CVE-2020-28915: Linux Kernel Buffer Overflow Vulnerability

CVE-2020-28915 Overview

CVE-2020-28915 is a buffer over-read vulnerability in the framebuffer console (fbcon) code of the Linux kernel before version 5.8.15. The flaw enables local attackers with physical access to read adjacent kernel memory. The issue is tracked under commit identifier CID-6735b4632def and is classified as [CWE-125: Out-of-bounds Read]. Memory disclosure through fbcon can expose sensitive kernel data structures, pointers useful for bypassing Kernel Address Space Layout Randomization (KASLR), or credentials residing in kernel memory.

Critical Impact
Local attackers with high privileges and physical access can read kernel memory through the fbcon framebuffer interface, potentially exposing sensitive kernel data and weakening other kernel protections.

Affected Products

Linux Kernel versions prior to 5.8.15
Distributions shipping the vulnerable fbcon framebuffer console driver
SUSE Linux distributions (tracked in SUSE Bug Report #1178886)

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

The vulnerability resides in the fbcon (framebuffer console) subsystem of the Linux kernel. fbcon provides the text-mode console rendering layer on top of the framebuffer device. The flaw is an out-of-bounds read at the framebuffer layer that allows an attacker to read kernel memory outside the intended buffer boundaries.

Because fbcon operates in kernel space, any memory disclosed through this code path originates from kernel address space. Disclosed contents can include kernel pointers, stack canaries, or cryptographic material. Such leaks can be chained with other vulnerabilities to defeat KASLR and stack protection mechanisms.

The fix landed in Linux 5.8.15 via commits 5af08640795b and 6735b4632def in the upstream tree.

Root Cause

The root cause is missing or incorrect bounds checking in the fbcon code path when handling framebuffer console data. The kernel reads beyond the allocated buffer, returning adjacent memory contents to the caller. This is a classic [CWE-125] out-of-bounds read where the read length is not constrained to the size of the source buffer.

Attack Vector

Exploitation requires local physical access and high privileges, as reflected in the CVSS vector AV:P/PR:H. An attacker interacting with the console subsystem on a vulnerable kernel can trigger the over-read condition. The flaw was identified through fuzzing by the Syzkaller project, with reproducer details available in Syzkaller Bug Report #08b8be45.

No public proof-of-concept exploits are available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. The EPSS score is 0.117%, indicating low expected exploitation likelihood. Refer to the Linux 5.8.15 ChangeLog for additional context on the fix.

Detection Methods for CVE-2020-28915

Indicators of Compromise

Unexpected console activity or framebuffer-related syscalls originating from non-administrative sessions
Kernel log entries referencing fbcon errors or anomalous framebuffer ioctl invocations
Local user processes interacting with /dev/fb* devices outside of normal display server operation

Detection Strategies

Audit kernel versions across the Linux fleet and flag systems running kernels older than 5.8.15 that include fbcon support
Monitor auditd for ioctl calls against framebuffer character devices by unexpected processes
Correlate console subsystem activity with privilege escalation attempts or unusual local logon events

Monitoring Recommendations

Enable kernel auditing for framebuffer device access and review logs centrally
Track dmesg output for fbcon warnings or KASAN reports if KASAN-enabled kernels are deployed in test environments
Inventory hosts where physical console access is feasible, such as kiosks and shared workstations, and apply elevated monitoring

How to Mitigate CVE-2020-28915

Immediate Actions Required

Upgrade affected systems to Linux kernel 5.8.15 or later, or apply the vendor backport for your distribution
Restrict physical access to systems running vulnerable kernels, since the attack vector is physical
Limit interactive console logons to trusted administrators on multi-user systems

Patch Information

The vulnerability is fixed in Linux kernel 5.8.15. The upstream fix is delivered via commits 5af08640795b and 6735b4632def. Distribution backports are tracked in vendor advisories such as SUSE Bug Report #1178886. Consult the Linux 5.8.15 ChangeLog for the full set of changes shipped with the fix.

Workarounds

Disable the fbcon framebuffer console where it is not required, for example by booting with nomodeset or blacklisting framebuffer drivers on headless servers
Enforce strict physical access controls and full-disk encryption to reduce exposure on portable devices
Use kernel lockdown mode where supported to limit access to low-level interfaces from privileged userspace

bash

# Verify the running kernel version and confirm it is patched
uname -r

# Example: install the fixed kernel on Debian/Ubuntu
sudo apt update && sudo apt install --only-upgrade linux-image-generic

# Example: install the fixed kernel on RHEL/CentOS/Fedora
sudo dnf update kernel

# Reboot to activate the patched kernel
sudo reboot