regs, u32 instr)
+{
+	if (user_mode(regs))
+		return 1;
+
+	if (instr & BIT(PSTATE_Imm_shift))
+		regs->pstate |= PSR_SSBS_BIT;
+	else
+		regs->pstate &= ~PSR_SSBS_BIT;
+
+	arm64_skip_faulting_instruction(regs, 4);
+	return 0;
+}
+
+static struct undef_hook ssbs_emulation_hook = {
+	.instr_mask	= ~(1U << PSTATE_Imm_shift),
+	.instr_val	= 0xd500401f | PSTATE_SSBS,
+	.fn		= ssbs_emulation_handler,
+};
+
+static enum mitigation_state spectre_v4_enable_hw_mitigation(void)
+{
+	static bool undef_hook_registered = false;
+	static DEFINE_RAW_SPINLOCK(hook_lock);
+	enum mitigation_state state;
+
+	/*
+	 * If the system is mitigated but this CPU doesn't have SSBS, then
+	 * we must be on the safelist and there's nothing more to do.
+	 */
+	state = spectre_v4_get_cpu_hw_mitigation_state();
+	if (state != SPECTRE_MITIGATED || !this_cpu_has_cap(ARM64_SSBS))
+		return state;
+
+	raw_spin_lock(&hook_lock);
+	if (!undef_hook_registered) {
+		register_undef_hook(&ssbs_emulation_hook);
+		undef_hook_registered = true;
+	}
+	raw_spin_unlock(&hook_lock);
+
+	if (spectre_v4_mitigations_off()) {
+		sysreg_clear_set(sctlr_el1, 0, SCTLR_ELx_DSSBS);
+		asm volatile(SET_PSTATE_SSBS(1));
+		return SPECTRE_VULNERABLE;
+	}
+
+	/* SCTLR_EL1.DSSBS was initialised to 0 during boot */
+	asm volatile(SET_PSTATE_SSBS(0));
+	return SPECTRE_MITIGATED;
 }
 
 /*
- * prctl interface for SSBD
+ * Patch a branch over the Spectre-v4 mitigation code with a NOP so that
+ * we fallthrough and check whether firmware needs to be called on this CPU.
  */
-static int ssbd_prctl_set(struct task_struct *task, unsigned long ctrl)
+void __init spectre_v4_patch_fw_mitigation_enable(struct alt_instr *alt,
+						  __le32 *origptr,
+						  __le32 *updptr, int nr_inst)
 {
-	int state = arm64_get_ssbd_state();
+	BUG_ON(nr_inst != 1); /* Branch -> NOP */
 
-	/* Unsupported */
-	if (state == ARM64_SSBD_UNKNOWN)
-		return -ENODEV;
+	if (spectre_v4_mitigations_off())
+		return;
 
-	/* Treat the unaffected/mitigated state separately */
-	if (state == ARM64_SSBD_MITIGATED) {
-		switch (ctrl) {
-		case PR_SPEC_ENABLE:
-			return -EPERM;
-		case PR_SPEC_DISABLE:
-		case PR_SPEC_FORCE_DISABLE:
-			return 0;
+	if (cpus_have_final_cap(ARM64_SSBS))
+		return;
+
+	if (spectre_v4_mitigations_dynamic())
+		*updptr = cpu_to_le32(aarch64_insn_gen_nop());
+}
+
+/*
+ * Patch a NOP in the Spectre-v4 mitigation code with an SMC/HVC instruction
+ * to call into firmware to adjust the mitigation state.
+ */
+void __init spectre_v4_patch_fw_mitigation_conduit(struct alt_instr *alt,
+						   __le32 *origptr,
+						   __le32 *updptr, int nr_inst)
+{
+	u32 insn;
+
+	BUG_ON(nr_inst != 1); /* NOP -> HVC/SMC */
+
+	switch (arm_smccc_1_1_get_conduit()) {
+	case SMCCC_CONDUIT_HVC:
+		insn = aarch64_insn_get_hvc_value();
+		break;
+	case SMCCC_CONDUIT_SMC:
+		insn = aarch64_insn_get_smc_value();
+		break;
+	default:
+		return;
 	}
+
+	*updptr = cpu_to_le32(insn);
+}
+
+static enum mitigation_state spectre_v4_enable_fw_mitigation(void)
+{
+	enum mitigation_state state;
+
+	state = spectre_v4_get_cpu_fw_mitigation_state();
+	if (state != SPECTRE_MITIGATED)
+		return state;
+
+	if (spectre_v4_mitigations_off()) {
+		arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_WORKAROUND_2, false, NULL);
+		return SPECTRE_VULNERABLE;
 	}
 
