[MODERATED] [PATCH SSBv11 3/3] seccomp 0

From: Kees Cook <keescook@chromium.org>
To: speck@linutronix.de
Subject: [MODERATED] [PATCH SSBv11 3/3] seccomp 0
Date: Tue, 1 May 2018 15:07:31 -0700	[thread overview]
Message-ID: <=?utf-8?q?=3C5f6464d53346d3939a81cab15e5d5b20ad90c05d=2E152530?= =?utf-8?q?8267=2Egit=2Ekeescook=40chromium=2Eorg=3E?=> (raw)
In-Reply-To: <cover.1525308267.git.keescook@chromium.org>

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>
---
 kernel/seccomp.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index dc77548167ef..88933d05e81b 100644
--- 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)
 {
@@ -239,6 +254,9 @@ static inline void seccomp_assign_mode(struct task_struct *task,
 	 */
 	smp_mb__before_atomic();
 	set_tsk_thread_flag(task, TIF_SECCOMP);
+
+	/* Assume seccomp processes want speculation flaw mitigation. */
+	spec_mitigate(task, PR_SPEC_STORE_BYPASS);
 }
 
 #ifdef CONFIG_SECCOMP_FILTER
-- 
2.17.0
