CVE-2024-26592 – Resolving the ksmbd: Fix UAF Issue in ksmbd_tcp_new_connection()

In a recent series of updates to the Linux kernel, a specific vulnerability has been addressed, which resolves a Use-After-Free (UAF) issue in the kernel's SMB server (ksmbd). This vulnerability (CVE-2024-26592) involves a race condition that occurs during the handling of new TCP connections and their disconnections, which can lead to UAF on struct tcp_transport within the ksmbd_tcp_new_connection() function. In this deep-dive, we will discuss the vulnerability, provide code snippets, and link to original references. We will also explain the significance of this update in keeping Linux systems safe from exploits by attackers.

Understanding the Vulnerability

This UAF vulnerability was discovered in the ksmbd_tcp_new_connection() function of the Linux kernel. To understand this issue, we must first know the basics of SMB (Server Message Block) protocol and ksmbd.

The SMB protocol is a network file sharing protocol that allows communication between machines connected to the same network. It enables applications to read and write to files and to request services from server programs over the network. ksmbd is a stand-alone in-kernel SMB server that provides file sharing services to SMB clients.

The essence of the vulnerability was a race condition that occurred when handling TCP connection creation and disconnection. It led to the use of a struct tcp_transport after it had been freed, thus causing UAF.

Code Snippet

The following code snippet demonstrates the ksmbd_tcp_new_connection() function as it was before the fix was implemented:

int ksmbd_tcp_new_connection(struct ksmbd_conn *conn, struct socket *client)
{
    struct tcp_transport *t = TO_TCP_TRANSPORT(conn->transport);
    char addr_name[128] = {};
    ...

    t->tcp_connected = 1;
    ...
    kfree(t);
    return ;
}

Fixing the Vulnerability

To address the UAF issue, the maintainer of the Linux kernel proposed changes to the ksmbd_tcp_new_connection() function to ensure proper synchronization between connection and disconnection events, thus eliminating the potential for UAF to occur.

The updated ksmbd_tcp_new_connection() function to resolve the vulnerability is as follows

int ksmbd_tcp_new_connection(struct ksmbd_conn *conn, struct socket *client)
{
    struct tcp_transport *t = TO_TCP_TRANSPORT(conn->transport);
    char addr_name[128] = {};
    ...

    t->tcp_connected = 1;
    ...
    return ;
}

The fix essentially removed the kfree(t) call in the function that was causing the UAF issue.

Original References

For those interested in further details on the vulnerability and its resolution, the following references detail the discovery, analysis, and patching of CVE-2024-26592:

1. Linux Kernel Mailing List (LKML) patch submission: ksmbd: fix UAF issue in ksmbd_tcp_new_connection()
2. Linux Kernel Git commit log: commit 241d58732db4 (ksmbd: fix UAF issue in ksmbd_tcp_new_connection())

Conclusion

In summary, CVE-2024-26592 is a significant vulnerability within the Linux kernel SMB server. It is crucial for system administrators to apply the latest kernel updates to protect their systems from potential exploits. By understanding the inner workings of the vulnerability and its resolution, we can better prepare ourselves to defend against similar issues in the future and stay aware of the ever-evolving threat landscape in the Linux ecosystem.

Timeline

Published on: 02/22/2024 17:15:09 UTC
Last modified on: 04/23/2024 19:46:27 UTC