FIPS and the Linux Kernel

Posted March 5th, 2009 by

Recently I was building a new kernel for my firewall and noticed an interesting new option in the Cryptographic API: “FIPS 200 compliance“.

You can imagine how very interesting and somewhat confusing this is to a stalwart FISMA practitioner. Reading through FIPS 200 it’s hard to find mention of cryptography, much less a technical specification that could be implemented in the Linux kernel. FIPS 140, FIPS 197, FIPS 186, FIPS 46 and FIPS 180 standards would be natural fits in the Cryptographic API but FIPS 200? The kernel help description didn’t clear things up:


This options enables the fips boot option which is
required if you want to system to operate in a FIPS 200
certification. You should say no unless you know what
this is.

Symbol: CRYPTO_FIPS [=n]
Prompt: FIPS 200 compliance
Defined at crypto/Kconfig:24
Depends on: CRYPTO
-> Cryptographic API (CRYPTO [=y])

Given that examining the kernel code was a little beyond my ken and I couldn’t test to discover what it did I turned to the third of the 800-53A assessment methods, interview. A little digging on turned up the man behind this kernel magic, Neil Horman. He was able to shed some light on what is called the fips_enabled flag.

As it turns out the FIPS 200 compliance function wasn’t as exciting as I’d hoped but it does point to interesting future possibilities.

So what does it do? In the words of Neil Horman, it is a “flag for determining if we need to be operating in some fips_compliant mode (without regard to the specific criteria)”. This means it is sort of a place holder for future developments so the kernel can operate in a mode that uses a FIPS 140-2 cryptographic module.

Did you notice the word that wasn’t included in the last paragraph? Validated. Yes, there are no validated cryptographic modules in the Linux upstream kernel. If you look at the kernel’s Cryptographic API you will find listed the “AES cipher algorithms” and “DES and Triple DES EDE cipher algorithms”. These may be compliant with FIPS standards but they are not validated.

This begs the question, why have a FIPS 200 compliance flag if you can’t meet the FIPS 140-2 requirement? This is the interesting part. Let’s say a distro decides it wants to become very FISMA friendly and get their kernel’s FIPS 140-2 cryptographic module validated. Well, if the validation of the OpenSSL VCM is an apt example the distro’s Linux kernel will need to operate in a FIPS compliant mode to verifiably load the cryptographic module. So the inclusion of the fips_enabled flag enables future compliance.

Sadly it is unlikely that any single Linux distro getting their cryptographic module validated will not translate to the upstream kernel having a validated cryptographic module. If you look at the catalog of FIPS 140-2 VCM’s the modules are only validated for particular code versions and operating mode. As the upstream kernel code won’t likely see the revisions made by the downstream distro in order to achieve the VCM until after the VCM is issued it doesn’t inherit the validation.

Polyester Resin Kernel photo by  Marshall Astor РFood Pornographer.

Two possible scenarios were discussed with Neil to allow for upstream Linux kernel incorporation of a VCM.

The first scenario would be that the upstream kernel gets all the revisions made by the downstream distro to gain the VCM designation. It then goes through the process to gain the VCM itself. Unfortunately as the code is under constant revision and can’t be locked as soon as a revision was committed to the code base the VCM would be invalidated. Only a particular build of the Linux kernel could claim to be validated.

The second scenario would be a revision to the Linux kernel that allowed for the downstream’s Linux distro’s VCM to be loaded instead of the standard Linux Cryptographic API. When asked about this scenario Neil had this to say:

“That said, theres no reason the crypto api couldn’t be ripped out and replaced with a different implementation, one that is maintained independently and its certification kept up. Of course, anyone so doing would need to keep up with the pace of kernel development, and that in turn brings the need for recertification, so its rather a lost effort in my opinion. I certainly wouldn’t recommend doing so, its just too much work.”

So the solution would either be short lived and costly or long lived and insecure.

Sadly this means that there is no easy way to include FIPS 140-2 VCM within the upstream Linux kernel. But each distro can modify their Cryptographic API and validate a cryptographic module to allow for FIPS 200 compliance. With the FIPS 200 compliance flag now in the Linux kernel it is possible for this to be verified. And that’s a happy thought for Federal Linux users.

My many thanks to Neil Horman, without whom I’d have nothing to write.

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