commit 00a02d0c502a06d15e07b857f8ff921e3e402675
author Kees Cook <keescook@chromium.org> Thu May 03 14:56:12 2018 -0700
committer Thomas Gleixner <tglx@linutronix.de> Sat May 05 00:51:44 2018 +0200
tree d1f2bd8ff6733e43840a199d11e455790d7c21c8
parent b849a812f7eb92e96d1c8239b06581b2cfd8b275

seccomp: Add filter flag to opt-out of SSB mitigation

If a seccomp user is not interested in Speculative Store Bypass mitigation
by default, it can set the new SECCOMP_FILTER_FLAG_SPEC_ALLOW flag when
adding filters.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/seccomp.c

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 2c819d6..53eb946 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -243,7 +243,8 @@ static inline void spec_mitigate(struct task_struct *task,
 }
 
 static inline void seccomp_assign_mode(struct task_struct *task,
-				       unsigned long seccomp_mode)
+				       unsigned long seccomp_mode,
+				       unsigned long flags)
 {
 	assert_spin_locked(&task->sighand->siglock);
 
@@ -253,8 +254,9 @@ 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);
+	/* Assume default seccomp processes want spec flaw mitigation. */
+	if ((flags & SECCOMP_FILTER_FLAG_SPEC_ALLOW) == 0)
+		spec_mitigate(task, PR_SPEC_STORE_BYPASS);
 	set_tsk_thread_flag(task, TIF_SECCOMP);
 }
 
@@ -322,7 +324,7 @@ static inline pid_t seccomp_can_sync_threads(void)
  * without dropping the locks.
  *
  */
-static inline void seccomp_sync_threads(void)
+static inline void seccomp_sync_threads(unsigned long flags)
 {
 	struct task_struct *thread, *caller;
 
@@ -363,7 +365,8 @@ static inline void seccomp_sync_threads(void)
 		 * allow one thread to transition the other.
 		 */
 		if (thread->seccomp.mode == SECCOMP_MODE_DISABLED)
-			seccomp_assign_mode(thread, SECCOMP_MODE_FILTER);
+			seccomp_assign_mode(thread, SECCOMP_MODE_FILTER,
+					    flags);
 	}
 }
 
@@ -486,7 +489,7 @@ static long seccomp_attach_filter(unsigned int flags,
 
 	/* Now that the new filter is in place, synchronize to all threads. */
 	if (flags & SECCOMP_FILTER_FLAG_TSYNC)
-		seccomp_sync_threads();
+		seccomp_sync_threads(flags);
 
 	return 0;
 }
@@ -835,7 +838,7 @@ static long seccomp_set_mode_strict(void)
 #ifdef TIF_NOTSC
 	disable_TSC();
 #endif
-	seccomp_assign_mode(current, seccomp_mode);
+	seccomp_assign_mode(current, seccomp_mode, 0);
 	ret = 0;
 
 out:
@@ -893,7 +896,7 @@ static long seccomp_set_mode_filter(unsigned int flags,
 	/* Do not free the successfully attached filter. */
 	prepared = NULL;
 
-	seccomp_assign_mode(current, seccomp_mode);
+	seccomp_assign_mode(current, seccomp_mode, flags);
 out:
 	spin_unlock_irq(&current->sighand->siglock);
 	if (flags & SECCOMP_FILTER_FLAG_TSYNC)