-	/*
-	 * Things are a bit backward here: the arm64 internal API
-	 * *enables the mitigation* when the userspace API *disables
-	 * speculation*. So much fun.
+	arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_WORKAROUND_2, true, NULL);
+
+	if (spectre_v4_mitigations_dynamic())
+		__this_cpu_write(arm64_ssbd_callback_required, 1);
+
+	return SPECTRE_MITIGATED;
+}
+
+void spectre_v4_enable_mitigation(const struct arm64_cpu_capabilities *__unused)
+{
+	enum mitigation_state state;
+
+	WARN_ON(preemptible());
+
+	state = spectre_v4_enable_hw_mitigation();
+	if (state == SPECTRE_VULNERABLE)
+		state = spectre_v4_enable_fw_mitigation();
+
+	update_mitigation_state(&spectre_v4_state, state);
+}
+
+static void __update_pstate_ssbs(struct pt_regs *regs, bool state)
+{
+	u64 bit = compat_user_mode(regs) ? PSR_AA32_SSBS_BIT : PSR_SSBS_BIT;
+
+	if (state)
+		regs->pstate |= bit;
+	else
+		regs->pstate &= ~bit;
+}
+
+void spectre_v4_enable_task_mitigation(struct task_struct *tsk)
+{
+	struct pt_regs *regs = task_pt_regs(tsk);
+	bool ssbs = false, kthread = tsk->flags & PF_KTHREAD;
+
+	if (spectre_v4_mitigations_off())
+		ssbs = true;
+	else if (spectre_v4_mitigations_dynamic() && !kthread)
+		ssbs = !test_tsk_thread_flag(tsk, TIF_SSBD);
+
+	__update_pstate_ssbs(regs, ssbs);
+}
+
+/*
+ * The Spectre-v4 mitigation can be controlled via a prctl() from userspace.
+ * This is interesting because the "speculation disabled" behaviour can be
+ * configured so that it is preserved across exec(), which means that the
+ * prctl() may be necessary even when PSTATE.SSBS can be toggled directly
+ * from userspace.
  */
+static int ssbd_prctl_set(struct task_struct *task, unsigned long ctrl)
+{
 	switch (ctrl) {
 	case PR_SPEC_ENABLE:
-		/* If speculation is force disabled, enable is not allowed */
-		if (state == ARM64_SSBD_FORCE_ENABLE ||
-		    task_spec_ssb_force_disable(task))
+		/* Enable speculation: disable mitigation */
+		/*
+		 * Force disabled speculation prevents it from being
+		 * re-enabled.
+		 */
+		if (task_spec_ssb_force_disable(task))
 			return -EPERM;
+
+		/*
+		 * If the mitigation is forced on, then speculation is forced
+		 * off and we again prevent it from being re-enabled.
+		 */
+		if (spectre_v4_mitigations_on())
+			return -EPERM;
+
 		task_clear_spec_ssb_disable(task);
 		clear_tsk_thread_flag(task, TIF_SSBD);
-		ssbd_ssbs_enable(task);
-		break;
-	case PR_SPEC_DISABLE:
-		if (state == ARM64_SSBD_FORCE_DISABLE)
-			return -EPERM;
-		task_set_spec_ssb_disable(task);
-		set_tsk_thread_flag(task, TIF_SSBD);
-		ssbd_ssbs_disable(task);
 		break;
 	case PR_SPEC_FORCE_DISABLE:
-		if (state == ARM64_SSBD_FORCE_DISABLE)
+		/* Force disable speculation: force enable mitigation */
+		/*
+		 * If the mitigation is forced off, then speculation is forced
+		 * on and we prevent it from being disabled.
+		 */
+		if (spectre_v4_mitigations_off())
 			return -EPERM;
-		task_set_spec_ssb_disable(task);
+
 		task_set_spec_ssb_force_disable(task);
+		fallthrough;
+	case PR_SPEC_DISABLE:
+		/* Disable speculation: enable mitigation */
+		/* Same as PR_SPEC_FORCE_DISABLE */
+		if (spectre_v4_mitigations_off())
+			return -EPERM;
+
+		task_set_spec_ssb_disable(task);
 		set_tsk_thread_flag(task, TIF_SSBD);
-		ssbd_ssbs_disable(task);
 		break;
 	default:
 		return -ERANGE;
 	}
 
+	spectre_v4_enable_task_mitigation(task);
 	return 0;
 }
 
@@ -409,22 +724,32 @@ int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
 
 static int ssbd_prctl_get(struct task_struct *task)
 {
-	switch (arm64_get_ssbd_state()) {
-	case ARM64_SSBD_UNKNOWN:
-		return -ENODEV;
-	case ARM64_SSBD_FORCE_ENABLE:
-		return PR_SPEC_DISABLE;
-	case ARM64_SSBD_KERNEL:
+	switch (spectre_v4_state) {
+	case SPECTRE_UNAFFECTED:
+		return PR_SPEC_NOT_AFFECTED;
+	case SPECTRE_MITIGATED:
+		if (spectre_v4_mitigations_on())
+			return PR_SPEC_NOT_AFFECTED;
+
+		if (spectre_v4_mitigations_dynamic())
+			break;
+
+		/* Mitigations are disabled, so we're vulnerable. */
+		fallthrough;
+	case SPECTRE_VULNERABLE:
+		fallthrough;
+	default:
+		return PR_SPEC_ENABLE;
+	}
+
+	/* Check the mitigation state for this task */
 	if (task_spec_ssb_force_disable(task))
 		return PR_SPEC_PRCTL | PR_SPEC_FORCE_DISABLE;
+
 	if (task_spec_ssb_disable(task))
 		return PR_SPEC_PRCTL | PR_SPEC_DISABLE;
+
 	return PR_SPEC_PRCTL | PR_SPEC_ENABLE;
-	case ARM64_SSBD_FORCE_DISABLE:
-		return PR_SPEC_ENABLE;
-	default:
-		return PR_SPEC_NOT_AFFECTED;
-	}
 }
 
 int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which)

arch/arm64/kernel/suspend.c

@@ -72,8 +72,7 @@ void notrace __cpu_suspend_exit(void)
 	 * have turned the mitigation on. If the user has forcefully
 	 * disabled it, make sure their wishes are obeyed.
 	 */
-	if (arm64_get_ssbd_state() == ARM64_SSBD_FORCE_DISABLE)
-		arm64_set_ssbd_mitigation(false);
+	spectre_v4_enable_mitigation(NULL);
 }
 
 /*