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Subject[PATCH v2] PR_SPEC_DISABLE_NOEXEC support for arm64.
From
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
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