[PATCH v2] PR_SPEC_DISABLE_NOEXEC support for arm64.

[Anthony Steinhauser <asteinhauser@google.com>]

Support of Spectre v4 PR_SPEC_DISABLE_NOEXEC mitigation mode for on arm64.

PR_SPEC_DISABLE_NOEXEC turns the mitigation on, but it is automatically
turned off whenever a new program is being execve'ed.

Signed-off-by: Anthony Steinhauser <asteinhauser@google.com>
---

I added the "#include <linux/sched/task_stack.h>" line to the
arch/arm64/kernel/proton-pack.c file just to make the kernel compilable.
It is not a part of the PR_SPEC_DISABLE_NOEXEC implementation.

 arch/arm64/kernel/process.c     |  7 +++++++
 arch/arm64/kernel/proton-pack.c | 35 +++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 9dbd35b95253..5ac43b743696 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -391,6 +391,13 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
 
 	ptrauth_thread_init_kernel(p);
 
+	if (task_spec_ssb_noexec(current)) {
+		clear_thread_flag(TIF_SSBD);
+		task_clear_spec_ssb_disable(current);
+		task_clear_spec_ssb_noexec(current);
+		spectre_v4_enable_task_mitigation(current);
+	}
+
 	if (likely(!(p->flags & PF_KTHREAD))) {
 		*childregs = *current_pt_regs();
 		childregs->regs[0] = 0;
diff --git a/arch/arm64/kernel/proton-pack.c b/arch/arm64/kernel/proton-pack.c
index b1ea935fd948..566c2304bba7 100644
--- a/arch/arm64/kernel/proton-pack.c
+++ b/arch/arm64/kernel/proton-pack.c
@@ -22,6 +22,7 @@
 #include <linux/device.h>
 #include <linux/nospec.h>
 #include <linux/prctl.h>
+#include <linux/sched/task_stack.h>
 
 #include <asm/spectre.h>
 #include <asm/traps.h>
@@ -681,6 +682,7 @@ static int ssbd_prctl_set(struct task_struct *task, unsigned long ctrl)
 			return -EPERM;
 
 		task_clear_spec_ssb_disable(task);
+		task_clear_spec_ssb_noexec(task);
 		clear_tsk_thread_flag(task, TIF_SSBD);
 		break;
 	case PR_SPEC_FORCE_DISABLE:
@@ -701,6 +703,36 @@ static int ssbd_prctl_set(struct task_struct *task, unsigned long ctrl)
 			return -EPERM;
 
 		task_set_spec_ssb_disable(task);
+		task_clear_spec_ssb_noexec(task);
+		set_tsk_thread_flag(task, TIF_SSBD);
+		break;
+	case PR_SPEC_DISABLE_NOEXEC:
+		/* Disable speculation (enable mitigation), but don't inherit
+		 * the mitigation when a new program is execve'd.
+		 *
+		 * Force disabled speculation prevents it from being
+		 * re-enabled even after exec.
+		 */
+		if (task_spec_ssb_force_disable(task))
+			return -EPERM;
+
+		/*
+		 * If the mitigation is forced on, then speculation is forced
+		 * of unconditionally and we revent it from being
+		 * re-enabled even after exec.
+		 */
+		if (spectre_v4_mitigations_on())
+			return -EPERM;
+
+		/*
+		 * 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_noexec(task);
 		set_tsk_thread_flag(task, TIF_SSBD);
 		break;
 	default:
@@ -746,6 +778,9 @@ static int ssbd_prctl_get(struct task_struct *task)
 	if (task_spec_ssb_force_disable(task))
 		return PR_SPEC_PRCTL | PR_SPEC_FORCE_DISABLE;
 
+	if (task_spec_ssb_noexec(task))
+		return PR_SPEC_PRCTL | PR_SPEC_DISABLE_NOEXEC;
+
 	if (task_spec_ssb_disable(task))
 		return PR_SPEC_PRCTL | PR_SPEC_DISABLE;
 
-- 
2.18.4