linux-kernel - Re: [PATCH 4/7] allow killing tasks in your own or child userns

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Message-ID: <20110110225151.GA18944@localhost>
Date:	Mon, 10 Jan 2011 16:51:51 -0600
From:	Serge Hallyn <serge.hallyn@...onical.com>
To:	Bastian Blank <bastian@...di.eu.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	containers@...ts.linux-foundation.org,
	kernel list <linux-kernel@...r.kernel.org>,
	LSM <linux-security-module@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Kees Cook <kees.cook@...onical.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH 4/7] allow killing tasks in your own or child userns

Quoting Bastian Blank (bastian@...di.eu.org):
> On Mon, Jan 10, 2011 at 09:13:34PM +0000, Serge E. Hallyn wrote:
> > +	const struct cred *cred = current_cred();
> > +	const struct cred *tcred = __task_cred(t);
> > +
> > +	if (cred->user->user_ns != tcred->user->user_ns) {
> > +		/* userids are not equivalent - either you have the
> > +		   capability to the target user ns or you don't */
> > +		if (ns_capable(tcred->user->user_ns, CAP_KILL))
> > +			return 1;
> > +		return 0;
> > +	}
> > +
> > +	/* same user namespace - usual credentials checks apply */
> > +	if ((cred->euid ^ tcred->suid) &&
> > +	    (cred->euid ^ tcred->uid) &&
> > +	    (cred->uid  ^ tcred->suid) &&
> > +	    (cred->uid  ^ tcred->uid) &&
> > +	    !ns_capable(tcred->user->user_ns, CAP_KILL))
> > +		return 0;
> > +
> > +	return 1;
> 
> Isn't that equal to this?
> 
> 	if (ns_capable(tcred->user->user_ns, CAP_KILL))
> 		return 1;
> 
> 	if (cred->user->user_ns == tcred->user->user_ns &&
> 	    (cred->euid == tcred->suid ||
> 	     cred->euid == tcred->uid ||
> 	     cred->uid == tcred->suid ||
> 	     cred->uid == tcred->uid))
> 		return 1;
> 
> 	return 0;
> 
> I would consider this much easier to read.

Unfortunately, it's actually not equivalent.  when capable()
returns success, then it sets the current->flags |= PF_SUPERPRIV.
If permission is granted based on userids and the capability
isn't needed, then we don't want to needlessly set PF_SUPERPRIV.

That's why I'm going to such lengths to call capable() as a last
resort.

I'm definately open to any ideas that'll get the code cleaner.

thanks,
-serge
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