Re: [dm-devel] [RFC PATCH v5 00/11] Integrity Policy Enforcement LSM (IPE)

[Chuck Lever]

> On Aug 5, 2020, at 2:15 PM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> 
> On Wed, 2020-08-05 at 09:59 -0700, James Morris wrote:
>> On Wed, 5 Aug 2020, James Bottomley wrote:
>> 
>>> I'll leave Mimi to answer, but really this is exactly the question that
>>> should have been asked before writing IPE.  However, since we have the
>>> cart before the horse, let me break the above down into two specific
>>> questions.
>> 
>> The question is valid and it was asked. We decided to first prototype what 
>> we needed and then evaluate if it should be integrated with IMA. We 
>> discussed this plan in person with Mimi (at LSS-NA in 2019), and presented 
>> a more mature version of IPE to LSS-NA in 2020, with the expectation that 
>> such a discussion may come up (it did not).
> 
> When we first spoke the concepts weren't fully formulated, at least to
> me.
>> 
>> These patches are still part of this process and 'RFC' status.
>> 
>>>   1. Could we implement IPE in IMA (as in would extensions to IMA cover
>>>      everything).  I think the answers above indicate this is a "yes".
>> 
>> It could be done, if needed.
>> 
>>>   2. Should we extend IMA to implement it?  This is really whether from a
>>>      usability standpoint two seperate LSMs would make sense to cover the
>>>      different use cases.
>> 
>> One issue here is that IMA is fundamentally a measurement & appraisal 
>> scheme which has been extended to include integrity enforcement. IPE was 
>> designed from scratch to only perform integrity enforcement. As such, it 
>> is a cleaner design -- "do one thing and do it well" is a good design 
>> pattern.
>> 
>> In our use-case, we utilize _both_ IMA and IPE, for attestation and code 
>> integrity respectively. It is useful to be able to separate these 
>> concepts. They really are different:
>> 
>> - Code integrity enforcement ensures that code running locally is of known 
>> provenance and has not been modified prior to execution.

My interest is in code integrity enforcement for executables stored
in NFS files.

My struggle with IPE is that due to its dependence on dm-verity, it
does not seem to able to protect content that is stored separately
from its execution environment and accessed via a file access
protocol (FUSE, SMB, NFS, etc).


>> - Attestation is about measuring the health of a system and having that 
>> measurement validated by a remote system. (Local attestation is useless).
>> 
>> I'm not sure there is value in continuing to shoe-horn both of these into 
>> IMA.
> 
> True, IMA was originally limited to measurement and attestation, but
> most of the original EVM concepts were subsequently included in IMA. 
> (Remember, Reiner Sailer wrote the original IMA, which I inherited.  I
> was originially working on EVM code integrity.)  From a naming
> perspective including EVM code integrity in IMA was a mistake.  My
> thinking at the time was that as IMA was already calculating the file
> hash, instead of re-calculating the file hash for integrity, calculate
> the file hash once and re-use it for multiple things - measurement, 
> integrity, and audit.   At the same time define a single system wide
> policy.
> 
> When we first started working on IMA, EVM, trusted, and encrypted keys,
> the general kernel community didn't see a need for any of it.  Thus, a
> lot of what was accomplished has been accomplished without the backing
> of the real core filesystem people.
> 
> If block layer integrity was enough, there wouldn't have been a need
> for fs-verity.   Even fs-verity is limited to read only filesystems,
> which makes validating file integrity so much easier.  From the
> beginning, we've said that fs-verity signatures should be included in
> the measurement list.  (I thought someone signed on to add that support
> to IMA, but have not yet seen anything.)

Mimi, when you and I discussed this during LSS NA 2019, I didn't fully
understand that you expected me to implement signed Merkle trees for all
filesystems. At the time, it sounded to me like you wanted signed Merkle
trees only for NFS files. Is that still the case?

The first priority (for me, anyway) therefore is getting the ability to
move IMA metadata between NFS clients and servers shoveled into the NFS
protocol, but that's been blocked for various legal reasons.

IMO we need agreement from everyone (integrity developers, FS
implementers, and Linux distributors) that a signed Merkle tree IMA
metadata format, stored in either an xattr or appended to an executable
file, will be the way forward for IMA in all filesystems.


> Going forward I see a lot of what we've accomplished being incorporated
> into the filesystems.  When IMA will be limited to defining a system
> wide policy, I'll have completed my job.
> 
> Mimi
> 
>> 
>>> I've got to say the least attractive thing
>>>      about separation is the fact that you now both have a policy parser.
>>>       You've tried to differentiate yours by making it more Kconfig
>>>      based, but policy has a way of becoming user space supplied because
>>>      the distros hate config options, so I think you're going to end up
>>>      with a policy parser very like IMAs.