pid_namespace: pidns_get() should check task_active_pid_ns() != NULL

pidns_get()->get_pid_ns() can hit ns == NULL. This task_struct can't
go away, but task_active_pid_ns(task) is NULL if release_task(task)
was already called. Alternatively we could change get_pid_ns(ns) to
check ns != NULL, but it seems that other callers are fine.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Eric W. Biederman ebiederm@xmission.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 06c62de9c711..db95d8eb761b 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -318,7 +318,9 @@ static void *pidns_get(struct task_struct *task)
         struct pid_namespace *ns;
 
         rcu_read_lock();
-        ns = get_pid_ns(task_active_pid_ns(task));
+        ns = task_active_pid_ns(task);
+        if (ns)
+                get_pid_ns(ns);
         rcu_read_unlock();
 
         return ns;