[patch 06/24] kernel/exit.c: release ptraced tasks before zap_pid_ns_processes

[akpm@linux-foundation.org]

From: Andrei Vagin <avagin@gmail.com>
Subject: kernel/exit.c: release ptraced tasks before zap_pid_ns_processes

Currently, exit_ptrace() adds all ptraced tasks in a dead list, then
zap_pid_ns_processes() waits on all tasks in a current pidns, and only
then are tasks from the dead list released.

zap_pid_ns_processes() can get stuck on waiting tasks from the dead list.  In
this case, we will have one unkillable process with one or more dead
children.

Thanks to Oleg for the advice to release tasks in find_child_reaper().

Link: http://lkml.kernel.org/r/20190110175200.12442-1-avagin@gmail.com
Fixes: 7c8bd2322c7f ("exit: ptrace: shift "reap dead" code from exit_ptrace() to forget_original_parent()")
Signed-off-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 kernel/exit.c |   12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

--- a/kernel/exit.c~kernel-release-ptraced-tasks-before-zap_pid_ns_processes
+++ a/kernel/exit.c
@@ -558,12 +558,14 @@ static struct task_struct *find_alive_th
 	return NULL;
 }
 
-static struct task_struct *find_child_reaper(struct task_struct *father)
+static struct task_struct *find_child_reaper(struct task_struct *father,
+						struct list_head *dead)
 	__releases(&tasklist_lock)
 	__acquires(&tasklist_lock)
 {
 	struct pid_namespace *pid_ns = task_active_pid_ns(father);
 	struct task_struct *reaper = pid_ns->child_reaper;
+	struct task_struct *p, *n;
 
 	if (likely(reaper != father))
 		return reaper;
@@ -579,6 +581,12 @@ static struct task_struct *find_child_re
 		panic("Attempted to kill init! exitcode=0x%08x\n",
 			father->signal->group_exit_code ?: father->exit_code);
 	}
+
+	list_for_each_entry_safe(p, n, dead, ptrace_entry) {
+		list_del_init(&p->ptrace_entry);
+		release_task(p);
+	}
+
 	zap_pid_ns_processes(pid_ns);
 	write_lock_irq(&tasklist_lock);
 
@@ -668,7 +676,7 @@ static void forget_original_parent(struc
 		exit_ptrace(father, dead);
 
 	/* Can drop and reacquire tasklist_lock */
-	reaper = find_child_reaper(father);
+	reaper = find_child_reaper(father, dead);
 	if (list_empty(&father->children))
 		return;
 
_