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

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Date: Mon, 10 Jun 2024 01:47:13 -0700
From: Jonathan Calmels <jcalmels@...0.net>
To: "Serge E. Hallyn" <serge@...lyn.com>
Cc: Andrew Morgan <morgan@...nel.org>, brauner@...nel.org,
 	ebiederm@...ssion.com, Jonathan Corbet <corbet@....net>,
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Subject: Re: [PATCH v2 1/4] capabilities: Add user namespace capabilities

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,
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.