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Date:   Fri, 12 May 2017 19:11:31 +0300
From:   Kirill Tkhai <>
To:     <>, <>, <>,
        <>, <>,
        <>, <>,
        <>, <>,
        <>, <>,
        <>, <>
Subject: [PATCH v2] pid_ns: Fix race between setns'ed fork() and

Imagine we have a pid namespace and a task from its parent's pid_ns,
which made setns() to the pid namespace. The task is doing fork(),
while the pid namespace's child reaper is dying. We have the race
between them:

Task from parent pid_ns             Child reaper
copy_process()                      ..
  alloc_pid()                       ..
  ..                                zap_pid_ns_processes()
  ..                                  disable_pid_allocation()
  ..                                  read_lock(&tasklist_lock)
  ..                                  iterate over pids in pid_ns
  ..                                    kill tasks linked to pids
  ..                                  read_unlock(&tasklist_lock)
  write_lock_irq(&tasklist_lock);   ..
  attach_pid(p, PIDTYPE_PID);       ..
  ..                                ..

So, just created task p won't receive SIGKILL signal,
and the pid namespace will be in contradictory state.
Only manual kill will help there, but does the userspace
care about this? I suppose, the most users just inject
a task into a pid namespace and wait a SIGCHLD from it.

The patch fixes the problem. It simply checks for
(pid_ns->nr_hashed & PIDNS_HASH_ADDING) in copy_process().
We do it under the tasklist_lock, and can't skip
PIDNS_HASH_ADDING as noted by Oleg:

"zap_pid_ns_processes() does disable_pid_allocation()
and then takes tasklist_lock to kill the whole namespace.
Given that copy_process() checks PIDNS_HASH_ADDING
under write_lock(tasklist) they can't race;
if copy_process() takes this lock first, the new child will
be killed, otherwise copy_process() can't miss
the change in ->nr_hashed."

If allocation is disabled, we just return -ENOMEM
like it's made for such cases in alloc_pid().

v2: Do not move disable_pid_allocation(), do not
introduce a new variable in copy_process() and simplify
the patch as suggested by Oleg Nesterov.
Account the problem with double irq enabling
found by Eric W. Biederman.

Signed-off-by: Kirill Tkhai <>
CC: Andrew Morton <>
CC: Ingo Molnar <>
CC: Peter Zijlstra <>
CC: Oleg Nesterov <>
CC: Mike Rapoport <>
CC: Michal Hocko <>
CC: Andy Lutomirski <>
CC: "Eric W. Biederman" <>
CC: Andrei Vagin <>
CC: Cyrill Gorcunov <>
CC: Serge Hallyn <>
 kernel/fork.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index bfd91b180778..d2cade9b6ad8 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1845,11 +1845,13 @@ static __latent_entropy struct task_struct *copy_process(
 	if (signal_pending(current)) {
-		spin_unlock(&current->sighand->siglock);
-		write_unlock_irq(&tasklist_lock);
 		retval = -ERESTARTNOINTR;
 		goto bad_fork_cancel_cgroup;
+	if (unlikely(!(ns_of_pid(pid)->nr_hashed & PIDNS_HASH_ADDING))) {
+		retval = -ENOMEM;
+		goto bad_fork_cancel_cgroup;
+	}
 	if (likely(p->pid)) {
 		ptrace_init_task(p, (clone_flags & CLONE_PTRACE) || trace);
@@ -1907,6 +1909,8 @@ static __latent_entropy struct task_struct *copy_process(
 	return p;
+	spin_unlock(&current->sighand->siglock);
+	write_unlock_irq(&tasklist_lock);

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