[Patch v7 16/18] 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.c...@linux.intel.com>
---
 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