Quoting Eric W. Biederman ([email protected]):
> Kyle Moffett <[email protected]> writes:
>
> > On Oct 04, 2007, at 21:44:02, Eric W. Biederman wrote:
> >> What we want from the LSM is the ability to say -EPERM when we can clearly
> >> articulate that we want to disallow something.
> >
> > This sort of depends on perspective; typically with security infrastructure you
> > actually want "... the ability to return success when we can clearly articulate
> > that we want to *ALLOW* something". File permissions work this way; we don't
> > have a list of forbidden users attached to each file, we have an owner, a
> > group, and a mode representing positive permissions. With that said in certain
> > high-
> > risk environments you need something even stronger that cannot be changed by the
> > "owner" of the file, if we don't entirely trust them,
>
> Yes. However last I looked at the LSM hooks we first do the normal unix
> permission checks. Then we run the hook. So it can only increase the
> number of times we say -EPERM.
>
> >> SElinux is not all encompassing or it is generally incomprehensible I don't
> >> know which. Or someone long ago would have said a better way to implement
> >> containers was with a selinux ruleset, here is a selinux ruleset that does
> >> that. Although it is completely possible to implement all of the isolation
> >> with the existing LSM hooks as Serge showed.
> >
> > The difference between SELinux and containers is that SELinux (and LSM as a
> > whole) returns -EPERM to operations outside the scope of the subject, whereas
> > containers return -ENOENT (because it's not even in the same namespace).
>
> Yes. However if you look at what the first implementations were. Especially
> something like linux-vserver. All they provided was isolation. So perhaps
> you would not see every process ps but they all had unique pid values.
>
> I'm pretty certain Serge at least prototyped a simplified version
> of that using the LSM hooks. Is there something I'm not remember in
> those hooks that allows hiding of information like processes?
Actually I had to introduce a new LSM hook to filter /proc pid listings.
> Yes. Currently with containers we are taking that one step farther as
> that solves a wider set of problems.
And I'm far happier with the pid namespaces :)
> >> We also have in the kernel another parallel security mechanism (for what is
> >> generally a different class of operations) that has been quite successful,
> >> and different groups get along quite well, and ordinary mortals can
> >> understand it. The linux firewalling code.
> >
> > Well, I wouldn't go so far as the "ordinary mortals can understand it" part;
> > it's still pretty high on the obtuse-o-meter.
>
> True. Probably a more accurate statement is:`unix command line power
> users can and do handle it after reading the docs. That's not quite
> ordinary mortals but it feels like it some days. It might all be
> perception...
>
> >> The linux firewalling codes has hooks all throughout the networking stack,
> >> just like the LSM has hooks all throughout the rest of linux kernel. There
> >> is a difference however. The linux firewalling code in addition to hooks has
> >> tables behind those hooks that it consults. There is generic code to walk
> >> those tables and consult with different kernel modules to decide if we should
> >> drop a packet. Each of those kernel modules provides a different capability
> >> that can be used to generate a firewall.
> >
> > This is almost *EXACTLY* what SELinux provides as an LSM module. The one
> > difference is that with SELinux some compromises and restrictions have been
> > made so that (theoretically) the resulting policy can be exhaustively analyzed
> > to *prove* what it allows and disallows. It may be that SELinux should be
> > split into 2 parts, one that provides the underlying table-matching and the
> > other that uses it to provide the provability guarantees. Here's a direct
> > comparison:
> >
> > netfilter:
> > (A) Each packet has src, dst, port, etc that can be matched
> > (B) Table of rules applied sequentially (MATCH => ACTION)
> > (C) Rules may alter the properties of packets as they are routed/
> > bridged/etc
> >
> > selinux:
> > (A) Each object has user, role, and type that can be matched
> > (B) Table of rules searched by object parameters (MATCH => allow/
> > auditallow/transition)
> > (C) Rules may alter the properties of objects through transition rules.
>
> Ok. There is something here.
>
> However in a generic setup, at least role would be an extended match
> criteria provided by the selinux module. It would not be a core
> attribute. It would need to depend on some extra functionality being
> compiled in.
>
> >> I'm not yet annoyed enough to go implement an iptables like interface to the
> >> LSM enhancing it with more generic mechanism to make the problem simpler, but
> >> I'm getting there. Perhaps next time I'm bored.
> >
> > I think a fair amount of what we need is already done in SELinux, and efforts
> > would be better spent in figuring out what seems too complicated in SELinux and
> > making it simpler. Probably a fair amount of that just means better tools.
>
> How about thinking of it another way.
>
> Perform the split up you talked about above and move the table
> matching into the LSM hooks.
>
> Use something like the iptables action and match to module mapping
> code so we can have multiple modules compiled in and useable at the
> same time with the LSM hooks.
>
> I think it is firmly established that selling SElinux to everyone is
> politically untenable. However enhancing the LSM (even if it is
> mostly selinux code movement down a layer) I think can be sold.
>
> If I could run Serge's isolation code and selinux rules at the same
> time that would be interesting.
But given that namespaces are making it upstream, what else is to be
gained from the bsdail module? What exactly are you looking for?
1. are you looking to cover all the corner cases - i.e. prevent killing
a process in another namespace through F_SETOWN or mqueue, etc?
2. are you looking for a potentially easier fix to the current absence
of isolation in the user namespace?
3. are you just generally looking to make lsm/selinux easier for
yourself to configure?
If 1, an selinux policy should cover you. So you can then skip to 3.
Or, alternatively, I do plan - as soon as my free time clears up a bit -
on demonstrating how to write some selinux policy to create a secure
container based on current -mm + your experimental network namespace
patches.
If 2, well, see 1. But for the long term solution, I don't think we
want that in an LSM. Rather we want to implement proper user namespace
enforcement in the kernel, then put 'policy' basically at the filesystem
layer and further in userspace. So maybe not exactly what I've outlined
before, but certainly similar. The filesystem can decide what user
namespace a file belongs to, whether based on cryptographic keys stored
with the file, or based on mount arguments, or something else. Users
can get access to those files using keys stored with their task_struct
if they are not in the right user namespace to otherwise access it.
Otherwise access to a file in another user namespace defaults to
the 3d mode bit, so you default to being 'other'. Further isolation
probably still belongs in selinux or another lsm.
If 3, then selinux policy modules may actually help you, else either
a new LSM (maybe like LIDS) or a userspace tool which is a front-end to
selinux policy, emulating the iptables rules formats, may be what you
want?
> My impression is that selinux is one monolithic blob that doesn't
> allow me to incrementally add matching or action features that I
> find interesting.
Actually with policy modules it gets much much better. I have in fact
been able to pretty easily write a short policy module to, say, create
an selinux user which ran as root and had full access to the system to
do system setup for automated testing. There is a learning curve in
having to look at existing modules for maybe a few days to get started,
but once you get started the policy modules do make it very easy to
add to current policy.
-serge
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