linux-kernel - Re: Allowing mapping supplemental groups in user namespace?

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Message-ID: <CAKdAkRSMCucS96ghj6vDtz31Vsa19AjuTsqAEYUTRUOZe7ZOwQ@mail.gmail.com>
Date:   Thu, 28 Mar 2019 11:43:05 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: Allowing mapping supplemental groups in user namespace?

On Thu, Mar 28, 2019 at 11:37 AM Serge E. Hallyn <serge@...lyn.com> wrote:
>
> On Thu, Mar 28, 2019 at 11:30:52AM -0700, Dmitry Torokhov wrote:
> > Hi Serge,
> >
> > On Thu, Mar 28, 2019 at 11:05 AM Serge E. Hallyn <serge@...lyn.com> wrote:
> > >
> > > On Thu, Feb 28, 2019 at 11:27:38AM -0800, Dmitry Torokhov wrote:
> > > > Hi Eric,
> > > >
> > > > Currently, unless caller has CAP_SETGID in parent namespace, we can
> > > > only map effective group id in the new user namespace. Would it be
> > > > possible to relax this rule to also allow mapping of supplemental
> > > > groups (1:1) of the caller?
> > > >
> > > > Thanks.
> > > >
> > > > --
> > > > Dmitry
> > >
> > > Hi,
> > >
> > > Is there a use case where adding those to /etc/subgid is onerous?
> > > (There probably is, just would like to see yours)
> >
> > We on Chrome OS limit number of suid binaries installed on the system,
> > so newgidmap does not have necessary privileges to carry out this
>
> <shrug> good goal in general so long as you don't take a few huge
> monolithic suid binaries instad of more simpler ones :)
>
> > operation. Also we are looking for a solution that we can use with our
> > minijail package where spawning additional binary is challenging even
> > if it was suid.
>
> Ok.  So fwiw I think what you propose should be ok.  I think you should
> post a patch to do it.  It's very possible that seeing that patch will
> remind us of the reason why it *is* a bad idea, but seeing the patch may
> be a required shock to elicit that memory.

OK, I will cook up something.

Thanks.

-- 
Dmitry