linux-kernel - Re: [PATCH 2/9] security: Make capabilities relative to the user namespace.

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Date:	Wed, 23 Feb 2011 12:01:57 +0000
From:	David Howells <dhowells@...hat.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	dhowells@...hat.com, LSM <linux-security-module@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>,
	James Morris <jmorris@...ei.org>,
	Kees Cook <kees.cook@...onical.com>,
	containers@...ts.linux-foundation.org,
	kernel list <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Michael Kerrisk <mtk.manpages@...il.com>, xemul@...allels.com
Subject: Re: [PATCH 2/9] security: Make capabilities relative to the user namespace.

David Howells <dhowells@...hat.com> wrote:

> >  	int (*capable) (struct task_struct *tsk, const struct cred *cred,
> > -			int cap, int audit);
> > +			struct user_namespace *ns, int cap, int audit);
> 
> Hmmm...  A chunk of the contents of the cred struct are user-namespaced.
> Could you add the user_namespace pointer to the cred struct and thus avoid
> passing it as an argument to other things.

Ah, no...  Ignore that, I think I see that you do need it.

> +int cap_capable(struct task_struct *tsk, const struct cred *cred,
> +		struct user_namespace *targ_ns, int cap, int audit)
>  {
> -	return cap_raised(cred->cap_effective, cap) ? 0 : -EPERM;
> +	for (;;) {
> +		/* The creator of the user namespace has all caps. */
> +		if (targ_ns != &init_user_ns && targ_ns->creator == cred->user)
> +			return 0;

Why is that last comment so?  Why should the creating namespace sport all
possible capabilities?  Do you have to have all capabilities available to you
to be permitted create a new user namespace?

Also, would it be worth having a separate cap_ns_capable()?  Wouldn't most
calls to cap_capable() only be checking the caps granted in the current user
namespace?

David
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