[PATCH 3.16 019/131] seccomp: Enable speculation flaw mitigations

[Ben Hutchings]

3.16.59-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Kees Cook <keescook@xxxxxxxxxxxx>

commit 5c3070890d06ff82eecb808d02d2ca39169533ef upstream.

When speculation flaw mitigations are opt-in (via prctl), using seccomp
will automatically opt-in to these protections, since using seccomp
indicates at least some level of sandboxing is desired.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
[bwh: Backported to 3.16:
 - Apply to current task
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 kernel/seccomp.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -16,6 +16,8 @@
 #include <linux/atomic.h>
 #include <linux/audit.h>
 #include <linux/compat.h>
+#include <linux/nospec.h>
+#include <linux/prctl.h>
 #include <linux/sched.h>
 #include <linux/seccomp.h>
 #include <linux/syscalls.h>
@@ -205,9 +207,24 @@ static inline bool seccomp_may_assign_mo
 	return true;
 }
 
+/*
+ * If a given speculation mitigation is opt-in (prctl()-controlled),
+ * select it, by disabling speculation (enabling mitigation).
+ */
+static inline void spec_mitigate(struct task_struct *task,
+				 unsigned long which)
+{
+	int state = arch_prctl_spec_ctrl_get(task, which);
+
+	if (state > 0 && (state & PR_SPEC_PRCTL))
+		arch_prctl_spec_ctrl_set(task, which, PR_SPEC_DISABLE);
+}
+
 static inline void seccomp_assign_mode(unsigned long seccomp_mode)
 {
 	current->seccomp.mode = seccomp_mode;
+	/* Assume seccomp processes want speculation flaw mitigation. */
+	spec_mitigate(current, PR_SPEC_STORE_BYPASS);
 	set_tsk_thread_flag(current, TIF_SECCOMP);
 }