How much of a mess does OpenVZ make? ;) Was: What can OpenVZ do?

Alexey Dobriyan adobriyan at gmail.com
Fri Mar 13 12:35:00 PDT 2009


On Fri, Mar 13, 2009 at 10:27:54AM -0700, Linus Torvalds wrote:
> On Thu, 12 Mar 2009, Sukadev Bhattiprolu wrote:
> 
> > Ying Han [yinghan at google.com] wrote:
> > | Hi Serge:
> > | I made a patch based on Oren's tree recently which implement a new
> > | syscall clone_with_pid. I tested with checkpoint/restart process tree
> > | and it works as expected.
> > 
> > Yes, I think we had a version of clone() with pid a while ago.
> 
> Are people _at_all_ thinking about security?
> 
> Obviously not.

For the record, OpenVZ always have CAP_SYS_ADMIN check on restore.
And CAP_SYS_ADMIN will be in version to be sent out.

Not having it is one big security hole.

> There's no way we can do anything like this. Sure, it's trivial to do 
> inside the kernel. But it also sounds like a _wonderful_ attack vector 
> against badly written user-land software that sends signals and has small 
> races.
> 
> Quite frankly, from having followed the discussion(s) over the last few 
> weeks about checkpoint/restart in various forms, my reaction to just about 
> _all_ of this is that people pushing this are pretty damn borderline. 
> 
> I think you guys are working on all the wrong problems. 
> 
> Let's face it, we're not going to _ever_ checkpoint any kind of general 
> case process. Just TCP makes that fundamentally impossible in the general 
> case, and there are lots and lots of other cases too (just something as 
> totally _trivial_ as all the files in the filesystem that don't get rolled 
> back).

What do you mean here? Unlinked files?

> So unless people start realizing that
>  (a) processes that want to be checkpointed had better be ready and aware 
>      of it, and help out

This is not going to happen. Userspace authors won't do anything
(nor they shouldn't).

>  (b) there's no way in hell that we're going to add these kinds of 
>      interfaces that have dubious upsides (just teach the damn program 
>      you're checkpointing that pids will change, and admit to everybody 
>      that people who want to be checkpointed need to do work) and are 
>      potential security holes.

I personally don't understand why on earth clone_with_pid() is again
with us.

As if pids are somehow unique among other resources.

It was discussed when IPC objects creation with specific parameters were
discussed.

"struct pid" and "struct pid_namespace" can be trivially restored
without leaking to userspace.

People probably assume that task should be restored with clone(2) which
is unnatural given relations between task_struct, nsproxy and individual
struct foo_namespace's

>  (c) if you are going to play any deeper games, you need to have 
>      privileges. IOW, "clone_with_pid()" is ok for _root_, but not for 
>      some random user. And you'd better keep that in mind EVERY SINGLE 
>      STEP OF THE WAY.
> 
> I'm really fed up with these discussions. I have seen almost _zero_ 
> critical thinking at all. Probably because anybody who is in the least 
> doubtful about it simply has tuned out the discussion. So here's my input: 
> start small, start over, and start thinking about other issues than just 
> checkpointing.


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