CVE-2020-28915: Linux Kernel Buffer Overflow Vulnerability

CVE-2020-28915 Overview

A buffer over-read vulnerability exists in the framebuffer console (fbcon) code within the Linux kernel before version 5.8.15. This out-of-bounds read vulnerability (CWE-125) in the framebuffer layer can be exploited by local attackers with physical access to read sensitive kernel memory, potentially exposing confidential information stored in kernel space.

Critical Impact

Local attackers with physical access and high privileges can leverage this vulnerability to read kernel memory contents, potentially exposing sensitive system information and cryptographic materials.

Affected Products

• Linux Kernel versions prior to 5.8.15
• Systems using framebuffer console (fbcon) functionality
• Linux-based systems with physical console access enabled

Discovery Timeline

• 2020-11-18 - CVE-2020-28915 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-28915

Vulnerability Analysis

This vulnerability is an out-of-bounds read (CWE-125) in the framebuffer console subsystem of the Linux kernel. The fbcon code, which provides the kernel's text-mode console interface over framebuffer devices, fails to properly validate buffer boundaries when handling certain operations at the framebuffer layer. This allows an attacker with physical access to the system to trigger memory reads beyond the intended buffer boundaries, potentially disclosing sensitive kernel memory contents.

The vulnerability was identified through automated fuzzing via the Syzkaller kernel fuzzer, which detected the improper memory access patterns. The Linux kernel maintainers addressed this issue through commits 5af08640 and 6735b463.

Root Cause

The root cause of this vulnerability lies in insufficient bounds checking within the fbcon framebuffer layer code. When processing framebuffer operations, the kernel fails to properly validate that memory access operations stay within allocated buffer boundaries. This missing validation allows read operations to extend beyond the legitimate buffer, accessing adjacent kernel memory regions.

Attack Vector

The attack vector requires physical access to the vulnerable system. An attacker with high privileges and physical access can interact with the framebuffer console to trigger the buffer over-read condition. While the physical access requirement limits the attack surface, successful exploitation could result in disclosure of kernel memory contents, integrity violations, and potential system instability. The vulnerability mechanism involves manipulating framebuffer console operations to cause the kernel to read beyond allocated buffer boundaries. For detailed technical analysis, refer to the Syzkaller Bug Report #08b8be45 and the associated kernel commits.

Detection Methods for CVE-2020-28915

Indicators of Compromise

• Unusual framebuffer console activity or error messages in kernel logs
• Kernel oops or warnings related to fbcon or framebuffer operations
• Unexpected memory access violations in dmesg output referencing fbcon subsystem
• Signs of local privilege escalation attempts on systems with physical access

Detection Strategies

• Monitor kernel logs (dmesg) for buffer overflow or out-of-bounds access warnings
• Implement kernel live patching detection to verify fbcon patches are applied
• Use kernel integrity monitoring tools to detect tampering with framebuffer subsystem
• Deploy SentinelOne Singularity to monitor for exploitation attempts and anomalous kernel behavior

Monitoring Recommendations

• Enable kernel auditing for framebuffer device access (/dev/fb*)
• Configure system logging to capture all kernel warnings and errors
• Implement physical access controls and logging for server rooms and console terminals
• Monitor for unauthorized kernel module loading or modification attempts

How to Mitigate CVE-2020-28915

Immediate Actions Required

• Update Linux kernel to version 5.8.15 or later immediately
• Restrict physical access to vulnerable systems
• Disable framebuffer console if not required using kernel boot parameter fb=off
• Review and audit physical access logs for potentially affected systems

Patch Information

The Linux kernel development team has released patches to address this vulnerability. Users should update to kernel version 5.8.15 or later. The fix is documented in the Linux 5.8.15 ChangeLog. Distribution-specific updates may be available through vendor security advisories, including the SUSE Bug Report #1178886.

Workarounds

• Disable framebuffer console by adding fb=off or nofb to kernel boot parameters
• Restrict physical console access to trusted administrators only
• Use serial console or SSH for remote administration instead of framebuffer console
• Implement network segmentation to limit exposure of affected systems

# Configuration example
# Disable framebuffer console in GRUB configuration
# Edit /etc/default/grub and add to GRUB_CMDLINE_LINUX:
GRUB_CMDLINE_LINUX="fb=off"

# Update GRUB configuration
sudo update-grub

# Alternatively, blacklist framebuffer modules
echo "blacklist fbcon" | sudo tee /etc/modprobe.d/blacklist-fbcon.conf