New LSM hooks

[Paul Moore]

On Tue, Feb 5, 2019 at 12:40 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
> Full disclosure: This is all about me and my interests.
>
> Developers propose new LSM hooks for many reasons. Often,
> it's in support of an exotic feature such as Infiniband.
> Sometimes it's because someone got clever when they optimized
> an otherwise mundane feature and bypassed the usual hooks,
> as is being proposed for kernfs. Usually there is a particular
> security module (i.e. SELinux) that is targeted for the hook.
> In almost all cases a hook is provided for that LSM, but not
> for any other. In many cases the developers don't even include
> the LSM email list in the discussions. LSM maintainers find
> out about the new hooks after the fact.

I think it is each LSM maintainer's job to ensure that patches which
affect the LSM hooks, either through modification or addition, are
CC'd to the LSM list for discussion.  I've tried to be good about
this, but I'm sure I've missed some over the years.

> Prior to 2008, when there was only one LSM, this made perfect
> sense. Since then, it's been a regular practice justified by
> the notion that it's the LSM maintainer's option to use the
> hook or not. That also makes sense in cases where the use is
> strictly optional, as it is in binder. It does not make sense,
> however, when a core system facility like kernfs is involved.

The term "optional" is tricky here.  In all the cases I can think of
where a LSM hook has been added to an existing bit of kernel
functionality, the hook has almost always been optional if for no
other reason than it didn't exist in the previous kernel.  I suppose
one could argue that there is a functional disparity between LSMs if
one LSM implements the hook and the others do not, but that is a
different issue.

> I get a double whammy on these new LSM hooks. I have to try
> to figure out if Smack cares, and if it does, whether to proposed
> hook will solve the problem for Smack. Because Smack uses
> xattrs and process attributes differently from SELinux there are
> often problems with hooks that are provided with only an SELinux
> implementation. I also have to work out how the new hook will
> work in the stacked security module case. There are some existing
> hooks that are a special challenge there, and when a new hook is
> proposed that does the same kind of things (e.g. use of secid,
> secctx or list of xattrs) without considering the consequences
> for other modules.

Once again, I think it is important to CC the LSM list on these sorts
of patchsets, and to get the patch author to consider other LSMs, but
I think asking the original author to preemptively add support to each
LSM for every new addition is too high a bar.  I would encourage the
individual LSMs (both maintainers and contributors) to work with patch
authors in a constructive manner to add support for new hooks when
they make sense for the given LSM.

> What do I want, I hear you cry? I want some sanity in the way
> LSM hooks are introduced. I want some standards or conventions
> on what is appropriate to pass into and out of LSM hooks. I
> want push back on special purpose hooks that are required to
> fix the deficiencies of a filesystem or bizarre hardware
> implementation. I want to stop spending all my time trying to
> deal with new, crazy LSM hooks. There are enough old ones to
> keep me entertained, thank you very much.

Yeah, good luck with that.  I think trying to legislate this too much
is a sure path to failure.  Our saving grace is that the LSM hook
boundary is not a kernel/userspace boundary so we are free to change
it as needed, we just need to make sure if we change an existing hook
we don't break any of the existing in-tree LSMs.  We've done this in
the past without too much problem, even other subsystems have done
this for us (without much notification to the LSMs) and it has
generally worked out okay.

I'm all for increased collaboration between LSMs, e.g. requiring hooks
changes to CC the LSM list, but I think mandating anything beyond that
is a fool's errand.

-- 
paul moore
www.paul-moore.com