Re: [PATCH 3/4] capability: Create a new capability CAP_SIGNED

Serge E. Hallyn
2013-03-19T21:00:54Z

Quoting Eric W. Biederman (ebiederm@xmission.com):
>
> Adding Serge as he is the sometimes capabilities maintainer to this
> discussion.

Thanks, Eric.

> Casey Schaufler <casey@schaufler-ca.com> writes:
>
> > On 3/18/2013 11:30 AM, Vivek Goyal wrote:
> >> On Mon, Mar 18, 2013 at 10:50:21AM -0700, Casey Schaufler wrote:
> >>> On 3/18/2013 10:05 AM, Vivek Goyal wrote:
> >>>> On Fri, Mar 15, 2013 at 02:12:59PM -0700, Casey Schaufler wrote:
> >>>>> On 3/15/2013 1:35 PM, Vivek Goyal wrote:
> >>>>>> Create a new capability CAP_SIGNED which can be given to signed executables.
> >>>>> This would drive anyone who is trying to use
> >>>>> capabilities as the privilege mechanism it is
> >>>>> intended to be absolutely crazy.
> >>>> Will calling it CAP_SIGNED_SERVICES help. I intend to use it as
> >>>> capability (and not just as a flag for task attribute).
> >>> No, the name is not the issue.
> >>>
> >>>> I think primary difference here is that this capability is controlled
> >>>> by kernel and only validly signed processes get it.
> >>> Applications are allowed to manipulate their capability sets
> >>> in well defined ways. The behavior of file based capabilities
> >>> is also explicitly defined. The behavior you are proposing would
> >>> violate both of these mechanisms.
> >>>
> >>>>> Capabilities aren't just random attribute bits. They
> >>>>> indicate that a task has permission to violate a
> >>>>> system policy (e.g. change the mode bits of a file
> >>>>> the user doesn't own). Think about how this will
> >>>>> interact with programs using file based capabilities.
> >>>> It is a separate capability. I am not sure why it would
> >>>> interfere with other capabilities or functionality out there.
> >>> The behavior of capabilities is uniform. You can't have one
> >>> capability that behaves differently from the others. If a
> >>> file is unsigned but has CAP_SIGNED in the file capability
> >>> set what do you expect to happen? Do you want a signed
> >>> application to be able to drop and raise the fact that it
> >>> is signed?
> >> I have already removed this capability from bounding set. Behavior
> >> I am looking for is that nobody should be able to set CAP_SIGNED
> >> as file capability. I will look into that.
> >
> > No! You are not listening. All capabilities work the same way.
> > If the file capabilities say ALL that means ALL. You do not get
> > to put a hole in the middle of the file based capabilities.
> >
> >
> >> I am thinking of this more as kernel managed capability. It is
> >> not in bounding set of any process and it can not be set as file
> >> capability.
> >
> > I heard that. No, you don't get to do that. All capabilities
> > work the same way. Your attribute does not behave the way
> > capabilities do, so you have to implement it some other way.
> >
> >
> >> It is a new capability, so no existing user application should
> >> be trying to set it.
> >
> > There are (and will be) applications that raise and drop all
> > capabilities, and that do so for good reasons.
> >
> >> I think the only surprise would be that they can't drop it. If
> >> that's a concern, may be we can allow dropping the capability.
> >> But the side affect is that there is no way to gain it back for
> >> the life time of process.
> >
> > Right. And that is a change to the capability mechanism. No, you
> > don't get to do that.
> >
> > You don't want a new capability. You want a new attribute that
> > behaves differently than capabilities do. You need to come up
> > with a different way to implement your attribute. You do not get
> > to change the way capabilities work.
> >
> >>> I expect that you don't want your attribute that indicates
> >>> that the binary was signed to behave the same way that
> >>> capabilities do. Like I said, capabilities are not just
> >>> attribute bits. You need a different kind of process attribute
> >>> to indicate that the binary was signed.
> >> I think I need more than process attribute. One of the things
> >> I am looking for is that signed processes run locked in memory
> >> and nobody (i think no unsigned process) is able to do ptrace() on it.
> >> Using the notion of capability might help here.
> >
> > There are already capabilities associated with ptrace. It would
> > be simple to add a check for signatures in cap_ptrace_access_check.
> >
> >
> >>> When (if ever) we have multiple LSM support you might consider
> >>> doing this as a small LSM. Until then, you're going to need a
> >>> different way to express the signature attribute.
> >> I am not sure why you are viewing it as necessarily as attribute only.
> >> I am thinking more in terms of that in certain situations, user space
> >> processes can't perform certain operations (like kexec) untile and
> >> unless process has the capability CAP_SIGNED_SERVICES. And this capability
> >> is granted if upon exec() process signature are verified.
> >
> > Sigh. You need the process attribute to make the checks against. The
> > process capability set, uids and groups are all examples of process
> > attributes that exist today.
> >
> >> So yes it is little different from how capabilities are managed
> >> currently. But is it very hard to extend the current capability definition
> >> and include the fact that kernel can give additional capabilities to
> >> processes based on some other factors.
> >
> > Yes. That is correct. That is why we have the LSM facility. The
> > unfortunate fact is that you only get one LSM at a time today. I
> > am working on fixing that, but there is still work to be done
> > before it will be ready for upstream.
> >
> > If signed application controls are deemed sufficiently important
> > and your implementation sound you should be able to get the signature
> > attribute and the checks on that attribute into the base system.
>
> Vivek the desired semantics for today for kexec is that you have an
> application that is allowed CAP_SYS_BOOT in it's file capabilities.
>
> In a context where root is not trusted with all capabilities by default
> you want one or a couple of capabilities to only be possible when coming
> from file capabilities. So that you can say. "I trust you oh great and
> blessed executable do what you will."
>
> I don't think those are contentious semantics.

[ keeping all context as I've just cc:d amorgan who may have input -
though i needed to skim the thread to understand ]

There are many ways that come to mind that we could use knowledge of a
signed binary. We might actually want a securebit which says "you need
to be running a signed executable to get capabilities." Or a capset
akin to the bounding set, saying you can only have the caps in this set
if the running binary was a signed one.

But I don't think CAP_SIGNED is the right way to expose and use this
state.

-serge