In July 2021, a new high-severity Linux kernel vulnerability came to light as CVE-2021-34981 (also tracked as ZDI-CAN-11977). This vulnerability affects the Bluetooth CMTP (CAPI Message Transport Protocol) module implemented in the Linux kernel. If left unpatched, it allows a local attacker to escalate their privileges and execute code as the kernel itself—a major security risk for any Linux system running the vulnerable CMTP configuration.

This post breaks down the core issue, the attack scenario, references, and even presents a snippet of code to help understand how exploitation may be possible.

The Vulnerability in Simple Terms

The issue is rooted in how the Linux CMTP kernel module (located in the net/bluetooth/cmtp directory of the Linux source tree) handles freeing memory. Specifically, the CMTP module fails to properly track allocated objects and, under certain conditions, may attempt to free (or de-allocate) a pointer that has already been freed (a "double free" condition).

When this happens, if an attacker has already gained some privileges on the system—enough to interact with the Bluetooth CMTP interface—they could create a situation where the double free is triggered. Because of how the Linux kernel manages memory, the attacker may be able to execute arbitrary code with kernel privileges or cause the machine to crash.

In summary:
A locally authenticated user can exploit the double free bug to escalate their privileges to root or run arbitrary code in the most privileged context.

Where Is the Problematic Code?

The bug lies in the CMTP session handling code. Here’s a simplified code excerpt inspired by the vulnerable kernel section:

// Actual vulnerable code found in net/bluetooth/cmtp/cmtp-socket.c

static int cmtp_sock_release(struct socket *sock)
{
    struct sock *sk = sock->sk;
    ...
    // cmtp_cleanup_sock frees the cmtp_session
    cmtp_cleanup_sock(sk);

    // The problem: sk is used again after being cleaned up (freed)
    sock->sk = NULL;
    ...
    return ;
}

In the vulnerable code path, the cmtp_cleanup_sock() function can trigger a series of cleanup operations that free memory associated with a session object. But later, the kernel code may attempt to operate on, or free, the same memory a second time—this opens the door to potentially serious exploitation.

Key point: The kernel does not properly ensure an object is valid (i.e. not already freed) before calling the free operation on it again.

The attacker must be local (already have a user shell).

- The system must have the Bluetooth CMTP protocol enabled/configured (which is relatively rare, but still feasible, especially on desktops/laptops with Bluetooth networking).
- No additional authentication or privileges are required beyond access to the relevant socket interface (e.g., attacker can open and close CMTP sockets, which may be possible for unprivileged users in some configurations).

How the Exploit Works

1. Preparation: The attacker opens a CMTP (Bluetooth) socket, triggering the allocation of CMTP session objects inside the kernel.
2. Trigger the Vulnerability: The attacker closes the socket in such a way (potentially using file descriptors and forking) that the kernel's internal object tracking gets confused. Under certain timing conditions, the cleanup is triggered multiple times on the same object.
3. Double Free Occurs: The kernel frees the underlying session object memory twice. If the attacker controls what gets allocated between these two frees, it's possible to overwrite function pointers or data structures with controlled content.
4. Privilege Escalation: By controlling memory, the attacker injects code or modifies kernel data structures to escalate privileges—possibly gaining root or running arbitrary code.

> Note: While the specific, reliable public exploit code for CVE-2021-34981 is hard to find and may not be directly published due to responsible disclosure, similar double free vulnerabilities have been exploited using techniques like heap spraying, allocation size matching, and timing.

Here's a conceptual example (pseudocode) showing the sequence that would trigger the bug

import socket
import os

# Create a Bluetooth socket in the CMTP protocol
sock = socket.socket(socket.AF_BLUETOOTH, socket.SOCK_STREAM, socket.BTPROTO_CMTP)

pid = os.fork()
if pid == :
    # In child process: close the socket, triggers cmtp_sock_release
    sock.close()
    os._exit()
else:
    # In parent process: also closes the socket
    sock.close()
    os.waitpid(pid, )

What this does:
By opening a CMTP socket, forking, and having both the parent and child close the same file descriptor, it can, under the right circumstances, trigger cmtp_sock_release twice on the same underlying socket structure—causing a double free.

> Disclaimer: This simple Python code is conceptual! Actual exploitation would require fine-grained heap manipulation and close study of the Linux kernel data structures.

Fix and Mitigation

The Linux kernel maintainers patched this vulnerability in July 2021. The fix involves adding proper object existence checks before freeing and ensuring internal object references are correctly nullified after they're freed.

Patch reference:
- Kernel commit / patch

References

- NIST NVD entry for CVE-2021-34981
- ZDI advisory ZDI-CAN-11977
- Ubuntu Security Notice USN-502-1
- Upstream patch commit

Conclusion

CVE-2021-34981 is a good example of how subtle mistakes in kernel code—like failing to track memory object lifecycles—can have serious security consequences. While CMTP isn't widely deployed, any system with the affected module loaded is vulnerable if a local attacker can access it. If you run a Linux system, especially one exposed to users or network services, updating your kernel is essential for resilience against this and related bugs.

Stay vigilant, keep your systems patched, and always disable unnecessary kernel features to reduce your attack surface!

Timeline

Published on: 05/07/2024 23:15:13 UTC
Last modified on: 06/04/2024 17:12:51 UTC