New LSM hooks

[Casey Schaufler]

On 2/5/2019 10:15 AM, Paul Moore wrote:
> 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.

It's often not the LSM maintainer, but the developer of
the feature that introduces the hook. That's the case that
has been a problem.


>> 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.

Agreed that there's ambiguity about what should be done for
an "optional" feature. If RedHat adds support for a feature
they have every reason to make sure it works correctly with
SELinux and no incentive to implement AppArmor hooks. But is
overlayfs an "optional" feature? If the container developers
who wanted overlayfs only cared about an SELinux environment
does that excuse them from introducing a feature that didn't
work with Smack?

>> 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.

Two edged sword. If someone introduced an LSM that worked
with btrfs but broke ext4 and xfs it wouldn't be likely to
make it upstream. I wouldn't expect the developer who wants
to introduce a new hook to provide all the LSM versions, but
I would like that developer to work with the LSM maintainers
before the patch is submitted.

>   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.

Yes.

>> 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.

Hey, if you don't play, you can't win. ;)

> I think trying to legislate this too much
> is a sure path to failure.

There's not doubt that "too much" legislation would be a problem.
Right now, we have *no* guidance or guidelines.


> 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.

Sure, but it's a royal pain to deal with the wild inconsistency
of interfaces we have today. I would like to discourage new
hooks that do things like expose filesystem or network protocol
implementation details to the LSM interface.

> 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.

VFS manages this, and I wouldn't call Al a fool.