Re: IMA: How to manage user space signing policy with others

From: Mimi Zohar
Date: Thu Feb 28 2013 - 14:24:16 EST

On Thu, 2013-02-28 at 10:13 -0500, Vivek Goyal wrote:
> Hi Mimi,
> I am running into issues w.r.t IMA policy management and user space
> signing. So thought of dropping a mail and gather some ideas.
> Currently IMA seems to able to one policy only which does not contain
> conflicting rules. We have tcb policies in-built and they don't have
> conflicting rules. User can put its own policy and that will replace
> kernel policy (default policy). And then user is responsible for making
> sure conflicting rules are not present.

The default integrity policy is a null policy, which can be replaced by
specifying a boot command line option(s). The integrity policy can be
replaced once, with a policy of your choice. Normally this would be
done in the initramfs, before pivoting root.

> Now with user space signing and secureboot, I have another set of rules
> which are not compatible with existing tcb policies.

I know. We've already discussed this

> This is how my
> rules look like as of today. These can change based on config options.
> appraise func=BPRM_CHECK appraise_type=optional
> appraise func=BPRM_POST_LOAD appraise_type=optional
> These rules are not compatible with tcp appraise rule.
> .action = APPRAISE,.fowner = GLOBAL_ROOT_UID,.flags = IMA_FOWNER
> That means in current scheme of things, multiple policies can't co-exist
> together. It has few disadvantages.

No, it just means the merged policy needs to make sense. The example you give

I've already suggested that you define a builtin secureboot integrity
policy, which is a subset of the ima_appraise_tcb policy. If secureboot
is enabled, then the secureboot integrity policy would be enabled by
default, in lieu of the default null policy.

I would suggest that the ima_appraise_tcb, which is more restrictive, be
permitted to replace the secureboot policy.

> - If we want IMA to be central point for all integrity measurement
> needs, then having one policy only is very limiting.

So far, you haven't made a convincing case for supporting more than one
policy. If anything, you've made it very clear that merging policies
needs to be well thought out.

> The fact that
> user can overirde that policy makes it worse as then kernel can
> not impose any policy at all.

The kernel IS enforcing the specified integrity policy.

> IOW, if user enables user space signign in kernel, say CONFIG_BIN_SIGN=y,
> then I need a way so that kernel can make sure IMA rules needed to
> ensure integrity of binaries are present and can not be overruled.

If you want to prohibit userspace from replacing the builtin policy, then
provide a Kconfig option based on secureboot.

> - Disabling policy can disable certain features in kernel. So in this
> case if user overides default policy, it will disable binary signing
> feature also (that too in a very unintutive way).

Or, you can look at Kconfig options as a way of defining what is
included in the build. The policy defines what is enabled.

> One possible way could be that we allow execution of all the relevant
> rules in a policy and return the ANDed results of all the rules. But
> this does not go well with the result caching. Atleast current IMA
> infrastructure does not allow it and might require overhaul.

> In general I am concerned about increased performance overhead if we
> allow multiple policies to co-exist.
> Performance overhead is a concern even without multiple policies. For
> user space signing, IMA hooks will be called for file operations like
> open(), mmap() etc and we don't require those to be called. I am not
> sure if performance overhead is significant or not. Once things start
> working, I will do some benchmarking.
> But coming back to the point, how to go about making sure user space
> signing policies can't be overridden if user has enabled user space
> signing feature in kernel.

Vivek, there are multiple concurrent threads to this discussion. One
thread discusses adding the 'optional' flag. The other thread discusses
defining a new hook to appraise the kernel image being loaded. If the
decision is to go forward with the latter, then this thread, using the
'optional' flag, becomes moot.

With a new hook to verify kernel images, you would be able to define a
single integrity rule, lets call the hook
security/ima_kexec_image_check(), that would appraise the integrity of
kernel images being loaded. There wouldn't be a convoluted policy that
verifies the signature of binaries, if it exists, yet permits all
binaries to execute, except those that failed signature verification.
You wouldn't need a new capability, nor rely on the signed binary to
appraise the images being loaded. If secureboot is enabled, the single
secureboot integrity rule could be merged safely with other policies.



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