CVE-2023-0461 - Use-After-Free in Linux Kernel Leads to Local Privilege Escalation (with Exploit Details)

Published: 2024-06  
Author: [ChatGPT Security Research Exclusive]  
Tags: Linux Kernel, Use-After-Free, Privilege Escalation, CVE-2023-0461, Exploit Development

Summary

CVE-2023-0461 uncovers a dangerous use-after-free bug in the Linux kernel, impacting systems with CONFIG_TLS or CONFIG_XFRM_ESPINTCP enabled. This bug is exploitable by local users without any special privileges, paving the way for local privilege escalation — a scenario where a basic user can obtain root or kernel-level access.

The vulnerability happens due to mismanagement of the icsk_ulp_data field in the struct inet_connection_sock, particularly when a TCP socket is reused from a listener state with lingering ULP (Upper Layer Protocol, such as TLS) contexts.

This post will break down the issue, show a code snippet illustrating the flaw, share how attackers can exploit it, and provide clear recommendations for mitigation.

What’s Affected

- Linux Kernel: Any version before commit 2c02d41d71f90a5168391b6a5f2954112ba2307c
- Configurations: CONFIG_TLS or CONFIG_XFRM_ESPINTCP enabled (common in modern distributions for performance features)

Technical Details (How the Bug Happens)

1. Enable ULP (TLS): A user creates and connects a TCP socket, then enables the TLS protocol (using setsockopt with TCP_ULP).

Context Attachment: This attaches a struct tls_context to the socket.

3. Socket Reuse: If that TCP socket is then switched into a listening state (listen()), the TCP stack does NOT clear the existing tls_context.
4. New Connection: A new connection accepted by this listener inherits the old (now unsafe) tls_context.
5. Use-After-Free: The inherited context can now be used after it’s already been freed, causing use-after-free and allowing attackers to manipulate kernel memory.

No privileges are needed for any of these socket operations. This can be triggered in a standard user process, making it ripe for exploitation.

Code Snippet: Reproducing the Use-After-Free

Here’s a (simplified) C code snippet that shows the dangerous socket flow.  

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <linux/tcp.h>

int main() {
    int sock = socket(AF_INET, SOCK_STREAM, );
    if (sock < ) { perror("socket"); return 1; }

    // Step 1: Connect to something (127...1:12345)
    // Fill sockaddr_in as server 
    // connect(sock, ...);

    // Step 2: Enable TLS ULP with setsockopt
    const char *ulp = "tls";
    if (setsockopt(sock, IPPROTO_TCP, TCP_ULP, ulp, strlen(ulp)) != ) {
        perror("setsockopt TCP_ULP");
        close(sock);
        return 1;
    }

    // Step 3: Change state to listening
    // This is the tricky part. In reality, must trick the kernel into
    // reusing this socket as a listener! (details omitted for brevity)

    // Step 4: Accept new connections or forcibly reuse the socket.
    // The vulnerable context is now inherited and can be used after free!

    printf("Vulnerability triggered!\n");
    close(sock);
    return ;
}

Note: The above is a simplified illustration. Real-world exploitation requires more steps (see exploit references below).

Install TLS context using setsockopt + TCP_ULP.

3. Close the connection and reuse the socket as a listener, without the kernel clearing the old context.
4. Accept a new connection on this now-listening socket — the child socket gets the dangling pointer (tls_context).
5. Manipulate the freed memory (using heap spraying or similar techniques) to take control of the kernel’s behavior or escalate privileges.

This bug gives attackers a primitive for arbitrary read/write within kernel memory by controlling freed objects, leading to local privilege escalation.

Visual Walkthrough

+---------------------+           +---------------------+
|    User program     |  <--->    |   Linux Kernel      |
+---------------------+           +---------------------+
        |                                   |
        |     socket, connect                |
        |----------------------------------->|
        |                                   |
        |     setsockopt TCP_ULP ("tls")     |
        |----------------------------------->|  // TLS context created
        |                                   |
        |     somehow listen() + accept()    |
        |----------------------------------->|  // context not cleared!
        |                                   |
        |     Child socket inherits pointer  |  // use-after-free
        |                                   |

Original References

- CVE Page (NVD)
- Linux Kernel Patch
- OSS Security mailing list post
- Security advisory by Red Hat
- Original discoverer: Peter Xu / Google Security

Upgrade your kernel:

Update to any kernel version including/after commit 2c02d41d71f90a5168391b6a5f2954112ba2307c (Feb 2023)

Final Words

CVE-2023-0461 highlights the risks in reusing privileged kernel objects without cleanup. Any user on an affected system can turn a trivial socket operation into full root access. Fix it by upgrading ASAP!

If you maintain Linux servers or distribute Linux, check your kernel config and patch!

Share and stay safe.  
*(This is an exclusive summary by ChatGPT based on public advisory and Linux kernel sources.)*

Timeline

Published on: 02/28/2023 15:15:00 UTC
Last modified on: 03/10/2023 04:57:00 UTC