[Patch v6 14/16] x86/speculation: Use STIBP to restrict speculation on non-dumpable task

[Tim Chen]

When a task changes its dumpability, arch_update_spec_ctrl_restriction()
is called to place restriction on the task's speculative execution
according to dumpability changes.

Implements arch_update_spec_restriction() for x86.  Use STIBP to
restrict speculative execution when running a task set to non-dumpable,
or clear the restriction if the task is set to dumpable.

Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt |  3 ++-
 arch/x86/kernel/cpu/bugs.c                      | 23 ++++++++++++++++++++---
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 89b193c..3979b12 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4229,7 +4229,8 @@
 				  If the CPU is vulnerable, the default mitigation
 				  is architecture and Kconfig dependent. See below.
 			prctl   - Enable mitigations per thread by restricting
-				  indirect branch speculation via prctl.
+				  indirect branch speculation via prctl or setting
+				  the thread as non-dumpable.
 				  Mitigation for a thread is not enabled by default to
 				  avoid mitigation overhead. The state of
 				  of the control is inherited on fork.
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index f349b3f..6cd64445 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -14,6 +14,7 @@
 #include <linux/module.h>
 #include <linux/nospec.h>
 #include <linux/prctl.h>
+#include <linux/coredump.h>
 
 #include <asm/spec-ctrl.h>
 #include <asm/cmdline.h>
@@ -153,8 +154,8 @@ static const char *spectre_v2_strings[] = {
 static const char *spectre_v2_app2app_strings[] = {
 	[SPECTRE_V2_APP2APP_NONE]    = "App-App Vulnerable",
 	[SPECTRE_V2_APP2APP_STRICT]  = "App-App Mitigation: Full app to app attack protection",
-	[SPECTRE_V2_APP2APP_PRCTL]   = "App-App Mitigation: Protect branch speculation restricted tasks",
-	[SPECTRE_V2_APP2APP_SECCOMP] = "App-App Mitigation: Protect branch speculation restricted and seccomp tasks",
+	[SPECTRE_V2_APP2APP_PRCTL]   = "App-App Mitigation: Protect non-dumpable and branch speculation restricted tasks",
+	[SPECTRE_V2_APP2APP_SECCOMP] = "App-App Mitigation: Protect non-dumpable, branch speculation restricted and seccomp tasks",
 };
 
 /* Lightweight mitigation: mitigate only tasks with TIF_SPEC_INDIR_BRANCH */
@@ -792,13 +793,29 @@ static void set_task_restrict_indir_branch(struct task_struct *tsk, bool restric
 
 	if (restrict_on)
 		update = !test_and_set_tsk_thread_flag(tsk, TIF_SPEC_INDIR_BRANCH);
-	else
+	else if (!task_spec_indir_branch_disable(tsk))
 		update = test_and_clear_tsk_thread_flag(tsk, TIF_SPEC_INDIR_BRANCH);
 
 	if (tsk == current && update)
 		speculation_ctrl_update_current();
 }
 
+int arch_update_spec_restriction(struct task_struct *task)
+{
+	if (!static_branch_unlikely(&spectre_v2_app_lite))
+		return 0;
+
+	if (!task->mm)
+		return -EINVAL;
+
+	if (get_dumpable(task->mm) != SUID_DUMP_USER)
+		set_task_restrict_indir_branch(task, true);
+	else
+		set_task_restrict_indir_branch(task, false);
+
+	return 0;
+}
+
 static int indir_branch_prctl_set(struct task_struct *task, unsigned long ctrl)
 {
 	switch (ctrl) {
-- 
2.9.4