Kflat: Kernel memory flattening module

Kflat is a Linux kernel implementation of the library for fast serialization of C structures. It works by making a copy of the kernel memory for indicated variables and structures. Such copy can be used to recreate the layout of kernel memory in userspace process, for instance in Auto off-Target project.

Uflat is a userspace port of Kflat code, compiled as shared library and intended to be used as a fast serialization engine for Linux applications.

Currently supported architectures are x86_64 and ARM64.

Quick start guides

If you'd like to start using KFLAT, check out quick start guides available for:

• Linux (x86_64) - doc/Startup_Linux.md
• Android (emulator) - doc/Startup_Android.md

For more general desciption, refer to this README file.

If you're interesting in UFLAT, checkout docs in lib/ directory.

Building

In order to build kflat framework you're gonna need:

• the source of targeted linux kernel
• the C compiler used to build this kernel
• the C++ compiler for the target architecture - for instance, aarch64-linux-gnu-g++

After collecting the above requirements, you can build kflat for your target architecture by using commands below:

ARM64:

export KERNEL_DIR="<path to kernel source>"
export CLANG_DIR="<path to clang directory>"
make KDIR=$KERNEL_DIR CCDIR=$CLANG_DIR ARCH=arm64

x86_64:

export KERNEL_DIR="<path to kernel source>"
export CLANG_DIR="<path to clang directory>"
make KDIR=$KERNEL_DIR CCDIR=$CLANG_DIR ARCH=x86_64

There are some extra build parameters than can be set:

• KFLAT_OPTS - enable extra/testing features in kflat_core module, like KFLAT_GET_OBJ_SUPPORT,
• KLEE_LIBCXX_INSTALL - if you wish to build kflat library with support for KLEE symbolic execution engine, specify here the path to libc++ library built for KLEE.

Project layout

Project directory presents as follow:

.
├── core            // Main implementation of kflat module
├── doc             // Project documentation
├── include         // Shared include files
├── lib             // Userspace library for (un)flattening images
│   └── include
├── recipes         // Collection of example kflat recipes
│   └── random_read
├── tests           // Examples and unit tests
├── tools           // Userspace tools used with kflat
└── utils           // Miscellaneous utilities

How to use it?

Below you can find general instruction for using kflat kernel module. For more detailed information head out to markdown files in doc/ directory.

Load kernel modules

The first step is to upload compiled kernel modules to your target machine. Built modules are located in: core/kflat_core.ko and recipes/*/*.ko files. To load copied files into the kernel, use insmod command:

insmod kflat_core.ko
insmod kflat_recipe_1.ko
insmod kflat_recipe_2.ko
# ...

Keep in mind that insmod accepts only a single module at the time - if you provide multiple files in the args list, it will load only the first one and ignore the rest. If loading module fails with error File exists, you need to unload currently loaded kflat from the kernel with rmmod command (first you need to unload all modules with kflat recipes, before kernel lets you unload the core driver).

In case any other error occurs, please refer to dmesg content, which should describe the source of the issue. If kernel version mismatch is reported, make sure that environment variable KERNEL_DIR used during build is set to directory from which kernel running on the target machine has been built.

After successfully loading copied modules with insmod command, the file /sys/kernel/debug/kflat should appear on debugfs. On newer Android versions, you might need to manually mount debugfs:

mount -t debugfs none /sys/kernel/debug
ls /sys/kernel/debug

For security reasons, access to this node is restricted only to processes with CAP_SYS_RAWIO capability. In case, your application failed to interact with kflat due to EPERM (Permission Denied) error, ensure you have the necessary capabilities.

Run kflat tests

Kflat is equipped with set of tests to ensure that all the functionalities are working as expected. In order to run the test, use ./kflattest app located in tools/ directory.

# List all available tests
./kflattest -l

# Run test CIRCLE
./kflattest CIRCLE

# Run all available tests
./kflattest ALL

# Run test CIRCLE and save its output to directory `output`
./kflattest -o output CIRCLE

./kflattest automatically validates obtained memory dump and informs whether it's correct (SUCCESS) or not (FAILED). All tests can be found in tests/ subdirectory.

Prepare kflat recipe

Kflat requires a description of target memory called kflat recipe. Vast and extensive documentation of recipes format is pending. For sample recipes, refer to directory recipes/.

Script for automatic generation of such recipes for any given kernel structure is under development. The current revision can be found in utils/ directory.

Execute kflat recipe

To execute selected kflat recipe, use executor app located in tools/ directory. The basic usage looks as follow:

# Run kflat recipe named random_read_iter and trigger it by AUTOmatically READing from `/dev/random` and save
#  the dumped memory to file image.kflat
# Executor will automatically 
./executor -o image.kflat AUTO random_read_iter READ /dev/random

# Run kflat recipe named do_init_module and wait for an external MANUAL trigger by user. By default, the dump is saved to "dump.kflat". 
./executor MANUAL do_init_module 
# In another terminal
insmod some_module.ko

Recipe identifier is the name of function for which assigned recipe will be triggered. IDs are insensitive to case. For details on available program arguments, please refer to ./executor --help:

Process kflat image

After executing kflat recipe, dumped slice of kernel memory can be loaded into userspace process address space with help from Unflatten library located in lib/ directory.

Detailed example of how to use Unflatten library can be found in the source code of imginfo app. Basic example is also listed below.

#include "unflatten.hpp"

int main(int argc, char** argv) {
    Flatten flatten;

    FILE* in = fopen(argv[1], "r");
    assert(in != NULL);

    assert(flatten.load(in, NULL) == 0);

    const struct A* pA = (const struct A*) flatten.get_next_root();
}

Refer to README file in lib/ directory for details regarding API reference and C bindings usage.

Talks

Talk about KFLAT from the Open Source Summit NA 2023 can be found here

Documentation

KFLAT - Selective Kernel Memory Serialization for Security and Debugging