CVE-2023-6121: Linux Kernel Buffer Overflow Vulnerability

CVE-2023-6121 Overview

An out-of-bounds read vulnerability was discovered in the NVMe-oF/TCP subsystem within the Linux kernel. This flaw allows a remote attacker with low-level privileges to send a specially crafted TCP packet to a vulnerable system, triggering a heap-based buffer overflow. The exploitation of this vulnerability results in kmalloc data being printed and potentially leaked to the kernel ring buffer (dmesg), leading to information disclosure.

Critical Impact

Remote attackers can exploit this vulnerability to leak sensitive kernel memory contents through the dmesg ring buffer, potentially exposing cryptographic keys, memory addresses, or other sensitive information useful for further attacks.

Affected Products

• Red Hat Enterprise Linux 6.0
• Red Hat Enterprise Linux 7.0
• Red Hat Enterprise Linux 8.0
• Red Hat Enterprise Linux 9.0

Discovery Timeline

• 2023-11-16 - CVE-2023-6121 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-6121

Vulnerability Analysis

The vulnerability resides in the NVMe over Fabrics (NVMe-oF) TCP transport subsystem of the Linux kernel. NVMe-oF allows NVMe commands and data to be transported over a network fabric, with TCP being one of the supported transport protocols. The flaw occurs when the subsystem improperly handles incoming TCP packets, failing to validate buffer boundaries before reading data.

When a maliciously crafted TCP packet is received by the NVMe-oF/TCP target, the kernel attempts to read beyond the allocated buffer boundaries. This out-of-bounds read operation accesses uninitialized or adjacent kernel heap memory (kmalloc data), which is then inadvertently written to the kernel ring buffer accessible via dmesg. An attacker positioned on the network can exploit this to extract potentially sensitive kernel memory contents.

This vulnerability is classified as CWE-125 (Out-of-bounds Read), which occurs when software reads data past the end, or before the beginning, of the intended buffer.

Root Cause

The root cause of CVE-2023-6121 lies in insufficient bounds checking within the NVMe-oF/TCP packet parsing routines. When processing incoming TCP packets, the code fails to properly validate the length fields against the actual allocated buffer size before performing read operations. This missing validation allows an attacker to specify crafted length values that cause the kernel to read beyond the intended buffer boundaries, accessing adjacent heap memory regions.

Attack Vector

The attack can be executed remotely over the network by sending specially crafted TCP packets to a system running the NVMe-oF/TCP target service. The attacker requires low-level authentication or network access to the NVMe-oF service. No user interaction is required for exploitation.

The attack flow involves:

1. Identifying a target system running NVMe-oF/TCP services
2. Crafting a TCP packet with manipulated length fields designed to trigger the out-of-bounds read
3. Sending the malicious packet to the target NVMe-oF/TCP port
4. The kernel processes the packet and leaks heap memory contents to dmesg
5. If the attacker has access to dmesg output (directly or through other means), they can extract the leaked information

Since no verified exploitation code is available, organizations should refer to the Red Hat Bug Report #2250043 for additional technical details about the vulnerability mechanism.

Detection Methods for CVE-2023-6121

Indicators of Compromise

• Unusual or malformed packets targeting NVMe-oF/TCP ports (typically port 4420)
• Unexpected kernel messages in dmesg containing heap memory dumps or unusual data patterns
• Increased network traffic to NVMe-oF/TCP services from untrusted sources
• Evidence of information disclosure attempts in network traffic analysis

Detection Strategies

• Monitor dmesg output for unusual memory dumps or unexpected hexadecimal data leakage patterns
• Implement network intrusion detection rules to identify malformed NVMe-oF/TCP packets
• Deploy endpoint detection and response (EDR) solutions to monitor for kernel-level anomalies
• Review NVMe-oF/TCP service logs for connection attempts from unauthorized IP addresses

Monitoring Recommendations

• Enable enhanced kernel logging for NVMe subsystem operations
• Configure SIEM rules to alert on dmesg entries containing potential memory leakage indicators
• Monitor network flows to NVMe-oF/TCP services for anomalous packet sizes or frequencies
• Implement baseline monitoring for normal NVMe-oF/TCP traffic patterns to detect deviations

How to Mitigate CVE-2023-6121

Immediate Actions Required

• Apply the latest kernel security updates from Red Hat addressing CVE-2023-6121
• Restrict network access to NVMe-oF/TCP services to trusted hosts only using firewall rules
• Review and audit systems for any signs of exploitation attempts
• Consider temporarily disabling NVMe-oF/TCP services on non-critical systems until patches are applied

Patch Information

Red Hat has released security advisories addressing this vulnerability. Organizations should apply the appropriate patches based on their Red Hat Enterprise Linux version:

• Red Hat Security Advisory RHSA-2024:2394
• Red Hat Security Advisory RHSA-2024:2950
• Red Hat Security Advisory RHSA-2024:3138

For Debian-based systems, refer to the Debian LTS Announcement January 2024 for applicable patches.

Workarounds

• Implement network segmentation to isolate NVMe-oF/TCP services from untrusted networks
• Use firewall rules to restrict access to NVMe-oF TCP port 4420 to authorized clients only
• Disable the nvme-tcp kernel module if NVMe-oF/TCP functionality is not required: modprobe -r nvme-tcp
• Deploy intrusion prevention systems (IPS) to filter potentially malicious NVMe-oF traffic

# Restrict NVMe-oF/TCP access using iptables
# Allow only trusted hosts to access NVMe-oF/TCP port 4420
iptables -A INPUT -p tcp --dport 4420 -s TRUSTED_IP_RANGE -j ACCEPT
iptables -A INPUT -p tcp --dport 4420 -j DROP

# Disable nvme-tcp module if not needed
echo "blacklist nvme-tcp" >> /etc/modprobe.d/blacklist-nvme-tcp.conf
modprobe -r nvme-tcp