A new security vulnerability, identified as CVE-2024-56645, has been discovered in the Linux kernel, specifically within the J1939 protocol implementation. The issue concerns improper reference counting, which can lead to a use-after-free condition and subsequently system crashes or other unpredictable behavior. This post provides a detailed overview of the vulnerability, a code snippet to demonstrate the issue, links to original references, and a brief explanation of the exploit.

Vulnerability Description

The Controller Area Network (CAN) protocol is a widely-used communication system for various industries, including automotive and industrial automation. In the Linux kernel, the J1939 protocol is a higher-level protocol based on the CAN protocol, which provides a standardized way of exchanging data between different devices.

The vulnerability is found in the j1939_session_new() function, which is responsible for creating a new J1939 session and initializing the corresponding reference counting. Specifically, the function does not properly handle reference counting for the initial message, leading to a refcount underflow.

The problem occurs because the j1939_session_skb_queue() function, which is used to enqueue a new message to the queue, increments the reference count for each new message using the skb_get() function. However, the initial reference count for the first message does not include this increment, leading to incorrect reference counting.

The full details of the vulnerability and updates can be found in the official Linux Kernel Mailing List Archive.

Code Snippet

The following code snippet demonstrates the issue and the corresponding fix in the j1939_session_new() function:

// Original implementation
static struct j1939_session *j1939_session_new(struct j1939_sock *jsk, struct sk_buff *skb)
{
	return j1939_session_new_attr(jsk, skb, JSK_SESSION_STATE_PENDING, ktime_now() + msecs_to_ktime());
}

// Fixed implementation
static struct j1939_session *j1939_session_new(struct j1939_sock *jsk, struct sk_buff *skb)
{
	struct j1939_session *session;

	// Additional skb_get() for the initial skb
	skb_get(skb);

	session = j1939_session_new_attr(jsk, skb, JSK_SESSION_STATE_PENDING, ktime_now() + msecs_to_ktime());

	return session;
}

In the function's *fixed implementation*, an additional call to skb_get(skb) is added to increment the reference count for the initial message, thus resolving the issue.

Exploit Details

Exploiting this vulnerability can potentially lead to a use-after-free condition, allowing an attacker to cause a system crash or execute arbitrary code with kernel privileges. A proof-of-concept (PoC) has yet to be released, but developers and system administrators are encouraged to patch the vulnerability as soon as possible.

While an attack exploiting this vulnerability could be far-fetched, due to the attacker needing a specific environment to trigger the bug, any unpatched system is potentially at risk and should be updated immediately.

Conclusion

The Linux kernel has resolved a vulnerability (CVE-2024-56645) concerning reference counting in J1939 sessions. Developers and administrators are recommended to apply the patch as soon as possible to mitigate any potential risks.

For further information, consult the official references provided

- Linux Kernel Mailing List Archive - Fix
- Linux Kernel Mailing List Archive - Vulnerability Announcement

Timeline

Published on: 12/27/2024 15:15:24 UTC
Last modified on: 01/20/2025 06:24:57 UTC