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SKP - Modern x86 Linux Kernel Patching

This is a PoC tool to patch x86 Linux kernel bzImages to load a kSHELF. This is a modern version of a the idea from Phrack 60-8 [1], but doing a very different style of patches.

This supports 5.15+ for both UEFI and BIOS, tested up to 6.11 (which was released today at time of writing).

Primarily tested with kernel images from Ubuntu, and my testing KConfig is derived from the default Ubuntu configuration. Other distros might do something that breaks this, though I hope not :) Make sure ftrace is enabled and you have CONFIG_REGULATOR as that is used in the initcall hook.

Currently relying on a private repo for the payloads, which will be GPL2'd soon (tm). You can probably get by with my older public version of klude, just know I have fixed the major bugs in it now, and have made it use multiple sections due to STRICT_RWX being default in modern kernels.

I have set this up to not need you to provide a payload, as the default behaviour is just to print some info during the kernels boot. You can modify the code in src/runtime/kshelf-loader/ if you want to do anything more.

Usage

Please read the source, as this is not something simple to use.

You will need to install the just command runner.

Help:

Available recipes:
    [Listing]
    default       # List Commands

    [build]
    clean         # Clean the Project
    patch-kernel kernel=env("SOURCE_KERNEL") payload=env("PAYLOAD", "") # Patch a kernel

    [run]
    gdb           # Connect to the GDB server
    run-bios      # Run a Kernel via BIOS
    run-grub-bios # Run the kernel via a BIOS grub rescue imagea
    run-grub-uefi # Run the Kernel via UEFI GRUB
    run-uefi      # Run a Kernel via UEFI with OVMF

    [setup]
    easylkb version kconfig=(BASEDIR / "configs/test.KConfig") # Use easylkb to build a kernel
    get-grub-uefi # Download the Ubuntu's UEFI build of GRUB
    get-rootfs    # Download OpenWRTs rootfs
    setup         # Install dependencies to build the project

    [testing]
    test-batch test_kernel_list payload=env("PAYLOAD") # Test a list of kernels

To setup the virtualenv and dependencies:

just setup

Then you need to the the following environment variables before you can patch a kernel:

export SOURCE_KERNEL=./sample-kernels/vmlinuz-6.8.0-41-generic
export PAYLOAD=../artifacts/main.bin

The following envvars exist:

  • PATCHED_KERNEL is the output kernel bzImage
  • SOURCE_KERNEL is the kernel image you are modifying,
  • ROOTFS is a rootfs to use for testing.
  • PAYLOAD is the kSHELF you want to load that was built with klude2.
  • OVMFFW is the OVMF firmware build you want to run in your tests.
  • EXTRA_PATCH is flags to src/patch-bzimage. You can disable uefi and bios patching with --no-uefi and --no-bios respectively.
  • EXTRA_STAGE2_DEFINE can be used to unset the DIRECT_PATCHING feature.

(You can also set these by an a per command basis, see the Justfile for internal names, then set those before the command you are trying to run!)

With those, you can run just patch-kernel and the patched kernel will be created. There is also support for two positional arguments to change the source kernel and payload instead of via the envvars.

You can then the following to test it out:

  • just run-uefi
  • just run-bios
  • just run-grub-uefi
  • just run-grub-bios

The default configuration requires one of the following to start the VM:

  • attaching gdb with gdb -ex "target remote localhost:1234"
  • connecting to localhost:55555 with netcat to start the virtual machine.

Prefixing the command with extra_qemu="", e.g just extra_qemu="" run-uefi will also autostart the kernel (and also allows extra qemu arguments)

If you:

  • need a rootfs, run just get-rootfs to download one from OpenWRT.
  • want to run this under uefi GRUB, run just get-grub-uefi to setup Ubuntu's UEFI GRUB (Note that the grub version you install limits which kernels you can boot!)

A build cache is in intermediate/SHASUM_OF_KERNEL which stores a copy of the kernels kallsyms and internal ELF.

If you need a kernel, easylkb is integrated, which you can use it via just easylkb 6.8 and get a working 6.8 kernel to test with. Adjust the version to try other versions, and you can also change the kconfig as well. The output kernel will be in ./tools/easylkb/kernel/linux-VERSION/arch/x86/boot/bzImage.

Testing

If you want to test a batch of kernels and see if they all boot correctly with the patch you can use the just test-batch command.

Create a file containing the paths to each kernel you want to test, then run just test-batch kernels.lst payload.bin and it will use ./src/scripts/verify.py to tell you which kernels failed to boot.

A timeout of 30 seconds is used for each kernel, which might not be enough. You can change it in ./src/test-batch.sh.

An example list of kernels looks like:

./samples/kernels/vmlinuz-6.8.0-41-generic
./tools/easylkb/kernel/linux-6.10/arch/x86/boot/bzImage
./tools/easylkb/kernel/linux-6.9/arch/x86/boot/bzImage
./tools/easylkb/kernel/linux-6.0/arch/x86/boot/bzImage

Bisecting Kernels

You'll probably end up needing to bisect changes to determine which changes broke things.

First, setup a script (wrapper.sh) in the directory of the kernel:

cd PATH_TO_SKP && ./src/scripts/bisect.sh $1 uefi

Then you can do a bisect like so:

export PAYLOAD=path_to_payload
git bisect start
git bisect old v5.15
git bisect new v5.16
git bisect run ./wrapper.sh `pwd`

Which should find the commit that introduced / fixed the issue in a hour or two.

