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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>
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1 parent a49f0d1 commit 8382fcac1b813ad0a4e68a838fc7ae93fa39eda0 @ebiederm ebiederm committed Dec 20, 2012
Showing with 8 additions and 0 deletions.
  1. +8 −0 kernel/fork.c
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8 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;

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