[PATCH v6 00/15] integrity: Introduce the Integrity Digest Cache

[Roberto Sassu]

On Tue, 2024-12-03 at 20:06 +0000, Eric Snowberg wrote:
> 
> > On Nov 26, 2024, at 3:41 AM, Roberto Sassu <roberto.sassu at huaweicloud.com> wrote:
> > 
> > On Tue, 2024-11-26 at 00:13 +0000, Eric Snowberg wrote:
> > > 
> > > > On Nov 19, 2024, at 3:49 AM, Roberto Sassu <roberto.sassu at huaweicloud.com> wrote:
> > > > 
> > > > From: Roberto Sassu <roberto.sassu at huawei.com>
> > > > 
> > > > The Integrity Digest Cache can also help IMA for appraisal. IMA can simply
> > > > lookup the calculated digest of an accessed file in the list of digests
> > > > extracted from package headers, after verifying the header signature. It is
> > > > sufficient to verify only one signature for all files in the package, as
> > > > opposed to verifying a signature for each file.
> > > 
> > > Is there a way to maintain integrity over time?  Today if a CVE is discovered 
> > > in a signed program, the program hash can be added to the blacklist keyring. 
> > > Later if IMA appraisal is used, the signature validation will fail just for that 
> > > program.  With the Integrity Digest Cache, is there a way to do this?  
> > 
> > As far as I can see, the ima_check_blacklist() call is before
> > ima_appraise_measurement(). If it fails, appraisal with the Integrity
> > Digest Cache will not be done.
> 
> 
> It is good the program hash would be checked beforehand and fail if it is 
> contained on the list. 
> 
> The .ima keyring may contain many keys.  If one of the keys was later 
> revoked and added to the .blacklist, wouldn't this be missed?  It would 
> be caught during signature validation when the file is later appraised, but 
> now this step isn't taking place.  Correct?

For files included in the digest lists, yes, there won't be detection
of later revocation of a key. However, it will still work at package
level/digest list level, since they are still appraised with a
signature.

We can add a mechanism (if it does not already exist) to invalidate the
integrity status based on key revocation, which can be propagated to
files verified with the affected digest lists.

> With IMA appraisal, it is easy to maintain authenticity but challenging to 
> maintain integrity over time. In user-space there are constantly new CVEs.  
> To maintain integrity over time, either keys need to be rotated in the .ima 
> keyring or program hashes need to be frequently added to the .blacklist.   
> If neither is done, for an end-user on a distro, IMA-appraisal basically 
> guarantees authenticity.
> 
> While I understand the intent of the series is to increase performance, 
> have you considered using this to give the end-user the ability to maintain 
> integrity of their system?  What I mean is, instead of trying to import anything 
> from an RPM, just have the end-user provide this information in some format 
> to the Digest Cache.  User-space tools could be built to collect and format 

This is already possible, digest-cache-tools
(https://github.com/linux-integrity/digest-cache-tools) already allow
to create a digest list with the file a user wants.

But in this case, the user is vouching for having taken the correct
measure of the file at the time it was added to the digest list. This
would be instead automatically guaranteed by RPMs or other packages
shipped with Linux distributions.

To mitigate the concerns of CVEs, we can probably implement a rollback
prevention mechanism, which would not allow to load a previous version
of a digest list.

> the data needed by the Digest Cache.  This data  may allow multiple versions 
> of the same program.  The data would then be signed by one of the system 
> kernel keys (either something in the secondary or machine keyring), to maintain 
> a root of trust.  This would give the end-user the ability to have integrity however 
> they see fit.  This leaves the distro to provide signed programs and the end-user 
> the ability to decide what level of software they want to run on their system.  If 
> something isn't in the Digest Cache, it gets bumped down to the traditional 
> IMA-appraisal.  I think it would simplify the problem you are trying to solve, 

All you say it is already possible. Users can generate and sign their
digest lists, and add enroll their key to the kernel keyring.

> especially around the missing kernel PGP code required for all this to work, 
> since it wouldn't be necessary.   With this approach, besides the performance 
> gain, the end-user would gain the ability to maintain integrity that is enforced by
> the kernel.

For what I understood, Linus would not be against the