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.

Signed-off-by: Marc Zyngier <redacted>
---
 arch/arm64/kernel/Makefile |   1 +
 arch/arm64/kernel/ssbd.c   | 107 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 108 insertions(+)
 create mode 100644 arch/arm64/kernel/ssbd.c

diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index bf825f38d206..0025f8691046 100644
--- a/arch/arm64/kernel/Makefile
+++ b/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)

diff --git a/arch/arm64/kernel/ssbd.c b/arch/arm64/kernel/ssbd.c
new file mode 100644
index 000000000000..34e3c430176b
--- /dev/null
+++ b/arch/arm64/kernel/ssbd.c

@@ -0,0 +1,107 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2018 ARM Ltd, All Rights Reserved.
+ */
+
+#include <linux/sched.h>
+#include <linux/thread_info.h>
+
+#include <asm/cpufeature.h>
+
+/*
+ * prctl interface for SSBD
+ * FIXME: Drop the below ifdefery once the common interface has been merged.
+ */
+#ifdef PR_SPEC_STORE_BYPASS
+static int ssbd_prctl_set(struct task_struct *task, unsigned long ctrl)
+{
+	int state = arm64_get_ssbd_state();
+
+	/* Unsupported or already mitigated */
+	if (state == ARM64_SSBD_UNKNOWN)
+		return -EINVAL;
+	if (state == ARM64_SSBD_MITIGATED)
+		return -EPERM;