CVE-2021-33655: Linux Kernel Buffer Overflow Vulnerability

CVE-2021-33655 Overview

CVE-2021-33655 is an out-of-bounds write vulnerability in the Linux kernel's framebuffer subsystem. When malicious data is sent to the kernel via the FBIOPUT_VSCREENINFO ioctl command, the kernel can be manipulated to write memory beyond allocated boundaries. This vulnerability affects the framebuffer device driver, which is responsible for managing the video display memory in Linux systems.

Critical Impact

Local attackers with high privileges can exploit this vulnerability to achieve complete system compromise through out-of-bounds memory writes, potentially leading to code execution, data corruption, or system crashes.

Affected Products

• Linux Kernel versions prior to 5.19
• Linux Kernel 5.19 RC1 through RC5
• Debian Linux 10.0 (Buster)
• Debian Linux 11.0 (Bullseye)

Discovery Timeline

• 2022-07-18 - CVE-2021-33655 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-33655

Vulnerability Analysis

This vulnerability resides in the Linux kernel's framebuffer (fbdev) subsystem, specifically in the handling of the FBIOPUT_VSCREENINFO ioctl command. The framebuffer device (/dev/fb0) provides a hardware-independent abstraction for the display, and the FBIOPUT_VSCREENINFO ioctl allows user-space applications to configure virtual screen parameters.

The vulnerability is classified as CWE-787 (Out-of-Bounds Write), indicating that the kernel fails to properly validate input parameters before performing memory operations. When a local attacker with sufficient privileges sends specially crafted data through this ioctl, the kernel can be tricked into writing data outside the intended memory boundaries.

Root Cause

The root cause of this vulnerability lies in insufficient bounds checking within the framebuffer driver's ioctl handler. When processing the FBIOPUT_VSCREENINFO command, the kernel fails to adequately validate the virtual screen resolution parameters (xres_virtual, yres_virtual) and related offsets before using them to calculate memory addresses for write operations. This allows an attacker to specify values that cause the kernel to compute addresses outside the allocated framebuffer memory region.

Attack Vector

Exploitation requires local access to the system with high privileges. The attacker must be able to open the framebuffer device (typically /dev/fb0) and issue ioctl commands. The attack proceeds by crafting malicious fb_var_screeninfo structure values and passing them to the FBIOPUT_VSCREENINFO ioctl.

The vulnerability is exploited through the following mechanism:

1. The attacker opens the framebuffer device with appropriate permissions
2. A maliciously crafted fb_var_screeninfo structure is prepared with out-of-bounds values for virtual screen dimensions or pixel offsets
3. The FBIOPUT_VSCREENINFO ioctl is called with the malicious data
4. The kernel processes these values without adequate validation, resulting in memory writes beyond the framebuffer boundaries

For technical implementation details, refer to the Linux Kernel Commit Details which contains the patch addressing this issue.

Detection Methods for CVE-2021-33655

Indicators of Compromise

• Unexpected kernel panics or system crashes related to framebuffer operations
• Suspicious ioctl calls to /dev/fb0 or other framebuffer devices with unusual parameters
• Anomalous memory access patterns in kernel logs associated with fbdev subsystem
• Unexpected privilege escalation attempts on systems with exposed framebuffer devices

Detection Strategies

• Monitor system calls for unusual ioctl activity targeting framebuffer devices (/dev/fb*)
• Implement auditd rules to track access to framebuffer device files and related ioctl operations
• Deploy kernel-level monitoring to detect out-of-bounds memory access attempts in the fbdev subsystem
• Utilize SentinelOne's behavioral analysis to identify anomalous framebuffer device interactions

Monitoring Recommendations

• Enable kernel debugging and logging for framebuffer-related operations
• Configure system audit logging to capture ioctl calls with the FBIOPUT_VSCREENINFO command
• Monitor for kernel oops or panic messages referencing framebuffer code paths
• Deploy endpoint detection solutions capable of identifying kernel-level exploitation attempts

How to Mitigate CVE-2021-33655

Immediate Actions Required

• Update Linux kernel to version 5.19 or later where the vulnerability is patched
• Apply vendor-provided security patches from distribution maintainers (Debian, etc.)
• Restrict access to framebuffer devices (/dev/fb*) to only necessary users and processes
• Consider disabling the framebuffer subsystem if not required for system operation

Patch Information

The vulnerability has been addressed in the upstream Linux kernel. The fix adds proper bounds checking for virtual screen parameters in the framebuffer ioctl handler. The patch is available via commit 086ff84617185393a0bbf25830c4f36412a7d3f4 in the Linux kernel git repository.

For Debian-based systems, security updates are available:

• Debian Security Advisory DSA-5191 for Debian stable releases
• Debian LTS Announcement for Debian LTS users

Workarounds

• Restrict permissions on /dev/fb* devices to prevent unauthorized access
• Use Linux kernel module blacklisting to disable unnecessary framebuffer drivers
• Implement mandatory access control (SELinux/AppArmor) policies to limit ioctl access
• Remove physical console access for untrusted users on affected systems

# Restrict framebuffer device permissions
chmod 600 /dev/fb0

# Blacklist framebuffer modules if not needed (add to /etc/modprobe.d/blacklist.conf)
blacklist fbcon
blacklist vesafb
blacklist uvesafb