Re: [RFC PATCH v4 0/2] Add capabilities file to securityfs

[Paul Moore]

On Wed, Aug 17, 2022 at 12:19 PM Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> On Wed, Aug 17, 2022 at 12:10:25PM -0400, Paul Moore wrote:
> > On Wed, Aug 17, 2022 at 11:50 AM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
> > > On 8/17/2022 7:52 AM, Paul Moore wrote:
> > > > On Wed, Aug 17, 2022 at 7:53 AM Francis Laniel
> > > > <flaniel@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx> 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, an integer is just as
> > meaningful in that regard.  What might be useful is if the application
>
> Maybe it's important to point out that an integer value capability in
> kernel will NEVER change its string value (or semantic meaning).
>
> The libcap tools like capsh accept integer capabilities, other tools
> probably should as well.  (see man 3 cap_from_text)

Seems like a reasonable thing to me, I would much prefer that than the
approach in this patchset.

-- 
paul-moore.com