seccomp: release task filters when the task exits

Previously, seccomp filters were released in release_task(), which
required the process to exit and its zombie to be collected. However,
exited threads/processes can't trigger any seccomp events, making it
more logical to release filters upon task exits.

This adjustment simplifies scenarios where a parent is tracing its child
process. The parent process can now handle all events from a seccomp
listening descriptor and then call wait to collect a child zombie.

seccomp_filter_release takes the siglock to avoid races with
seccomp_sync_threads. There was an idea to bypass taking the lock by
checking PF_EXITING, but it can be set without holding siglock if
threads have SIGNAL_GROUP_EXIT. This means it can happen concurently
with seccomp_filter_release.

This change also fixes another minor problem. Suppose that a group
leader installs the new filter without SECCOMP_FILTER_FLAG_TSYNC, exits,
and becomes a zombie. Without this change, SECCOMP_FILTER_FLAG_TSYNC
from any other thread can never succeed, seccomp_can_sync_threads() will
check a zombie leader and is_ancestor() will fail.
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrei Vagin <avagin@google.com>
Link: https://lore.kernel.org/r/20240628021014.231976-3-avagin@google.comReviewed-by: Tycho Andersen <tandersen@netflix.com>
Signed-off-by: Kees Cook <kees@kernel.org>

kernel/exit.c

@@ -277,7 +277,6 @@ void release_task(struct task_struct *p)
 	}
 
 	write_unlock_irq(&tasklist_lock);
-	seccomp_filter_release(p);
 	proc_flush_pid(thread_pid);
 	put_pid(thread_pid);
 	release_thread(p);
@@ -832,6 +831,8 @@ void __noreturn do_exit(long code)
 	io_uring_files_cancel();
 	exit_signals(tsk);  /* sets PF_EXITING */
 
+	seccomp_filter_release(tsk);
+
 	acct_update_integrals(tsk);
 	group_dead = atomic_dec_and_test(&tsk->signal->live);
 	if (group_dead) {

kernel/seccomp.c

@@ -502,6 +502,9 @@ static inline pid_t seccomp_can_sync_threads(void)
 		/* Skip current, since it is initiating the sync. */
 		if (thread == caller)
 			continue;
+		/* Skip exited threads. */
+		if (thread->flags & PF_EXITING)
+			continue;
 
 		if (thread->seccomp.mode == SECCOMP_MODE_DISABLED ||
 		    (thread->seccomp.mode == SECCOMP_MODE_FILTER &&
@@ -563,18 +566,21 @@ static void __seccomp_filter_release(struct seccomp_filter *orig)
  * @tsk: task the filter should be released from.
  *
  * This function should only be called when the task is exiting as
- * it detaches it from its filter tree. As such, READ_ONCE() and
- * barriers are not needed here, as would normally be needed.
+ * it detaches it from its filter tree. PF_EXITING has to be set
+ * for the task.
  */
 void seccomp_filter_release(struct task_struct *tsk)
 {
-	struct seccomp_filter *orig = tsk->seccomp.filter;
+	struct seccomp_filter *orig;
 
-	/* We are effectively holding the siglock by not having any sighand. */
-	WARN_ON(tsk->sighand != NULL);
+	if (WARN_ON((tsk->flags & PF_EXITING) == 0))
+		return;
 
+	spin_lock_irq(&tsk->sighand->siglock);
+	orig = tsk->seccomp.filter;
 	/* Detach task from its filter tree. */
 	tsk->seccomp.filter = NULL;
+	spin_unlock_irq(&tsk->sighand->siglock);
 	__seccomp_filter_release(orig);
 }
 
@@ -602,6 +608,13 @@ static inline void seccomp_sync_threads(unsigned long flags)
 		if (thread == caller)
 			continue;
 
+		/*
+		 * Skip exited threads. seccomp_filter_release could have
+		 * been already called for this task.
+		 */
+		if (thread->flags & PF_EXITING)
+			continue;
+
 		/* Get a task reference for the new leaf node. */
 		get_seccomp_filter(caller);