In early 2024, a new vulnerability was found and fixed in the Linux kernel binder subsystem, referenced as CVE-2024-26926. The bug involves improper offset alignment handling in the binder_get_object() function. This post will explain what the issue was, how it was fixed, and what risks it created for users and devices, using simple language and clear code samples. This is a complete, exclusive guide to CVE-2024-26926.

What is the Linux Binder?

Linux Binder is a key part of Android. It lets processes communicate securely (IPC: inter-process communication). If you use Android devices, the Binder is always running under the hood.

What Happened? (The Vulnerability)

A change in the Linux kernel’s binder code was introduced to avoid data leaks:

> Commit 6d98eb95b450 ("binder: avoid potential data leakage when copying txn") introduced changes to how binder objects are copied. In doing so, it unintentionally removed an offset alignment check done through calls to binder_alloc_copy_from_buffer() -> check_buffer().

Before this commit, there was a built-in check to make sure that when copying binder objects, the offset (where to copy from) was properly aligned (e.g., on a 4 or 8 byte boundary). Removing the alignment check with the code change meant that an attacker could trick the kernel into reading or writing data at misaligned offsets.

Why is Offset Alignment Important?

Offset alignment is crucial for data integrity and security. Think of memory as a series of mailboxes; if you try to access a mailbox between two legitimate boxes, you might get the wrong mail—or someone else's mail! Misaligned reads could lead to:

Details: Where Was the Problem?

The problematic code was here, after commit 6d98eb95b450:

// Code before the fix
if (copy_from_user(&object, (void __user *)(uintptr_t)
    buffer->user_data + off, sizeof(object)))
    return -EFAULT;
// Omitted offset alignment check

Notice how the code copies an object from user space using the copy_from_user() function, but it doesn’t check if off (the offset) is properly aligned.

To fix CVE-2024-26926, the kernel developers added the missing alignment check explicitly

// Code after the fix
if (off % sizeof(u64) != ) {
    binder_user_error("Invalid object offset: misaligned\n");
    return -EINVAL;
}
if (copy_from_user(&object,
    (void __user *)(uintptr_t) buffer->user_data + off,
    sizeof(object)))
    return -EFAULT;

Now, before copying data, the code checks the offset. If it’s not aligned with 8 bytes (common alignment for 64-bit systems), it fails. This blocks attackers from tricking the binder kernel code.

How Could Attackers Exploit CVE-2024-26926?

An attacker (e.g., a malicious Android app) could use a custom crafted transaction to pass a misaligned offset. The binder code would then possibly read or write memory it shouldn’t, leading to memory leaks or kernel crashes:

# Pseudo-exploit code for educational purposes
import fcntl, struct

BINDER_DEVICE = "/dev/binder"
BINDER_WRITE_READ = xc0306201  # ioctl code for binder

fd = open(BINDER_DEVICE, "rb+")

# Suppose off is intentionally misaligned
off = 3  # Not 4 or 8 aligned
object_data = b"\x00" * 40  # crafting potentially invalid object

# send bad object
try:
    fcntl.ioctl(fd, BINDER_WRITE_READ, object_data)
except OSError as e:
    print("Error from kernel: ", e)

This is simplified, but illustrates how a bad app might poke the vulnerability.

- Kernel commit 6d98eb95b450: github.com/torvalds/linux/commit/6d98eb95b450
- CVE-2024-26926 page: nvd.nist.gov/vuln/detail/CVE-2024-26926
- Fix in Linux 6.9-rc1: lore.kernel.org/patch/cover.1710954056.git.christian@brauner.io/

Conclusion: What Should You Do?

- If you maintain Android devices or Linux systems using Binder IPC, update to the latest kernel that includes this fix.

Stay safe—don't let unaligned offsets open the door for attackers!

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Timeline

Published on: 04/25/2024 06:15:57 UTC
Last modified on: 05/04/2025 12:55:13 UTC