[PATCH v2 5/5] pid_namespace: prevent TIF_NOTIFY_SIGNAL from interrupting the reaper

[Christian Brauner <brauner@kernel.org>]

Since commit 7fea700e04bd ("zap_pid_ns_processes: clear
TIF_NOTIFY_SIGNAL along with TIF_SIGPENDING") zap_pid_ns_processes()
clears TIF_NOTIFY_SIGNAL before every kernel_wait4() call. Otherwise
kernel_wait4() doesn't sleep and returns -ERESTARTSYS if
signal_pending() is true and the reaper busy-waits.

But clearing TIF_NOTIFY_SIGNAL doesn't fix it. It can get raised again.
For example, klp_send_signals() sets it raw against any task that hasn't
transitioned yet. And it keeps sending it until the livepatch finishes.

The sleep loop waiting for pid_allocated to drop has the same problem
and it doesn't even clear anything. So a schedule() in
TASK_INTERRUPTIBLE returns immediately.

Use a no_notify_signal guard. signal_pending() will ignore
TIF_NOTIFY_SIGNAL and the pid namespace reaper sleeps until a child
exits or free_pid() wakes it.

Fixes: 7fea700e04bd ("zap_pid_ns_processes: clear TIF_NOTIFY_SIGNAL along with TIF_SIGPENDING")
Cc: stable@vger.kernel.org
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 kernel/pid_namespace.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index d36afc58ee1d..8bc9edb40b78 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -238,9 +238,10 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
 	 * kernel_wait4() will also block until our children traced from the
 	 * parent namespace are detached and become EXIT_DEAD.
 	 */
+	/* Task work must not busy-loop the reaper, see signal_pending(). */
+	guard(no_notify_signal)();
 	do {
 		clear_thread_flag(TIF_SIGPENDING);
-		clear_thread_flag(TIF_NOTIFY_SIGNAL);
 		rc = kernel_wait4(-1, NULL, __WALL, NULL);
 	} while (rc != -ECHILD);
 

-- 
2.53.0