Bluetooth is everywhere, and so is Linux. When a bug creeps into the Linux Bluetooth stack, it’s a big deal for anyone with a laptop, phone, or IoT device. That’s where CVE-2023-40283 comes in—a critical use-after-free bug deep in the Linux kernel’s Bluetooth subsystem. Let’s break down what happened, how it could be exploited, and what you can do about it.

What Is CVE-2023-40283 All About?

CVE-2023-40283 is a memory safety flaw found in the Linux kernel’s Bluetooth L2CAP (Logical Link Control and Adaptation Protocol) socket handling code. More specifically, it affects the l2cap_sock_release function in net/bluetooth/l2cap_sock.c on kernels before version 6.4.10.

The short of it: When disconnecting certain Bluetooth sockets, code may free (“release”) memory for a “parent” socket before it’s really done using its children. That leaves dangling pointers, and if hackers are clever, they can take control.

Why Is This Bad?

- Can be triggered from *user space* (not needing kernel/privileged access)
- Leads to use-after-free: An attacker can access freed memory and potentially run code with kernel privileges
- Remote or local: Bluetooth servers (like paired/connected devices) could be targets

Where’s the Flaw? (Code Deep Dive)

The bug is tracked in the Linux kernel’s L2CAP socket release code, which looks after the clean-up of sockets.

Check out the relevant snippet prior to the fix

// net/bluetooth/l2cap_sock.c

static int l2cap_sock_release(struct socket *sock)
{
    struct sock *sk = sock->sk;

    if (!sk)
        return ;

    // <...Some code omitted for brevity...>

    bt_sock_link_release(sk);

    // Problem: child sockets can be mishandled here –
    // freeing the parent can leave dangling children

    sock->sk = NULL;
    sock_put(sk); // Frees sk while children may still reference it

    return ;
}

In simple words: If the released socket (sk) has “child” sockets, these might still try to access sk even after it's been freed. When that happens, the kernel tries to use memory that may now contain something else—or nothing—leading to a crash or, worse, a hack.

The Patch

The fix (as of Linux kernel 6.4.10) walks through each child, correctly disconnects, and only then releases the parent socket’s memory.

Patch reference:

Git commit: “Bluetooth: L2CAP: Fix use-after-free caused by l2cap_sock_release”

Relevant snippet from the patch

    lock_sock(sk);
    // Properly cleanup children
    sock_set_flag(sk, SOCK_DEAD);
    release_children(sk);
    release_sock(sk);

Exploitation Demo: How Could an Attacker Abuse This?

Use-after-free bugs are powerful. They’re the bread-and-butter for kernel exploit writers because they let you reuse freed memory however you want. Here’s a simplified workflow an attacker might follow (hypothetical proof-of-concept):

// Pseudocode to trigger use-after-free on a vulnerable kernel

int l2cap_fd = socket(AF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_L2CAP);
bind(l2cap_fd, ...); // Bind to local Bluetooth address

int child_fd = accept(l2cap_fd, ...); // Get child socket

// Trick: Close parent while child is still open
close(l2cap_fd);

// Now, operating on child_fd may lead to use-after-free in kernel
write(child_fd, data, len); // This access may hit freed memory!

*If you can race the kernel to allocate your own data in the freed region, you might hijack the kernel!*

Note: Real-world exploitation requires expert knowledge, and stakes are high—this is proof-of-concept. But published research shows that use-after-free bugs are commonly weaponized for privilege escalation and even remote code execution.

Original Patch:

Git commit link

CVE Record:

NVD entry for CVE-2023-40283

Linux Bluetooth codebase:

net/bluetooth/l2cap_sock.c (GitHub mirror)

Linux kernel security mailing list:

oss-security Archive

What Should You Do?

- Update your kernel. If you run Linux (especially as a Bluetooth device/server), upgrade to *at least* 6.4.10.
- Use trusted devices. Only pair with devices you know and trust until you are sure your OS is patched.
- Monitor system logs. If you suspect attacks or instability near Bluetooth devices, investigate quickly.

In Short

CVE-2023-40283 isn’t just a technical glitch—it’s a reminder that even kernel developers can slip up, and real attackers are watching. Take your updates seriously!

Stay safe and keep your systems patched.

*Exclusive breakdown and plain-English analysis by an AI security enthusiast (2024). Please do not adapt or republish without permission.*

Timeline

Published on: 08/14/2023 03:15:00 UTC
Last modified on: 10/11/2023 19:15:00 UTC