linux-kernel - Re: [capabilities] Allow normal inheritance for a configurable set of capabilities

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Message-ID: <54D10A50.5030707@schaufler-ca.com>
Date:	Tue, 03 Feb 2015 09:50:08 -0800
From:	Casey Schaufler <casey@...aufler-ca.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Christoph Lameter <cl@...ux.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Jonathan Corbet <corbet@....net>,
	Aaron Jones <aaronmdjones@...il.com>, Ted Ts'o <tytso@....edu>,
	LSM List <linux-security-module@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...uxfoundation.org>,
	Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [capabilities] Allow normal inheritance for a configurable set
 of capabilities

On 2/3/2015 9:28 AM, Serge E. Hallyn wrote:
> Quoting Casey Schaufler (casey@...aufler-ca.com):
>> On 2/3/2015 7:51 AM, Serge E. Hallyn wrote:
>>> Quoting Casey Schaufler (casey@...aufler-ca.com):
>>>> On 2/2/2015 12:37 PM, Andy Lutomirski wrote:
>>>>> On Mon, Feb 2, 2015 at 10:08 AM, Serge Hallyn <serge.hallyn@...ntu.com> wrote:
>>>>>> Quoting Casey Schaufler (casey@...aufler-ca.com):
>>>>>>> I'm game to participate in such an effort. The POSIX scheme
>>>>>>> is workable, but given that it's 20 years old and hasn't
>>>>>>> developed real traction it's hard to call it successful.
>>>>>> Over the years we've several times discussed possible reasons for this
>>>>>> and how to help.  I personally think it's two things:  1. lack of
>>>>>> toolchain and fs support.  The fact that we cannot to this day enable
>>>>>> ping using capabilities by default because of cpio, tar and non-xattr
>>>>>> filesystems is disheartening.  2. It's hard for users and applications
>>>>>> to know what caps they need.  yes the API is a bear to use, but we can
>>>>>> hide that behind fancier libraries.  But using capabilities requires too
>>>>>> much in-depth knowledge of precisely what caps you might need for
>>>>>> whatever operations library may now do when you asked for something.
>>>>> None of this could address the problem here, though: if I hold a
>>>>> capability and I want to pass that capability to an exec'd helper, I
>>>>> shouldn't need the fs's help to do this.
>>>> One of the holes in the 1003.1e spec is what to do with a program file
>>>> that does not have a capability set attached to it. The two options are
>>>> drop all capabilities and leave the capabilities alone. The latter gives
>>>> you what you're asking for. The former is arguably safer.
>>> Hm, so if we were to change that, what should we do in the case of (a)
>>> an fs which doesn't support xattrs,
>> You have two choices, really. The first is to treat the files on that
>> filesystem as having no xattrs, thus they have the inheritable behavior.
>> The alternative is to default to some value for the filesystem (Smack
>> does this) which may or may not be provided in the mount options.
>>
>>>  (2) expanding a tarball/cpio which
>>> didn't have xattrs (should tar/cpio fill them in with empty sets?),
>>> and
>> Files get no capability sets, hence the inheriting behavior.
>>
>>> (3) do we add a default empty set in the case of an fs mounted with
>>> NOSUID?
>> No, I think that is the opposite of what NOSUID is trying to do.
>> For the capability behavior to match the setuid bit behavior all
>> files will be inheriting, as if they had no capability set. It would
>> be safer to pretend there is an empty set, but that's not what
>> NOSUID does.
>>
>>> It's an interesting notion.
>> It's what we did in Trusted Irix. It made life much easier.
> Is there any chance you'd have time to write a patch to implement this?

Woof. I'll at least take a look.

>
> (I wasn't going to ask bc I assumed not, but heck maybe you're bored
> on a desert island or snowed in and just looking for an excuse to hack :)

Not at all bored, but I think this could be important.

>
> -serge
>

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