CVE-2023-5633: Linux Kernel Privilege Escalation Flaw

CVE-2023-5633 Overview

CVE-2023-5633 is a use-after-free vulnerability in the Linux kernel that was inadvertently introduced through reference count changes made as part of fixes for CVE-2023-33951 and CVE-2023-33952. The flaw exists in the way memory objects are handled when they are being used to store a surface. When running inside a VMware guest with 3D acceleration enabled, a local, unprivileged user could potentially exploit this vulnerability to escalate their privileges.

Critical Impact

Local privilege escalation in VMware guest environments with 3D acceleration enabled allows unprivileged users to gain elevated system access.

Affected Products

• Linux Kernel (versions prior to 6.6 and release candidates 6.6-rc1 through 6.6-rc6)
• Red Hat Enterprise Linux 8.x and 9.x (including EUS, AUS, TUS variants)
• Red Hat CodeReady Linux Builder 8.x and 9.x (all architectures including ARM64, IBM Z Systems, Power LE)

Discovery Timeline

• October 23, 2023 - CVE-2023-5633 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-5633

Vulnerability Analysis

This use-after-free (CWE-416) vulnerability manifests in the Linux kernel's graphics subsystem, specifically in the VMware virtual GPU driver (vmwgfx). The flaw was a regression introduced when developers modified reference counting mechanisms while addressing two prior vulnerabilities (CVE-2023-33951 and CVE-2023-33952). The improper handling of memory object reference counts during surface storage operations creates a condition where memory can be accessed after it has been freed.

The vulnerability requires local access to the system and affects VMware guest virtual machines that have 3D graphics acceleration enabled. While this limits the attack surface to virtualized environments with specific configurations, successful exploitation grants the attacker full system privileges.

Root Cause

The root cause stems from incorrect reference count management in the kernel's graphics memory handling code. When the fixes for CVE-2023-33951 and CVE-2023-33952 modified the reference counting logic for memory objects used to store surfaces, they inadvertently created a race condition where a memory object's reference count could reach zero while still being referenced by another part of the code. This allows the memory to be freed while still in use, leading to the classic use-after-free condition.

Attack Vector

Exploitation of CVE-2023-5633 requires local access to a VMware guest system running a vulnerable Linux kernel with 3D acceleration enabled. The attacker must be an authenticated user on the system, though no elevated privileges are required to trigger the vulnerability.

The attack flow involves:

1. The attacker interacts with the VMware 3D graphics driver (vmwgfx) through standard system interfaces
2. By manipulating memory object operations related to surface storage, the attacker triggers the use-after-free condition
3. The freed memory can be reallocated with attacker-controlled data
4. When the kernel subsequently accesses the freed memory, it uses the attacker's controlled data, potentially allowing arbitrary code execution in kernel context

For detailed technical information, refer to the Red Hat CVE Details and Red Hat Bugzilla ID 2245663.

Detection Methods for CVE-2023-5633

Indicators of Compromise

• Unexpected kernel crashes or panics related to the vmwgfx driver module
• Anomalous memory access patterns in kernel logs associated with graphics subsystem operations
• Suspicious privilege changes for local user accounts without corresponding administrative actions
• Kernel oops messages referencing freed memory in vmwgfx code paths

Detection Strategies

• Monitor kernel logs (dmesg, /var/log/kern.log) for use-after-free warnings or vmwgfx driver errors
• Deploy kernel-level monitoring tools to detect abnormal memory access patterns in graphics driver operations
• Implement file integrity monitoring on critical system binaries to detect post-exploitation modifications
• Use behavioral analysis to identify unusual process privilege transitions

Monitoring Recommendations

• Enable audit logging for system calls related to graphics device interactions
• Configure SIEM rules to alert on kernel panic events in VMware guest environments
• Monitor for unauthorized loading or unloading of kernel modules
• Track user privilege escalation events and correlate with graphics driver activity

How to Mitigate CVE-2023-5633

Immediate Actions Required

• Apply available kernel patches from your Linux distribution vendor immediately
• If patching is not immediately possible, disable VMware 3D acceleration as a temporary mitigation
• Restrict local access to systems running vulnerable kernel versions
• Implement the principle of least privilege to limit potential attacker capabilities

Patch Information

Red Hat has released security advisories addressing this vulnerability across multiple product versions:

• RHSA-2024:0113 - Security update for kernel
• RHSA-2024:0134 - Security update for kernel
• RHSA-2024:0461 - Security update for kernel
• RHSA-2024:1404 - Security update for kernel
• RHSA-2024:4823 - Security update for kernel
• RHSA-2024:4831 - Security update for kernel

Consult your specific Red Hat Enterprise Linux version to determine the appropriate patch. For upstream Linux kernels, the fix is included in kernel version 6.6 and later.

Workarounds

• Disable VMware 3D acceleration in the virtual machine settings until patches can be applied
• Blacklist the vmwgfx kernel module if 3D graphics functionality is not required
• Limit local user access to affected systems to trusted personnel only
• Consider migrating workloads to patched systems or alternative virtualization platforms temporarily

# Disable vmwgfx module (temporary workaround)
echo "blacklist vmwgfx" | sudo tee /etc/modprobe.d/blacklist-vmwgfx.conf
sudo update-initramfs -u
# Reboot required for changes to take effect

# Verify module is not loaded after reboot
lsmod | grep vmwgfx