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

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Date:	Mon, 10 Jan 2011 23:22:12 -0500
From:	Oren Laadan <orenl@...columbia.edu>
To:	Serge Hallyn <serge.hallyn@...onical.com>
CC:	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



On 01/10/2011 05:51 PM, Serge Hallyn wrote:
> 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.

A bit off-topic: does this means that c/r needs to save and 
restore this process flag ?
> 
> That's why I'm going to such lengths to call capable() as a last
> resort.

IMHO, worth a one line comment in the code ...

Oren.
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