seccomp: Enable speculation flaw mitigations

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@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
CVE-2018-3639 (x86)

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 5f0dfb2abb8d39542d72003f860e25eaff66c8f0..55f20aee7ea23a5a9497bc41800fcdc820ef468e 100644 (file)
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -19,6 +19,8 @@
 #include <linux/compat.h>
 #include <linux/coredump.h>
 #include <linux/kmemleak.h>
+#include <linux/nospec.h>
+#include <linux/prctl.h>
 #include <linux/sched.h>
 #include <linux/sched/task_stack.h>
 #include <linux/seccomp.h>
@@ -227,6 +229,19 @@ static inline bool seccomp_may_assign_mode(unsigned long seccomp_mode)
        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(struct task_struct *task,
                                       unsigned long seccomp_mode)
 {
@@ -238,6 +253,8 @@ static inline void seccomp_assign_mode(struct task_struct *task,
         * filter) is set.
         */
        smp_mb__before_atomic();
+       /* Assume seccomp processes want speculation flaw mitigation. */
+       spec_mitigate(task, PR_SPEC_STORE_BYPASS);
        set_tsk_thread_flag(task, TIF_SECCOMP);
 }