exit: fix the setns() && PR_SET_CHILD_SUBREAPER interaction

find_new_reaper() checks same_thread_group(reaper, child_reaper) to
prevent the cross-namespace reparenting but this is not enough if the
exiting parent was injected by setns() + fork().

Suppose we have a process P in the root namespace and some namespace X.
P does setns() to enter the X namespace, and forks the child C.
C forks a grandchild G and exits.

The grandchild G should be re-parented to X->child_reaper, but in this
case the ->real_parent chain does not lead to ->child_reaper, so it will
be wrongly reparanted to P's sub-reaper or a global init.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

1 files changed, 8 insertions, 5 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 8f14b866f9f6..5cfbd595f918 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -578,15 +578,18 @@ static struct task_struct *find_new_reaper(struct task_struct *father,
                 return thread;
 
         if (father->signal->has_child_subreaper) {
+                unsigned int ns_level = task_pid(father)->level;
                 /*
                  * Find the first ->is_child_subreaper ancestor in our pid_ns.
-                 * We start from father to ensure we can not look into another
-                 * namespace, this is safe because all its threads are dead.
+                 * We can't check reaper != child_reaper to ensure we do not
+                 * cross the namespaces, the exiting parent could be injected
+                 * by setns() + fork().
+                 * We check pid->level, this is slightly more efficient than
+                 * task_active_pid_ns(reaper) != task_active_pid_ns(father).
                  */
-                for (reaper = father;
-                     !same_thread_group(reaper, child_reaper);
+                for (reaper = father->real_parent;
+                     task_pid(reaper)->level == ns_level;
                      reaper = reaper->real_parent) {
-                        /* call_usermodehelper() descendants need this check */
                         if (reaper == &init_task)
                                 break;
                         if (!reaper->signal->is_child_subreaper)