linux-kernel - Re: [RFC PATCH v1 4/4] Allow to change the user namespace in which user rlimits are counted

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Date:   Mon, 2 Nov 2020 18:30:24 +0100
From:   Alexey Gladkov <gladkov.alexey@...il.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Christian Brauner <christian@...uner.io>
Subject: Re: [RFC PATCH v1 4/4] Allow to change the user namespace in which
 user rlimits are counted

On Mon, Nov 02, 2020 at 06:10:06PM +0100, Jann Horn wrote:
> On Mon, Nov 2, 2020 at 5:52 PM Alexey Gladkov <gladkov.alexey@...il.com> wrote:
> > Add a new prctl to change the user namespace in which the process
> > counter is located. A pointer to the user namespace is in cred struct
> > to be inherited by all child processes.
> [...]
> > +       case PR_SET_RLIMIT_USER_NAMESPACE:
> > +               if (!capable(CAP_SYS_RESOURCE))
> > +                       return -EPERM;
> > +
> > +               switch (arg2) {
> > +               case PR_RLIMIT_BIND_GLOBAL_USERNS:
> > +                       error = set_rlimit_ns(&init_user_ns);
> > +                       break;
> > +               case PR_RLIMIT_BIND_CURRENT_USERNS:
> > +                       error = set_rlimit_ns(current_user_ns());
> > +                       break;
> > +               default:
> > +                       error = -EINVAL;
> > +               }
> > +               break;
> 
> I don't see how this can work. capable() requires that
> current_user_ns()==&init_user_ns, so you can't use this API to bind
> rlimits to any other user namespace.
> 
> Fundamentally, if it requires CAP_SYS_RESOURCE, this probably can't be
> done as an API that a process uses to change its own rlimit scope. In
> that case I would implement this as part of clone3() instead of
> prctl(). (Then init_user_ns can set it if the caller has
> CAP_SYS_RESOURCE. If you want to have support for doing the same thing
> with nested namespaces, you'd also need a flag that the first-level
> clone3() can set on the namespace to say "further rlimit splitting
> should be allowed".)
> 
> Or alternatively, we could say that CAP_SYS_RESOURCE doesn't matter,
> and instead you're allowed to move the rlimit scope if your current
> hard rlimit is INFINITY. That might make more sense? Maybe?

I think you are right. CAP_SYS_RESOURCE is not needed here since you still
cannot exceed the rlimit in the parent user namespace.

-- 
Rgrds, legion