Hello Linux Gurus,
I am seeking divine inspiration.
I don’t understand the apparent lack of hypervisor-based kernel protections in desktop Linux. It seems there is a significant opportunity for improvement beyond the basics of KASLR, stack canaries, and shadow stacks. However, I don’t see much work in this area on Linux desktop, and people who are much smarter than me develop for the kernel every day yet have not seen fit to produce some specific advanced protections at this time that I get into below. Where is the gap in my understanding? Is this task so difficult or costly that the open source community cannot afford it?
Windows PCs, recent Macs, iPhones, and a few Android vendors such as Samsung run their kernels atop a hypervisor. This design permits introspection and enforcement of security invariants from outside or underneath the kernel. Common mitigations include protection of critical data structures such as page table entries, function pointers, or SELinux decisions to raise the bar on injecting kernel code. Hypervisor-enforced kernel integrity appears to be a popular and at least somewhat effective mitigation although it doesn’t appear to be common on desktop Linux despite its popularity with other OSs.
Meanwhile, in the desktop Linux world, users are lucky if a distribution even implements secure boot and offers signed kernels. Popular software packages often require short-circuiting this mechanism so the user can build and install kernel modules, such as NVidia and VirtualBox drivers. SELinux is uncommon, ergo root access is more or less equivalent to the kernel privileges including introduction of arbitrary code into the kernel on most installations. TPM-based disk encryption is only officially supported experimentally by Ubuntu and is usually linked to secure boot, while users are largely on their own elsewhere. Taken together, this feels like a missed opportunity to implement additional defense-in-depth.
It’s easy to put code in the kernel. I can do it in a couple of minutes for a “hello world” module. It’s really cool that I can do this, but is it a good idea? Shouldn’t somebody try and stop me?
Please insert your unsigned modules into my brain-kernel. What have I failed to understand, or why is this the design of the kernel today? Is it an intentional omission? Is it somehow contrary to the desktop Linux ethos?
A root use can modify the kernel on disk and then trigger a reboot. You need either containers or full virtualization to protect against that.
This cannot be done on most consumer OSs like Macs or Windows, or Android smartphones, because secure boot would refuse to load a modified kernel from the disk. It is possible on typical desktop Linux installations if they don’t implement secure boot.
Root access on any of these platforms would still result in persistent low level system access
On Android, secure boot causes boot loader validation, kernel validation, and subsequent validation by the kernel of all application code that is loaded into the system. You need an additional bug to obtain persistent access if the code has not been signed by an authorized party.
This is why iPhone jailbreaks are bifurcated into teathered and unteathered — many modern OSs require a second bug to survive a reboot and achieve persistence. The introduced code won’t pass signature check.
Secure boot is build on a model of proprietary software and antiuser freedom. For secure boot to do anything you first have to restrict what software the user can run which is already a no no. If you ignore that secure boot is often riddle with security problems and many companies use default keys. Your average device has multiple exploits.
Also, why would it matter if an adversary gain root vs kernel level access? Root can do anything so it wouldn’t matter much.