linux-kernel - Re: group ownership of tun devices -- nonfunctional?

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Message-ID: <46C8D124.4030701@home.nl>
Date:	Mon, 20 Aug 2007 01:24:20 +0200
From:	Rene Herman <rene.herman@...e.nl>
To:	Bodo Eggert <7eggert@....de>
CC:	Mike Mohr <akihana@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: group ownership of tun devices -- nonfunctional?

On 08/19/2007 11:42 PM, Bodo Eggert wrote:

> On Sun, 19 Aug 2007, Rene Herman wrote:
> 
>> On 08/19/2007 06:05 PM, Bodo Eggert wrote:
>>
>>> IMHO the check is broken:
>>>
>>> +               if (((tun->owner != -1 &&
>>> +                     current->euid != tun->owner) ||
>>> +                    (tun->group != -1 &&
>>> +                     current->egid != tun->group)) &&
>>> +                    !capable(CAP_NET_ADMIN))
>>>                         return -EPERM;
>>>
>>> It should be something like:
>>>
>>> +               if (!((tun->owner == tun->owner) ||
>>> +                     (tun->group == tun->group) ||
>> ???
> 
> Argh, I edited asuming the same order of variables. Substitute 
> current->e{uid,gid} for one of the sides.

Okay. Just had to ask. That looked so odd...

>>> +                     capable(CAP_NET_ADMIN)))
>>>                         return -EPERM;
> 
> The intended semantics is If the user is not
>  * the allowed user
> or
>  * member of the allowed group
> or
>  * cabable of CAP_NET_ADMIN
> then error out. I'm asuming

There is a short description of the desired semantics in the link that was 
posted:

http://lkml.org/lkml/2007/6/18/228

===
The user now is allowed to send packages if either his euid or his egid
matches the one specified via tunctl (via -u or -g respecitvely). If both
gid and uid are set via tunctl, both have to match.
===

Paraphrasing the original code above, it's saying:

if ((owner_is_set && does_not_match) || (group_is_set && does_not_match))
	bugger_off_unless(CAP_NET_ADMIN);

or reverting the logic:

if ((owner_is_unset || does_match) && (group_is_unset || does_match))
	good_to_go();

which probably matches the intention -- we're good to go only if the 
credentials that are set also match.

Rene.
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