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: cca22dc1202a ("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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/exit.c b/kernel/exit.c
index 3fb7be0019644e136a6c31e5bfd47884777ced02..2639a30a8aa5dd9054bc0af5951397efb1083713 100644 (file)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -558,12 +558,14 @@ static struct task_struct *find_alive_thread(struct task_struct *p)
        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_reaper(struct task_struct *father)
                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(struct task_struct *father,
                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;