linux-kernel - Re: [PATCH v2 1/4] capabilities: Add user namespace capabilities

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Message-ID: <20240610124855.GA2193924@mail.hallyn.com>
Date: Mon, 10 Jun 2024 07:48:55 -0500
From: "Serge E. Hallyn" <serge@...lyn.com>
To: Jonathan Calmels <jcalmels@...0.net>
Cc: "Serge E. Hallyn" <serge@...lyn.com>, Andrew Morgan <morgan@...nel.org>,
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Subject: Re: [PATCH v2 1/4] capabilities: Add user namespace capabilities

On Mon, Jun 10, 2024 at 01:47:13AM -0700, Jonathan Calmels wrote:
> On Sun, Jun 09, 2024 at 08:50:24PM GMT, Serge E. Hallyn wrote:
> > On Sun, Jun 09, 2024 at 03:43:34AM -0700, Jonathan Calmels wrote:
> > > Attackers often rely on user namespaces to get elevated (yet confined)
> > > privileges in order to target specific subsystems (e.g. [1]). Distributions
> > 
> > I'd modify this to say "in order to target *bugs* in specific subsystems" :)
> 
> Ack
> 
> > > This effectively mimics the inheritable set rules and means that, by
> > > default, only root in the user namespace can regain userns capabilities
> > > previously dropped:
> > 
> > Something about this last sentence feels wrong, but I'm not sure what
> > the best alternative would be.  As is, though, it makes it sound as though
> > root in the userns can always regain previously dropped capabilities, but
> > that's not true if dropped in ancestor ns, or if root also dropped the
> > bits from its bounding set (right?).
> 
> Right, the wording is a little bit confusing here I admit.
> What I meant to say is that if a cap is dropped in a *given* namespace,
> then it can only be regained by root there. But yes, caps can never be
> regained from ancestors ns. I'll try to rephrase it.
> 
> BTW, this is rather strict, but I think that's what we want right,

Yes,

> something simple? Alternative would be to have a new cap masked off by
> default, but if granted to a userns, allows you to regain ancestors
> caps.

we absolutely do not want to allow regaining caps dropped in an
ancestor namespace.

thanks,
-serge