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

[Dr. Greg]

On Tue, Nov 19, 2024 at 11:49:07AM +0100, Roberto Sassu wrote:

Hi Roberto, I hope the week is going well for you.

> From: Roberto Sassu <roberto.sassu at huawei.com>
> 
> Integrity detection and protection has long been a desirable feature, to
> reach a large user base and mitigate the risk of flaws in the software
> and attacks.
> 
> However, while solutions exist, they struggle to reach a large user base,
> due to requiring higher than desired constraints on performance,
> flexibility and configurability, that only security conscious people are
> willing to accept.
> 
> For example, IMA measurement requires the target platform to collect
> integrity measurements, and to protect them with the TPM, which introduces
> a noticeable overhead (up to 10x slower in a microbenchmark) on frequently
> used system calls, like the open().
> 
> IMA Appraisal currently requires individual files to be signed and
> verified, and Linux distributions to rebuild all packages to include file
> signatures (this approach has been adopted from Fedora 39+). Like a TPM,
> also signature verification introduces a significant overhead, especially
> if it is used to check the integrity of many files.
> 
> This is where the new Integrity Digest Cache comes into play, it offers
> additional support for new and existing integrity solutions, to make
> them faster and easier to deploy.
> 
> The Integrity Digest Cache can help IMA to reduce the number of TPM
> operations and to make them happen in a deterministic way. If IMA knows
> that a file comes from a Linux distribution, it can measure files in a
> different way: measure the list of digests coming from the distribution
> (e.g. RPM package headers), and subsequently measure a file if it is not
> found in that list.
> 
> The performance improvement comes at the cost of IMA not reporting which
> files from installed packages were accessed, and in which temporal
> sequence. This approach might not be suitable for all use cases.
> 
> 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.

Roberto, a big picture question for you, our apologies if we
completely misunderstand your patch series.

The performance benefit comes from the fact that the kernel doesn't
have to read a file and calculate the cryptographic digest when the
file is accessed.  The 'trusted' digest value comes from a signed list
of digests that a packaging entity provides and the kernel validates.
So there is an integrity guarantee that the supplied digests were the
same as when the package was built.

Is there a guarantee implemented, that we missed, that the on-disk
file actually has the digest value that was initially generated by the
packaging entity when the file is accessed operationally?

Secondly, and in a related issue, what happens in a container
environment when a pathname is accessed that is actually a different
file but with the same effective pathname as a file that is in the
vendor validated digest list?

Once again, apologies, if we completely misinterpret the issues
involved.

Have a good remainder of the week.

As always,
Dr. Greg

The Quixote Project - Flailing at the Travails of Cybersecurity
              https://github.com/Quixote-Project