CVE-2021-33656: Openatom Openeuler Buffer Overflow Flaw

CVE-2021-33656 Overview

CVE-2021-33656 is an out-of-bounds write vulnerability in the Linux kernel that occurs when setting font data through the PIO_FONT ioctl command. When malicious data is provided to this ioctl interface, the kernel fails to properly validate the input, leading to memory corruption by writing beyond allocated buffer boundaries. This vulnerability affects multiple Linux distributions including openEuler, Debian Linux, and the upstream Linux kernel.

Critical Impact

Successful exploitation of this vulnerability can lead to local privilege escalation, kernel memory corruption, or denial of service through physical access to the system.

Affected Products

• Linux Kernel (versions prior to the security fix)
• openEuler 20.03 LTS (including SP1 and SP3)
• Debian Linux 10.0

Discovery Timeline

• 2022-07-18 - CVE-2021-33656 published to NVD
• 2025-04-02 - Last updated in NVD database

Technical Details for CVE-2021-33656

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-bounds Write), a memory corruption issue that occurs within the Linux kernel's virtual terminal (VT) font handling subsystem. The flaw exists in the processing of the PIO_FONT ioctl command, which is used to set console fonts.

When a user with access to the console device issues a PIO_FONT ioctl call with specially crafted font data, the kernel copies the provided data into internal buffers without adequate bounds checking. This allows an attacker to write beyond the intended memory region, potentially corrupting adjacent kernel memory structures.

The vulnerability requires physical access to the system, as the attack vector involves direct interaction with console devices. However, once exploited, the consequences can be severe—ranging from kernel crashes and denial of service to potential privilege escalation if the attacker can precisely control the corrupted memory regions.

Root Cause

The root cause of this vulnerability lies in insufficient input validation in the legacy font ioctl handlers within the Linux kernel's VT subsystem. The PIO_FONT ioctl does not properly verify the size and boundaries of user-supplied font data before copying it into kernel memory. This oversight allows maliciously crafted input to exceed buffer boundaries, resulting in out-of-bounds memory writes.

The fix involves dropping support for the old, insecure font ioctls entirely, as indicated by the patch named vt-drop-old-font-ioctls.patch. This approach eliminates the vulnerable code path rather than attempting to add bounds checking to legacy interfaces.

Attack Vector

The attack requires physical access to the target system. An attacker with access to the console device (typically /dev/console or /dev/tty) can craft a malicious ioctl request targeting the PIO_FONT command. The attack flow proceeds as follows:

1. The attacker opens a console device with appropriate permissions
2. A specially crafted font structure with oversized or malformed data is prepared
3. The ioctl() system call is invoked with the PIO_FONT command
4. The kernel processes the request without proper bounds validation
5. Memory corruption occurs as data is written beyond buffer boundaries

This can potentially be leveraged to corrupt kernel data structures, crash the system, or in sophisticated attacks, achieve code execution in kernel context.

Detection Methods for CVE-2021-33656

Indicators of Compromise

• Unexpected kernel crashes or panics related to VT or font subsystems
• Unusual ioctl calls to console devices (/dev/console, /dev/tty*)
• System instability or memory corruption symptoms after console access
• Kernel log messages indicating memory violations in VT-related functions

Detection Strategies

• Monitor for unusual ioctl system calls targeting console devices using system call auditing
• Implement kernel-level monitoring for PIO_FONT ioctl invocations
• Deploy SentinelOne Singularity Platform to detect kernel-level exploitation attempts
• Enable kernel memory protection features such as KASLR and SMAP to complicate exploitation

Monitoring Recommendations

• Configure auditd to log all ioctl calls to /dev/console and /dev/tty* devices
• Review kernel logs for memory corruption warnings or VT subsystem errors
• Implement endpoint detection and response (EDR) solutions capable of detecting privilege escalation attempts
• Monitor for processes making unusual system calls related to terminal font operations

How to Mitigate CVE-2021-33656

Immediate Actions Required

• Update the Linux kernel to version 5.10.127 or later which includes the security fix
• Apply vendor-specific patches from your Linux distribution (Debian, openEuler, etc.)
• Restrict physical access to systems and console devices where possible
• Review and limit user permissions to console and TTY devices

Patch Information

The vulnerability has been addressed through the removal of legacy font ioctl handlers. The patch vt-drop-old-font-ioctls.patch is available in Linux kernel stable releases starting from version 5.10.127. Distribution-specific patches are available through the following resources:

• Linux Kernel Patch for 5.10.127 - Official kernel patch
• Debian LTS Announcement - Debian security updates
• OpenEuler CVE-2021-33656 Details - openEuler advisory

Workarounds

• Limit physical access to systems and restrict who can log in at the console
• Use SELinux or AppArmor policies to restrict ioctl operations on console devices
• Disable unused console/TTY devices where operationally feasible
• Consider deploying kernel hardening options such as CONFIG_HARDENED_USERCOPY

# Restrict console device permissions as a temporary mitigation
chmod 600 /dev/console
chmod 600 /dev/tty*

# Verify current kernel version
uname -r

# Update kernel on Debian-based systems
apt update && apt upgrade linux-image-$(uname -r)