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Subject[PATCH 4.14 87/92] arm64: ssbd: Add prctl interface for per-thread mitigation
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Marc Zyngier <marc.zyngier@arm.com>

commit 9cdc0108baa8ef87c76ed834619886a46bd70cbe upstream.

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>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/arm64/kernel/Makefile | 1
arch/arm64/kernel/ssbd.c | 108 +++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 109 insertions(+)
create mode 100644 arch/arm64/kernel/ssbd.c

--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -54,6 +54,7 @@ arm64-obj-$(CONFIG_KEXEC) += machine_ke
arm64-obj-$(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_ARM64_SSBD) += ssbd.o

ifeq ($(CONFIG_KVM),y)
arm64-obj-$(CONFIG_HARDEN_BRANCH_PREDICTOR) += bpi.o
--- /dev/null
+++ b/arch/arm64/kernel/ssbd.c
@@ -0,0 +1,108 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2018 ARM Ltd, All Rights Reserved.
+ */
+
+#include <linux/errno.h>
+#include <linux/prctl.h>
+#include <linux/sched.h>
+#include <linux/thread_info.h>
+
+#include <asm/cpufeature.h>
+
+/*
+ * prctl interface for SSBD
+ */
+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;
+ }
+}

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