Kconfig-Hardened-Check - A Tool For Checking The Hardening Options In The Linux Kernel Config

Motivation

There are plenty of Linux kernel hardening config options. A lot of them are not enabled by the major distros. We have to enable these options ourselves to make our systems more secure.

But nobody likes checking configs manually. So let the computers do their job!

kconfig-hardened-check.py helps me to check the Linux kernel Kconfig option list against my hardening preferences, which are based on the

KSPP recommended settings, CLIP OS kernel configuration, last public grsecurity patch (options which they disable), SECURITY_LOCKDOWN_LSM patchset, direct feedback from Linux kernel maintainers (Daniel Vetter in issue #38).

I also created Linux Kernel Defence Map that is a graphical representation of the relationships between these hardening features and the corresponding vulnerability classes or exploitation techniques.

Supported microarchitectures

X86_64 X86_32 ARM64 ARM

Installation

You can install the package:

pip install git+https://github.com/a13xp0p0v/kconfig-hardened-check

or simply run ./bin/kconfig-hardened-check from the cloned repository.

Usage

usage: kconfig-hardened-check [-h] [--version] [-p {X86_64,X86_32,ARM64,ARM}]
[-c CONFIG]
[-m {verbose,json,show_ok,show_fail}]

Checks the hardening options in the Linux kernel config

optional arguments:
-h, --help show this help message and exit
--version show program's version number and exit
-p {X86_64,X86_32,ARM64,ARM}, --print {X86_64,X86_32,ARM64,ARM}
print hardening preferences for selected architecture
-c CONFIG, --config CONFIG
check the kernel config file against these preferences
-m {verbose,json,show_ok,show_fail}, --mode {verbose,json,show_ok,show_fail}
choose the report mode

Output for Ubuntu 20.04 LTS (Focal Fossa) kernel config

kconfig-hardened-check versioning

I usually update the kernel hardening recommendations after each Linux kernel release.

So the version of kconfig-hardened-check is associated with the corresponding version of the kernel.

The version format is: [major_number].[kernel_version].[kernel_patchlevel]

Questions and answers

Q: How disabling CONFIG_USER_NS cuts the attack surface? It's needed for containers!

A: Yes, the CONFIG_USER_NS option provides some isolation between the userspace programs, but the tool recommends disabling it to cut the attack surface of the kernel.

The rationale:

A nice LWN article about the corresponding LKML discussion: https://lwn.net/Articles/673597/

A twitter thread about CONFIG_USER_NS and security: https://twitter.com/robertswiecki/status/1095447678949953541

A good overview of the trade-off between having user namespaces enabled, disabled and available only for root: https://github.com/NixOS/nixpkgs/pull/84522#issuecomment-614640601

Q: Why CONFIG_GCC_PLUGINS is automatically disabled during the kernel compilation?

A: It means that your gcc doesn't support plugins. For example, if you have gcc-7 on Ubuntu, try to install gcc-7-plugin-dev package, it should help.

Q: KSPP and CLIP OS recommend CONFIG_PANIC_ON_OOPS=y. Why doesn't this tool do the same?

A: I personally don't support this recommendation because it provides easy denial-of-service attacks for the whole system (kernel oops is not a rare situation). I think having CONFIG_BUG is enough here -- if we have a kernel oops in the process context, the offending/attacking process is killed.

Q: What about performance impact of these kernel hardening options?

A: Ike Devolder @BlackIkeEagle made some performance tests and described the results in this article.

Q: Why enabling CONFIG_STATIC_USERMODEHELPER breaks various things in my GNU/Linux system? Do I really need that feature?

A: Linux kernel usermode helpers can be used for privilege escalation in kernel exploits (example 1, example 2). CONFIG_STATIC_USERMODEHELPER prevents that method. But it requires the corresponding support in the userspace: see the example implementation by Tycho Andersen @tych0.

Q: Does my kernel have all those mitigations of Transient Execution Vulnerabilities in my hardware?

A: Checking the kernel config is not enough to answer this question. I highly recommend using spectre-meltdown-checker tool maintained by Stéphane Lesimple @speed47.

Kconfig-Hardened-Check - A Tool For Checking The Hardening Options In The Linux Kernel Config