A new feature added in test snapshots for OpenBSD releases will create a unique kernel every time an OpenBSD user reboots or upgrades his computer.
This feature is named KARL — Kernel Address Randomized Link — and works by relinking internal kernel files in a random order so that it generates a unique kernel binary blob every time.
Currently, for stable releases, the OpenBSD kernel uses a predefined order to link and load internal files inside the kernel binary, resulting in the same kernel for all users.
KARL is different from ASLR
Developed by Theo de Raadt, KARL will work by generating a new kernel binary at install, upgrade, and boot time. If the user boots up, upgrades, or reboots his machine, the most recently generated kernel will replace the existing kernel binary, and the OS will generate a new kernel binary that will be used on the next boot/upgrade/reboot, constantly rotating kernels on reboots or upgrades.
KARL should not be confused with ASLR — Address Space Layout Randomization — a technique that randomizes the memory address where application code is executed, so exploits can’t target a specific area of memory where an application or the kernel is known to run.
“It still loads at the same location in KVA [Kernel Virtual Address Space]. This is not kernel ASLR!,” said de Raadt.
Instead, KARL generates kernel binaries with random internal structures, so exploits cannot leak or attack internal kernel functions, pointers, or objects. A technical explanation is available below.
A unique kernel is linked such that the startup assembly code is kept in the same place, followed by randomly-sized gapping, followed by all the other .o files randomly re-organized. As a result the distances between functions and variables are entirely new. An info leak of a pointer will not disclose other pointers or objects. This may also help reduce gadgets on variable-sized architectures, because polymorphism in the instruction stream is damaged by nested offsets changing.
“As a result, every new kernel is unique,” de Raadt says.
Feature developed in the last two months
Work on this feature started in May and was first discussed in mid-June on the OpenBSD technical mailing list. KARL has recently landed in snapshot versions of OpenBSD 6.1.
“The situation today is that many people install a kernel binary from OpenBSD, and then run that same kernel binary for 6 months or more. Of course, if you booted that same kernel binary repeatedly, the layout would be the same. That is where we are today, for commited code,” de Raadt says.
“However, snapshots of -current contain a futher change, which I worked on with Robert Peichaer. That change is scaffolding to ensure you boot a newly-linked kernel upon every reboot.
KARL is a unique feature
Speaking to Bleeping Computer, Tiberiu C. Turbureanu, founder of Technoethical, a startup that sells privacy-focused hardware products, says this feature appears to be unique to OpenBSD.
“It’s not implemented in Linux,” Turbureanu said. “This looks like a great idea,” the expert added, regarding the possibility of having this feature ported to the Linux kernel.
Instead, the Linux project has just added support for Kernel Address Space Layout Randomization (KASLR), a feature that ports ASLR to the kernel itself, loading the kernel at a randomized memory address.
This feature was turned on by default in Linux 4.12, released last week. The difference between the two is that KARL loads a different kernel binary in the same place, while KASLR loads the same binary in random locations. Same goal, different paths.
As for Windows, KARL is not supported, but Microsoft has used KASLR for many years. Fabian Wosar, Chief Technical Officer for antivirus maker Emsisoft is all on board with adding KARL to the Windows kernel.
“OpenBSD’s idea would go even further [than current Windows kernel protections] as everyone would have a unique kernel binary as well,” Wosar said in a private conversation with Bleeping Computer.
“So even if you had the address where the kernel starts (which is randomised), you couldn’t use it to figure out where certain functions are located, as the location of the functions relative to the kernel start will be different from system to system as well,” Wosar added.
Having KARL on other OS platforms would greatly improve the security of both Windows and Linux users.