Date:   Wed, 17 Aug 2022 13:19:48 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Casey Schaufler <casey@...aufler-ca.com>
Cc:     Francis Laniel <flaniel@...ux.microsoft.com>,
        linux-security-module@...r.kernel.org,
        Eric Biederman <ebiederm@...ssion.com>,
        Serge Hallyn <serge@...lyn.com>,
        James Morris <jmorris@...ei.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:BPF [MISC]" <bpf@...r.kernel.org>
Subject: Re: [RFC PATCH v4 0/2] Add capabilities file to securityfs

On Wed, Aug 17, 2022 at 12:49 PM Casey Schaufler <casey@...aufler-ca.com> wrote:
> On 8/17/2022 9:10 AM, Paul Moore wrote:
> > On Wed, Aug 17, 2022 at 11:50 AM Casey Schaufler <casey@...aufler-ca.com> wrote:
> >> On 8/17/2022 7:52 AM, Paul Moore wrote:
> >>> On Wed, Aug 17, 2022 at 7:53 AM Francis Laniel
> >>> <flaniel@...ux.microsoft.com> wrote:
> >>>> Le mardi 16 août 2022, 23:59:41 CEST Paul Moore a écrit :
> >>>>> On Mon, Jul 25, 2022 at 8:42 AM Francis Laniel
> >>>>>
> >>>>> <flaniel@...ux.microsoft.com> wrote:
> >>>>>> Hi.
> >>>>>>
> >>>>>> First, I hope you are fine and the same for your relatives.
> >>>>> Hi Francis :)
> >>>>>
> >>>>>> A solution to this problem could be to add a way for the userspace to ask
> >>>>>> the kernel about the capabilities it offers.
> >>>>>> So, in this series, I added a new file to securityfs:
> >>>>>> /sys/kernel/security/capabilities.
> >>>>>> The goal of this file is to be used by "container world" software to know
> >>>>>> kernel capabilities at run time instead of compile time.
> >>>>> ...
> >>>>>
> >>>>>> The kernel already exposes the last capability number under:
> >>>>>> /proc/sys/kernel/cap_last_cap
> >>>>> I'm not clear on why this patchset is needed, why can't the
> >>>>> application simply read from "cap_last_cap" to determine what
> >>>>> capabilities the kernel supports?
> >>>> When you capabilities with, for example, docker, you will fill capabilities
> >>>> like this:
> >>>> docker run --rm --cap-add SYS_ADMIN debian:latest echo foo
> >>>> As a consequence, the "echo foo" will be run with CAP_SYS_ADMIN set.
> >>>>
> >>>> Sadly, each time a new capability is added to the kernel, it means "container
> >>>> stack" software should add a new string corresponding to the number of the
> >>>> capabilities [1].
> >>> Thanks for clarifying things, I thought you were more concerned about
> >>> detecting what capabilities the running kernel supported, I didn't
> >>> realize it was getting a string literal for each supported capability.
> >>> Unless there is a significant show of support for this
> >> I believe this could be a significant help in encouraging the use of
> >> capabilities. An application that has to know the list of capabilities
> >> at compile time but is expected to run unmodified for decades isn't
> >> going to be satisfied with cap_last_cap. The best it can do with that
> >> is abort, not being able to ask an admin what to do in the presence of
> >> a capability that wasn't around before because the name isn't known.
> > An application isn't going to be able to deduce the semantic value of
> > a capability based solely on a string value,
>
> True, but it can ask someone what to do, and in that case a string is
> much better than a number ...

If you are asking a user what to do, that user can just as easily look
up the capability list to translate numbers to intent.  If your
security approach requires a user knowing all of the subtle details
around a capability based on 10~15 character string, I wish you the
best of luck :)

-- 
paul-moore.com

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