Re: [PATCH 2/9] security: Move trivial IMA hooks into LSM

Mimi Zohar
2022-10-20T12:18:36Z

On Wed, 2022-10-19 at 16:41 -0700, Kees Cook wrote:
> On Wed, Oct 19, 2022 at 04:45:41PM -0400, Mimi Zohar wrote:
> > On Wed, 2022-10-19 at 11:59 -0700, Kees Cook wrote:
> > > On Wed, Oct 19, 2022 at 10:34:48AM -0400, Mimi Zohar wrote:
> > > > On Thu, 2022-10-13 at 15:36 -0700, Kees Cook wrote:
> > > > > This moves the trivial hard-coded stacking of IMA LSM hooks into the
> > > > > existing LSM infrastructure.
> > > >
> > > > The only thing trivial about making IMA and EVM LSMs is moving them to
> > > > LSM hooks. Although static files may be signed and the signatures
> > > > distributed with the file data through the normal distribution
> > > > mechanisms (e.g. RPM), other files cannot be signed remotely (e.g.
> > > > configuration files). For these files, both IMA and EVM may be
> > > > configured to maintain persistent file state stored as security xattrs
> > > > in the form of security.ima file hashes or security.evm HMACs. The LSM
> > > > flexibility of enabling/disabling IMA or EVM on a per boot basis breaks
> > > > this usage, potentially preventing subsequent boots.
> > >
> > > I'm not suggesting IMA and EVM don't have specific behaviors that need to
> > > be correctly integrated into the LSM infrastructure. In fact, I spent a
> > > lot of time designing that infrastructure to be flexible enough to deal
> > > with these kinds of things. (e.g. plumbing "enablement", etc.) As I
> > > mentioned, this was more of trying to provide a head-start on the
> > > conversion. I don't intend to drive this -- please take whatever is
> > > useful from this example and use it. :) I'm happy to help construct any
> > > missing infrastructure needed (e.g. LSM_ORDER_LAST, etc).
> > >
> > > As for preventing subsequent boots, this is already true with other LSMs
> > > that save state that affects system behavior (like SELinux tags, AppArmor
> > > policy). IMA and EVM are not special in that regard conceptually.
> >
> > > Besides, it also looks like it's already possible to boot with IMA or EVM
> > > disabled ("ima_appraise=off", or "evm=fix"), so there's no regression
> > > conceptually for having "integrity" get dropped from the lsm= list at
> > > boot. And if you want it not to be silent disabling, that's fine --
> > > just panic during initialization if "integrity" is disabled, as is
> > > already happening.
> >
> > Being able to specify "ima_appraise=" on the boot command line requires
> > IMA_APPRAISE_BOOTPARAM to be configured. Even when specified, if the
> > system is booted with secure-boot mode enabled, it also cannot be
> > modified. With the ability of randomly enabling/disabling LSMs, these
> > protections are useless.
>
> Sure, so let's get lsm= added to the lockdown list, etc.

I thought the move to "lsm=" was to allow different LSMs to be
enabled/disabled at run time. Adding "lsm=" to the lockdown list
doesn't seem like the correct solution to limiting which LSMs can be
enabled/disabled at runtime. As I recall, lockdown needs to be enabled
by userspace.

> My point is for
> us to work through each of these concerns and address them. I am not an
> IMA/EVM expert, but I do understand the LSM infrastructure deeply, so
> I'd like to help you get these changes made.

Sure

--
thanks,

Mimi