Re: Sandbox thoughts

[Andy Lutomirski <luto amacapital net>]

On Fri, Feb 20, 2015 at 4:31 PM, Eric W. Biederman
<ebiederm xmission com> wrote:
> Andy Lutomirski <luto amacapital net> writes:
>
> > On Thu, Feb 19, 2015 at 8:38 AM, Alexander Larsson <alexl redhat com> wrote:
> > > On Tue, 2015-02-17 at 13:23 -0800, Andy Lutomirski wrote:
> > >
> > > >  - setuid / privileged helper.  Why do you need a privileged helper?
> > > > You should be able to do all of this using user namespaces.  The
> > > > Sandstorm code linked above does exactly this.
> > >
> > > I tried this a bit, but i ran into two snags i don't understand.
> > >
> > > First of all, as uid/gid 1000 i can put "1000 1000 1"
> > > in /proc/self/uid_map from the child. However, i cannot put "1000 1000
> > > 1" into gid_map, as i get EPERM.
> > > I don't understand this, is this not supposed to work?
> >
> > You need newer manpages :-/  Try the attached variant.
>
> Yeah.  You need to disable setgroups for that to work.
>
> > > Secondly, i'm failing to mount another instance of devpts. It fails with
> > > EINVAL.
> >
> > Hmm.  Off the top of my head, there's no good reason that devpts with
> > the newinstance option couldn't be allowed in a userns.  Eric, any
> > thoughts here?  The patch would be straightforward.
>
> Looking at the code you have to have uid 0 and gid 0 mapped and you have
> to specify newinstance.  But devepts is mountable without being the
> global root user.'

Wow, my grepping skills are nonexistent today.

> The restriction of having uid 0 and gid 0 mapped is just that /dev/ptmx is
> and has alwasy been owned by root and so mknod_ptmx just won't let you
> create a device inode as with a uid or gid you can't map.

All we'd have to do is to add ptmx_uid and ptmx_gid options, right?
I'll send a patch.

--Andy

> Eric



-- 
Andy Lutomirski
AMA Capital Management, LLC