From 1f7f4dde5c945f41a7abc2285be43d918029ecc5 Mon Sep 17 00:00:00 2001 From: "Eric W. Biederman" Date: Thu, 14 Nov 2013 21:10:16 -0800 Subject: [PATCH] fork: Allow CLONE_PARENT after setns(CLONE_NEWPID) Serge Hallyn writes: > Hi Oleg, > > commit 40a0d32d1eaffe6aac7324ca92604b6b3977eb0e : > "fork: unify and tighten up CLONE_NEWUSER/CLONE_NEWPID checks" > breaks lxc-attach in 3.12. That code forks a child which does > setns() and then does a clone(CLONE_PARENT). That way the > grandchild can be in the right namespaces (which the child was > not) and be a child of the original task, which is the monitor. > > lxc-attach in 3.11 was working fine with no side effects that I > could see. Is there a real danger in allowing CLONE_PARENT > when current->nsproxy->pidns_for_children is not our pidns, > or was this done out of an "over-abundance of caution"? Can we > safely revert that new extra check? The two fundamental things I know we can not allow are: - A shared signal queue aka CLONE_THREAD. Because we compute the pid and uid of the signal when we place it in the queue. - Changing the pid and by extention pid_namespace of an existing process. From a parents perspective there is nothing special about the pid namespace, to deny CLONE_PARENT, because the parent simply won't know or care. From the childs perspective all that is special really are shared signal queues. User mode threading with CLONE_PARENT|CLONE_VM|CLONE_SIGHAND and tasks in different pid namespaces is almost certainly going to break because it is complicated. But shared signal handlers can look at per thread information to know which pid namespace a process is in, so I don't know of any reason not to support CLONE_PARENT|CLONE_VM|CLONE_SIGHAND threads at the kernel level. It would be absolutely stupid to implement but that is a different thing. So hmm. Because it can do no harm, and because it is a regression let's remove the CLONE_PARENT check and send it stable. Cc: stable@vger.kernel.org Acked-by: Oleg Nesterov Acked-by: Andy Lutomirski Acked-by: Serge E. Hallyn Signed-off-by: "Eric W. Biederman" --- kernel/fork.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/fork.c b/kernel/fork.c index 728d5be9548c..f82fa2ee7581 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1171,7 +1171,7 @@ static struct task_struct *copy_process(unsigned long clone_flags, * do not allow it to share a thread group or signal handlers or * parent with the forking task. */ - if (clone_flags & (CLONE_SIGHAND | CLONE_PARENT)) { + if (clone_flags & CLONE_SIGHAND) { if ((clone_flags & (CLONE_NEWUSER | CLONE_NEWPID)) || (task_active_pid_ns(current) != current->nsproxy->pid_ns_for_children))