pidns: Outlaw thread creation after unshare(CLONE_NEWPID)

The sequence:
unshare(CLONE_NEWPID)
clone(CLONE_THREAD|CLONE_SIGHAND|CLONE_VM)

Creates a new process in the new pid namespace without setting
pid_ns->child_reaper.  After forking this results in a NULL
pointer dereference.

Avoid this and other nonsense scenarios that can show up after
creating a new pid namespace with unshare by adding a new
check in copy_prodcess.

Pointed-out-by:  Oleg Nesterov <oleg@redhat.com>
Acked-by:  Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index a31b823b3c2d..65ca6d27f24e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1166,6 +1166,14 @@ static struct task_struct *copy_process(unsigned long clone_flags,
                                 current->signal->flags & SIGNAL_UNKILLABLE)
                 return ERR_PTR(-EINVAL);
 
+        /*
+         * If the new process will be in a different pid namespace
+         * don't allow the creation of threads.
+         */
+        if ((clone_flags & (CLONE_VM|CLONE_NEWPID)) &&
+            (task_active_pid_ns(current) != current->nsproxy->pid_ns))
+                return ERR_PTR(-EINVAL);
+
         retval = security_task_create(clone_flags);
         if (retval)
                 goto fork_out;