Messages in this thread | | | Date | Wed, 22 Oct 2008 12:03:25 -0500 | From | "Serge E. Hallyn" <> | Subject | Re: [RFC v7][PATCH 2/9] General infrastructure for checkpoint restart |
| |
Quoting Oren Laadan (orenl@cs.columbia.edu): > > > Serge E. Hallyn wrote: > > Quoting Oren Laadan (orenl@cs.columbia.edu): > > Just thinking aloud... > > > > Is read mode appropriate? The user can edit the statefile and restart > > it. Admittedly the restart code should then do all the appropriate > > checks for recreating resources, but I'm having a hard time thinking > > through this straight. > > > > Let's say hallyn is running passwd. ruid=500,euid=0. He quickly > > checkpoints. Then he restarts. Will restart say "ok, the /bin/passwd > > binary is setuid 0 so let hallyn take euid=0 for this?" I guess not. > > But are there other resources for which this is harder to get right? > > I'd say that checkpoint and restart are separate. > > In checkpoint, you read the state and save it somewhere; you don't > modify anything in the target task (container). This equivalent to > ptrace read-mode. If you could do ptrace, you could save all that > state. In fact, you could save it in a format that is suitable for > a future restart ... (or just forge one !)
Yeah, that's convincing.
> In restart, we either don't trust the user and keep everything to > be done with her credentials, of we trust the root user and allow > all operations (like loading a kernel module). > > We can actually have both modes of operations. How to decide that > we trust the user is a separate question: one option is to have > both checkpoint and restart executables setuid - checkpoint will > sign (in user space) the output image, and restart (in user space) > will validate the signature, before passing it to the kenrel. Surely > there are other ways...
Makes sense.
...
> > Hmm, so do you think we just always use the caller's credentials? > > Nope, since we will fail to restart in many cases. We will need a way > to move from caller's credentials to saved credentials, and even from > caller's credentials to privileged credentials (e.g. to reopen a file > that was created by a setuid program prior to dropping privileges).
Can we agree to worry about that much much later? :) Would you agree that for the majority of use-cases, restarting with caller's credentials will work? Or am I wrong about that?
> To do that, we will need to agree on a way to escalate/change the > credentials. This however belongs to user-space (and then the binaries > for checkpoint/restart will be setuid themselves).
Ok those are less scary, and I have no problem with those.
> There will also be the issue of mapping credentials: a user A may have > one UID/GID on once system and another UID/GID on another system, and > we may want to do the conversion. This, too, can be done in user space > prior to restart by using an appropriate filter through the checkpoint > stream.
User namespaces may help here too. So user A can create a new user namespace and restart as user B in that namespace. But right now that sounds like overkill.
-serge
| |