On June 2024, a critical Linux kernel vulnerability—CVE-2024-57791—was publicly disclosed and promptly patched. This flaw affects the SMC (Shared Memory Communications) network protocol implementation, specifically when handling CLC (Connection Layer Control) messages. Exploiting this bug could lead to resource exhaustion and denial-of-service (DoS) scenarios.

This exclusive read explains the CVE for system operators, developers, and Red Teamers—breaking down its workings, showing code snippets, detailing potential attacks, and summarizing how to stay secure.

1. Vulnerability Overview

The vulnerability lives in the kernel file net/smc/, responsible for IBM SMC protocol support. When a CLC msg is received, the length field in the message header (from the network) tells the kernel how much data to read. However, this value can’t be fully trusted—malicious users may set a bogus length that exceeds safe boundaries.

The bug: If length exceeds the kernel buffer size (buflen), the function smc_clc_wait_msg() attempts to drain extra bytes, but due to a missing check on the draining operation, if draining fails, the code can enter a deadloop, consuming CPU, and impacting system resources.

Vulnerable function snippet before the patch

// smc_clc_wait_msg implementation, simplified:

while (remaining > ) {
    struct msghdr msg = { ... };
    ret = sock_recvmsg(sock, &msg, flags);
    if (ret <= )
        break;
    // ... continue draining
}

Issue:
If sock_recvmsg() fails (e.g., due to a malformed message or malicious actor), the function doesn’t break out appropriately when draining, potentially causing an infinite loop.

The Patch

The fix is to check sock_recvmsg return value during draining. If it fails or returns (EOF), the loop breaks, preventing deadlock.

Patched code

while (remaining > ) {
    struct msghdr msg = { ... };
    ret = sock_recvmsg(sock, &msg, flags);
    if (ret <= ) // <-- Added check
        break;
    remaining -= ret;
}

Patch reference:
- Kernel.org Patch Commit
- LKML Post

3. How Can It Be Exploited?

An attacker with access to send network messages to a vulnerable system can craft a CLC message with an artificially large length field. When the kernel’s SMC code tries to process the message:

This can consume 100% CPU on the kernel thread, leading to DoS

The attack doesn’t require privileged access—just the ability to open SMC connections to the target (often unfiltered in certain enterprise environments).

Setup: Attacker establishes SMC connection to victim server.

2. Send Fake CLC Message: Craft and send a CLC message over the network with length set much higher than possible (xffffff or similar).
3. Close Connection: Make sure the socket stops sending bytes, causing sock_recvmsg() to return or -1.
4. Kernel Loop: The victim kernel’s SMC receiver gets stuck in a loop, consuming CPU, reducing overall system performance.

5. Proof-of-Concept (PoC) Code

Below is a Python snippet for a *fake* SMC server client interaction, simulating the sending of an overlength CLC msg (for lab testing only):

import socket, struct

# Fake CLC header: 16 bytes, last 4 are length
overlength = xFFFFFF
clc_header = b'SMC\x00' + b'\x00' * 8 + struct.pack('>I', overlength)

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(('target_ip', 7447)) # Example SMC port

sock.sendall(clc_header)
# Don't send the remaining data as per 'length', just close
sock.close()

*Note: Running such code against unauthorized systems may be illegal—use only in lab or with explicit permission!*

7. References

- Kernel.org Patch Commit
- CVE Record on cve.org
- LKML Disclosure Thread

8. Conclusion

CVE-2024-57791 demonstrates how trusting user-controlled network fields can lead to dangerous consequences—even in low-level kernel network code. The fix is straightforward but essential for all production Linux environments exposed to SMC traffic.

Action required: Update your kernel, control exposure to enterprise protocols, and always scrutinize network input.


*Have more questions about Linux kernel security? Drop them below or check out kernel.org security documentation.*

Timeline

Published on: 01/11/2025 13:15:29 UTC
Last modified on: 05/04/2025 10:04:52 UTC