Kernel Self Protection Project

Mission Statement

This project starts with the premise that kernel bugs have a very long lifetime, and that the kernel must be designed in ways to protect against these flaws. We must think of security beyond fixing bugs. As a community, we already find and fix individual bugs via static checkers (compiler flags, smatch, coccinelle, coverity) and dynamic checkers (kernel configs, trinity, KASan). Those efforts are important and on-going, but if we want to protect our billion Android phones, our cars, the International Space Station, and everything else running Linux, we must get proactive defensive technologies built into the upstream Linux kernel. We need the kernel to fail safely, instead of just running safely.

These kinds of protections have existed for years in the PaX and grsecurity patches, and in piles of academic papers. For various social, cultural, and technical reasons, they have not made their way into the upstream kernel, and this project seeks to change that. Our focus is on kernel self-protection, rather than kernel-supported userspace protections. The goal is to eliminate classes of bugs and eliminate methods of exploitation.

Principles

A short list of things to keep in mind when designing self-protection features:

• Patience and an open mind will be needed. We're all trying to make Linux better, so let's stay focused on the results.
• Upstream development is evolutionary, not revolutionary, which means it can sometimes take time for features to become fully realized.
• Features will be more than finding bugs, and should be active at run-time to catch previously unknown flaws.
• Features will not be developer-"opt-in". When a feature is enabled at build time, it should work for all code built into the kernel (which has the side-effect of also covering out-of-tree code, like in vendor forks).

Work Areas

While there are already a number of upstream kernel security features, we are still missing many. While the following is far from a comprehensive list, it's at least a starting point we can add to:

Bug Classes

• Stack overflow
• Integer overflow
• Heap overflow
• Format string injection
• Kernel pointer leak
• Uninitialized variables
• Use-after-free

Exploitation Methods

• Kernel location
• Text overwrite
• Function pointer overwrite
• Userspace execution
• Userspace data usage
• Reused code chunks

Specific TODO Items

Besides the general work outlined above, there are number of specific tasks that have either been asked about frequently or are otherwise in need some time and attention:

• Split thread_info off of kernel stack (Done: x86, arm64, s390. Needed on arm, powerpc and others?)
• Move kernel stack to vmap area (Done: x86, s390. Needed on arm, arm64, powerpc and others?)
• Implement kernel relocation and KASLR for ARM
• Write a plugin to clear struct padding
• Write a plugin to do format string warnings correctly (gcc's -Wformat-security is bad about const strings)
• Make CONFIG_STRICT_KERNEL_RWX and CONFIG_STRICT_MODULE_RWX mandatory (done for arm64 and x86, other archs still need it)
• Convert remaining BPF JITs to eBPF JIT (with blinding) (In progress: arm)
• Write lib/test_bpf.c tests for eBPF constant blinding
• Further restriction of perf_event_open (e.g. perf_event_paranoid=3)
• Extend HARDENED_USERCOPY to use slab whitelisting (in progress)
• Extend HARDENED_USERCOPY to split user-facing malloc()s and in-kernel malloc()svmalloc stack guard pages (in progress)
• protect ARM vector table as fixed-location kernel target
• disable kuser helpers on arm
• rename CONFIG_DEBUG_LIST better and default=y
• add WARN path for page-spanning usercopy checks (instead of the separate CONFIG)
• create UNEXPECTED(), like BUG() but without the lock-busting, etc
• create defconfig "make" target for by-default hardened Kconfigs (using guidelines below)
• provide mechanism to check for ro_after_init memory areas, and reject structures not marked ro_after_init in vmbus_register()
• expand use of __ro_after_init, especially in arch/arm64
• Add stack-frame walking to usercopy implementations (Done: x86. In progress: arm64. Needed on arm, others?)
• restrict autoloading of kernel modules (like GRKERNSEC_MODHARDEN) (In progress: Timgad LSM)

Documentation

For kernel protections already in upstream (or under active development) that have specific documentation:

refcount_t

Kernel reference counter overflow protection