arm64: ssbd: Add prctl interface for per-thread mitigation

If running on a system that performs dynamic SSBD mitigation, allow
userspace to request the mitigation for itself. This is implemented
as a prctl call, allowing the mitigation to be enabled or disabled at
will for this particular thread.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/Makefile

@@ -54,6 +54,7 @@ arm64-obj-$(CONFIG_ARM64_RELOC_TEST)	+= arm64-reloc-test.o
 arm64-reloc-test-y := reloc_test_core.o reloc_test_syms.o
 arm64-obj-$(CONFIG_CRASH_DUMP)		+= crash_dump.o
 arm64-obj-$(CONFIG_ARM_SDE_INTERFACE)	+= sdei.o
+arm64-obj-$(CONFIG_ARM64_SSBD)		+= ssbd.o
 
 obj-y					+= $(arm64-obj-y) vdso/ probes/
 obj-m					+= $(arm64-obj-m)

arch/arm64/kernel/ssbd.c

@@ -0,0 +1,110 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2018 ARM Ltd, All Rights Reserved.
+ */
+
+#include <linux/errno.h>
+#include <linux/sched.h>
+#include <linux/thread_info.h>
+
+#include <asm/cpufeature.h>
+
+/*
+ * prctl interface for SSBD
+ * FIXME: Drop the below ifdefery once merged in 4.18.
+ */
+#ifdef PR_SPEC_STORE_BYPASS
+static int ssbd_prctl_set(struct task_struct *task, unsigned long ctrl)
+{
+	int state = arm64_get_ssbd_state();
+
+	/* Unsupported */
+	if (state == ARM64_SSBD_UNKNOWN)
+		return -EINVAL;
+
+	/* 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;
+		}
+	}
+
+	/*
+	 * Things are a bit backward here: the arm64 internal API
+	 * *enables the mitigation* when the userspace API *disables
+	 * speculation*. So much fun.
+	 */
+	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))
+			return -EPERM;
+		task_clear_spec_ssb_disable(task);
+		clear_tsk_thread_flag(task, TIF_SSBD);
+		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);
+		break;
+	case PR_SPEC_FORCE_DISABLE:
+		if (state == ARM64_SSBD_FORCE_DISABLE)
+			return -EPERM;
+		task_set_spec_ssb_disable(task);
+		task_set_spec_ssb_force_disable(task);
+		set_tsk_thread_flag(task, TIF_SSBD);
+		break;
+	default:
+		return -ERANGE;
+	}
+
+	return 0;
+}
+
+int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
+			     unsigned long ctrl)
+{
+	switch (which) {
+	case PR_SPEC_STORE_BYPASS:
+		return ssbd_prctl_set(task, ctrl);
+	default:
+		return -ENODEV;
+	}
+}
+
+static int ssbd_prctl_get(struct task_struct *task)
+{
+	switch (arm64_get_ssbd_state()) {
+	case ARM64_SSBD_UNKNOWN:
+		return -EINVAL;
+	case ARM64_SSBD_FORCE_ENABLE:
+		return PR_SPEC_DISABLE;
+	case ARM64_SSBD_KERNEL:
+		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)
+{
+	switch (which) {
+	case PR_SPEC_STORE_BYPASS:
+		return ssbd_prctl_get(task);
+	default:
+		return -ENODEV;
+	}
+}
+#endif	/* PR_SPEC_STORE_BYPASS */