Introduction: The Peril of Kernel Use-After-Free (UAF)

Kernel exploitation on Android is a critical area of research and practical security. A Use-After-Free (UAF) vulnerability, specifically in the kernel context, represents one of the most potent primitives an attacker can leverage. It occurs when a program continues to use a pointer after the memory it points to has been freed, potentially allowing an attacker to reclaim that freed memory with malicious data. In the kernel, this can lead to arbitrary code execution, privilege escalation, or full system compromise. This hands-on lab will delve into the methodology of exploiting a UAF on an ARM64 Android kernel to achieve arbitrary read and write primitives, which are foundational for almost any advanced exploit.

Our primary goal is to demonstrate how a UAF can be transformed into arbitrary kernel memory read and write capabilities, a stepping stone to root privileges.

Setting the Stage: Your Android Kernel Lab

To follow along practically, you would typically need:

• An ARM64 Android device with root access, ideally running a custom-built kernel where KASLR is either disabled or a bypass is available.
• A Linux host machine with the Android NDK and toolchain for cross-compiling userspace exploits.
• Kernel source code corresponding to your device’s kernel version for understanding structures and symbols.
• Debugging tools like GDB with `qemu-system-aarch64` for emulated environments, or `kgdb` for physical devices.

For simplicity in this guide, we’ll assume we have a basic understanding of kernel memory management (SLAB/SLUB) and the ability to interact with a vulnerable kernel module via `ioctl`.

Understanding the UAF Primitive (Conceptual Example)

Imagine a vulnerable kernel module that manages a simple object. Let’s call it `my_object`. This object might contain data and a function pointer table (vtable or `ops` struct).

struct my_object { uint64_t data[8]; void (*cleanup)(struct my_object *obj);};

A typical vulnerable driver might expose `ioctl` commands:

1. `CMD_ALLOC`: Allocates `my_object`, initializes it.
2. `CMD_FREE`: Frees `my_object`.
3. `CMD_USE`: Performs an operation on `my_object`, potentially calling `cleanup`.

The UAF vulnerability arises if `CMD_FREE` is called, and then `CMD_USE` is called afterwards, or if two threads race, freeing and then using the same object. Here’s a simplified pseudo-code representation of the vulnerability:

static struct my_object *global_obj = NULL;long my_driver_ioctl(struct file *filp, unsigned int cmd, unsigned long arg){    switch (cmd) {        case CMD_ALLOC:            if (global_obj) return -EBUSY;            global_obj = kmalloc(sizeof(struct my_object), GFP_KERNEL);            if (!global_obj) return -ENOMEM;            memset(global_obj, 0, sizeof(struct my_object));            global_obj->cleanup = my_default_cleanup; // Initialize function pointer            printk(

        
        
        

            

                
            
            

                
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