[PATCH] exit: fix the setns() && PR_SET_CHILD_SUBREAPER interaction

[Oleg Nesterov <oleg@redhat.com>]

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>
---
 kernel/exit.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 76e263e..e8b0776 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -569,15 +569,18 @@ 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)
-- 
2.5.0