CVE-2023-3863 - Understanding the Use-After-Free Vulnerability in Linux NFC (A Deep Dive)

In the world of Linux security, kernel vulnerabilities pose significant risks—often leading to privilege escalation or information leaks. An example of such a vulnerability is CVE-2023-3863, a recently disclosed flaw in the NFC (Near Field Communication) subsystem of the Linux kernel. This bug is a *use-after-free* in the nfc_llcp_find_local function, and can allow a local attacker with special privileges to leak sensitive kernel memory.

In this exclusive post, we’ll break down what CVE-2023-3863 is, how it works, share code excerpts, links, and exploit details. All complex security concepts are simplified for everyone to understand.

Background: NFC in Linux

NFC is a set of communication protocols enabling wireless data transfer. Many modern devices (like Android phones) use NFC for payments or data exchange. In Linux, NFC support lives in the kernel under the net/nfc subsystem.

The Vulnerability in a Nutshell

The issue, (see Red Hat Advisory and kernel commit) happens in the nfc_llcp_find_local function in net/nfc/llcp_core.c. When a user with the right privileges interacts with the NFC socket interface, it's possible to manipulate the system into using a memory pointer *after* it has already been freed. This is called *use-after-free*.

Result: The attacker can leak information (such as kernel pointers), which could later help them escalate their privileges or craft more dangerous attacks.

Where’s the Problem?

To understand the bug, let’s look at a simplified chunk of code involved—inspired by the actual vulnerable function.

struct nfc_llcp_local *nfc_llcp_find_local(struct net *net, ...) {
    struct nfc_llcp_local *local;

    // Find the list entry (simplified)
    list_for_each_entry(local, &llcp_local_list, list) {
        if (some_condition)
            goto found;
    }
    return NULL;

found:
    // It's possible that 'local' is freed elsewhere
    // before it’s used here
    do_something_with(local);
    return local;
}

The problem here is subtle. If another thread or operation deletes (frees) the local object just after it’s found but before it's used, the kernel code will end up using a *dangling pointer*. This pointer might now refer to freed memory, now used for some other purpose, so accessing it can lead to a crash or an information leak.

What's "Use-After-Free"?

A use-after-free bug happens when software continues to use a memory area after it’s already been released. That opens the door to all sorts of abuses.

What Can Attackers Achieve?

Because this function is part of handling privileged NFC socket operations, only users with the capability to manipulate NFC devices or sockets can trigger the bug (usually root or a user in the nfc group). Here’s what an attacker could achieve:

- Kernel Information Leak: Sensitive data from the kernel could be exposed to a user-space process, revealing memory addresses, structures, or stack layouts.
- Help Crafting Other Exploits: While this bug alone is mainly a leak, it helps attackers weaken kernel protections intended to stop more damaging attacks (like KASLR bypass).

Proof-of-Concept: Exploiting the Flaw

A real exploit would be far more complex, but here’s a simplified proof-of-concept illustrating the problem logic. Do not run this on production systems.

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

int main() {
    int sock;
    
    // Step 1: Open an NFC socket (needs privileges)
    sock = socket(AF_NFC, SOCK_SEQPACKET, NFC_PROTO_LLCP);
    if (sock < ) {
        perror("socket");
        return 1;
    }

    // Step 2: Rapidly create and destroy LLCP devices or connections
    for (int i = ; i < 100; i++) {
        // Trigger operations racing the kernel's memory management
        // This is a placeholder step; actual exploitation requires
        // precise racing and use of setsockopt/ioctl/etc.
    }

    // Step 3: Attempt to read leaked kernel memory through NFC I/O
    char buf[256];
    ssize_t n = recv(sock, buf, sizeof(buf), );
    if (n > ) {
        printf("Leaked data: ");
        for (int i = ; i < n; i++)
            printf("%02x ", (unsigned char)buf[i]);
        printf("\n");
    }

    close(sock);
    return ;
}

*(Note: Above code omits many details, but the real attack is triggered via clever races and special socket operations.)*

Mitigation and Patch

The upstream Linux kernel maintainers patched the bug. The patch makes sure that any discovered object is properly referenced, and not just a raw pointer in a race.

Upgrade your Linux kernel. Distributions like Fedora, Ubuntu, and Red Hat have backported the fix to their supported releases.

Original References

- Red Hat CVE Page
- Kernel.org Commit diff
- Ubuntu Security Advisory (USN-XXXX-1, example)

Conclusion

CVE-2023-3863 is a textbook case showing why memory safety in kernel code is so important. While most regular users are not affected (since it needs special privileges), system administrators and anyone building Linux devices with NFC support must update as soon as possible.

Always keep your kernel up-to-date, especially if your infrastructure uses hardware features like NFC. For extra protection, restrict access to special groups like nfc and regularly audit system privileges.


Stay safe and happy hacking! If you have more technical questions or want to see full exploit code walk-throughs (for academic purposes), follow the links above or reach out to your Linux community.

Timeline

Published on: 07/24/2023 15:15:00 UTC
Last modified on: 08/19/2023 18:17:00 UTC