4.9-stable review patch. If anyone has any objections, please let me know.
------------------
From: Andrei Vagin avagin@gmail.com
commit 8fb335e078378c8426fabeed1ebee1fbf915690c upstream.
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 Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- kernel/exit.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-)
--- a/kernel/exit.c +++ b/kernel/exit.c @@ -525,12 +525,14 @@ 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; @@ -546,6 +548,12 @@ 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);
@@ -632,7 +640,7 @@ 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;