The bisect.sh script does assume old is the one that causes the issue, and new is the one where it is fixed. You can change this behaviour by removing the --invert in ./src/scripts/bisect.sh

Techniques

This modifies two paths to boot the kernel. Via UEFI and by the traditional BIOS bootloaders. Both routes will load a kSHELF, which is a type of kernel module I developed to avoid having a module loaded.

Information Gathering

I use vmlinux-to-elf[2] to extract kernel symbols and their offsets, and unpack the kernel to search for a cavity to use in the BIOS path. This gets passed down to the build process for the runtime component as that needs to know where to place the payload in the kernel and also where symbols like kallsyms_lookup_name() are relative to it.

UEFI

First, we hook the UEFI entrypoint with code that will install a hook on ExitBootServices(). This hook can do one of two things:

  • Setup a hook for the Runtime function GetVariable() which should be called during the boot by most kernels.
  • Directly patch the kernel as it is decompressed at this point on post 6.6 kernels.

The runtime hook has some advantages in terms of it allowing the use of payloads of arbitary sizes, while the direct patch only allows ~1MB, depending on the kernel image (see src/scripts/find_space.py where it is at the time of writing set to 65kb) and also working on older kernel versions as ExitBootServices() is called much earlier in the boot process. The primary disavantage is that you have to do a runtime hook, and the path is seperate from what the BIOS hook does.

The runtime hook is probably what you should use in most cases however. You can force it to be always used by unseting DIRECT_PATCHING in the envvar.

This hook then runs the kSHELF loader and gets the module going.

BIOS

This BIOS path installs a hook in code32_start (see advanced hooks in [3]), which patches the code to jump to the decompressed kernel to then call our code to patch the kernel. We jump to our code by going to 0x100_000 + offset - start of .text.

This is done as the kernel relocates itself at this point, and will cut off our appended data from being accessed relatively. But there remains the original mapping 0x100_000, which we can use if we set the pref_address and disable relocation in the x86 boot protocol [3].

Our code here will then copy the remaining data into a known cavity in the kernel image, and hook an initcall to transfer control to it.

Background / Notes

This is not the first attempt at this sort of thing. The original paper in Phrack 60-8 [1] is the first attempt of this that I'm aware of. More recently, I'm aware of two projects [4][5] (found via [6]) that did this for non-x86 kernel images, aiming to replace the kernel and adjust various offsets. I looked at doing that sort of approach, but ended up deciding it was easier to go my route to support a wider variety of kernel versions.

I do think the approach to get transfer control to the appended code in Phrack 60-8 is a bit bizare, as I went and reviewed kernels from that era and the initcall approach did exist back then. Would have made far more sense to use it than modify the syscall table.

Like the article I didn't bother removing the bss clearing code, as I just appended 2 extra pages of null bytes to fill up bss, which worked fine with this technique as it doesn't need to survive the whole boot process. Those do get corrupted on older kernels in the BIOS bootpath though.

The x86 boot path has undergone a bit of work in 2023 [7][8], which made a lot of good changes. The PE header got reworked which made adding an extra section easier (though you can just remove .reloc in older images, which this project does).

The code does assume the added section is writable, to use global variables, which might cause issues with some UEFI firmware. The recent kernel changes were meant to avoid them existing in the kernel.

Kernel Images do include their own checksum, as part of build.c, but AFAIK nothing verifies it so I did not bother reimplementing it.

One thing I find a bit confusing is that the pref_address in the x86 boot protocol is meant to be where the kernel is relocated to. On modern kernels it is set to 0x1_000_000, but it was fine for me to lower it down to 0x100_000. GRUB does use it if the relocatable flag (otherwise 0x100_000 is used) is set but I got away without it being changed with the default -kernel flag in qemu with it always using 0x100_000. I believe qemu ignoring this is a bug, and GRUB always using 0x100_000 if the kernel isn't relocatable is wrong as well as it seems to contradict [3]:

This field, if nonzero, represents a preferred load address for the kernel. A relocating bootloader should attempt to load at this address if possible.

A non-relocatable kernel will unconditionally move itself and to run at this address.

Though reading that, it sounds like the kernel is relocating itself to do in-place decompression, which matches up with arch/x86/boot/compressed/head_64.S (lines 310 to 450 in the 6.10 tree). So maybe GRUB is doing the right thing?

If you care about infecting LKMs, I did a seperate project reimplementing another old phrack article that can do that. [9]

I am obviously not the first person to come up with hooking a runtime service. pcileech, a project for performing DMA attacks, uses it to target older kernel versions [10]. It finds kallsyms_lookup_name() in that shellcode, which is what you should do if you want to make the patching code smaller and not require using vmlinux-to-elf.

If the kernel is compiled with KASAN, the patch sometimes seems sometimes trigger a global-out-of-bounds, when calling a function with one of the payloads strings as an argument. Noticed this only in 6.10, wasn't sure if there was some changes related to KASAN here as my builds for 6.9 also had KASAN enabled.

On my remaining list todo is the following:

  • Code cleanup, src/patch-bzimage is a bit of a mess right now.
  • Maybe support older kernels. Unsure if I'll bother, as the issue is dealing with things like symbols being renamed, etc. I only ever want to care about kernels released in the last 5 years. 20.04 is the furthest I want to go back to.
  • A proper writeup.

License

GPL2

References