CVE-2021-33631: Openatom Openeuler Integer Overflow Flaw

CVE-2021-33631 Overview

CVE-2021-33631 is an Integer Overflow or Wraparound vulnerability affecting the openEuler kernel on Linux, specifically within filesystem modules. This vulnerability enables attackers to trigger a Forced Integer Overflow condition, potentially leading to memory corruption, privilege escalation, or system instability. The flaw exists in openEuler kernel versions from 4.19.90 before 4.19.90-2401.3 and from 5.10.0-60.18.0 before 5.10.0-183.0.0.

Critical Impact

Local attackers with low privileges can exploit this integer overflow vulnerability to achieve high confidentiality, integrity, and availability impacts on affected systems, potentially gaining elevated privileges or causing system crashes.

Affected Products

• openAtom openEuler kernel 4.19.90 to 4.19.90-2401.3
• openAtom openEuler kernel 5.10.0-60.18.0 to 5.10.0-183.0.0
• openAtom openEuler Linux distributions using affected kernel versions

Discovery Timeline

• 2024-01-18 - CVE-2021-33631 published to NVD
• 2025-04-02 - Last updated in NVD database

Technical Details for CVE-2021-33631

Vulnerability Analysis

This vulnerability is classified under CWE-190 (Integer Overflow or Wraparound), a memory corruption flaw that occurs when arithmetic operations produce values that exceed the storage capacity of the integer data type. In the context of the openEuler kernel filesystem modules, the integer overflow can be triggered when processing specially crafted data, causing the computed value to wrap around and produce an unexpectedly small or negative value.

When exploited, this integer overflow can lead to subsequent memory corruption issues such as buffer overflows, incorrect memory allocations, or out-of-bounds memory access. Since this vulnerability resides in kernel-level filesystem code, successful exploitation could allow an attacker with local access and low privileges to escalate their privileges, corrupt kernel memory, or cause denial of service conditions.

The attack requires local access to the system, meaning the attacker must already have some level of access to the target machine. However, the low attack complexity and lack of user interaction requirements make this vulnerability particularly concerning for multi-user systems or environments where local privilege escalation is a threat vector.

Root Cause

The root cause of CVE-2021-33631 lies in insufficient integer boundary validation within the openEuler kernel's filesystem modules. When processing certain filesystem operations, the code fails to properly check whether arithmetic operations on integer values will result in overflow conditions. Without proper validation, these overflow conditions cause the resulting value to wrap around, leading to incorrect size calculations, improper memory allocations, or buffer overflows when the corrupted value is subsequently used in memory operations.

Attack Vector

The attack vector for CVE-2021-33631 requires local access to the vulnerable system. An attacker with low-privilege access can exploit this vulnerability without requiring any user interaction. The exploitation process involves triggering filesystem operations that cause the vulnerable integer arithmetic to overflow. This could be accomplished through:

1. Mounting specially crafted filesystem images
2. Performing filesystem operations that trigger the vulnerable code path
3. Manipulating filesystem metadata to cause integer overflow conditions during processing

The vulnerability affects the local system's confidentiality, integrity, and availability, as successful exploitation can lead to privilege escalation, memory corruption, or system crashes.

Detection Methods for CVE-2021-33631

Indicators of Compromise

• Unexpected kernel crashes or panics related to filesystem operations
• Anomalous memory allocation patterns in kernel logs
• Suspicious local privilege escalation attempts following filesystem operations
• System instability when mounting or accessing filesystem images

Detection Strategies

• Monitor kernel logs (dmesg) for filesystem-related errors, memory corruption warnings, or unexpected kernel panics
• Implement kernel-level intrusion detection to identify anomalous system calls to filesystem functions
• Deploy file integrity monitoring to detect unauthorized changes to filesystem structures
• Use SentinelOne's behavioral detection capabilities to identify privilege escalation attempts following filesystem operations

Monitoring Recommendations

• Enable comprehensive kernel logging to capture filesystem operation details and potential exploitation attempts
• Implement system call auditing for filesystem-related operations using auditd or similar tools
• Monitor for unusual process behavior following filesystem mounts or operations
• Regularly review system logs for signs of exploitation attempts or memory corruption events

How to Mitigate CVE-2021-33631

Immediate Actions Required

• Update openEuler kernel to version 4.19.90-2401.3 or later for 4.19.x branch
• Update openEuler kernel to version 5.10.0-183.0.0 or later for 5.10.x branch
• Restrict local access to systems running vulnerable kernel versions
• Monitor systems for signs of exploitation while patches are being applied

Patch Information

Security patches are available through multiple channels. The fix has been committed to the upstream Linux kernel repository with commit ID 5c099c4fdc438014d5893629e70a8ba934433ee8. For openEuler-specific patches, refer to the Linux Kernel Commit Details and the openEuler security bulletins including openEuler-SA-2024-1030 through openEuler-SA-2024-1035. Additional pull requests for the kernel patches are available on Gitee at Pull Request #1389 and Pull Request #1396.

Workarounds

• Restrict local user access to minimize the attack surface for this vulnerability
• Implement strict access controls for filesystem mount operations
• Disable or restrict access to kernel modules where feasible until patches can be applied
• Consider using SELinux or AppArmor policies to restrict filesystem operations for untrusted users

# Check current kernel version
uname -r

# Verify if system is running vulnerable kernel
# For 4.19.x branch: vulnerable if < 4.19.90-2401.3
# For 5.10.x branch: vulnerable if < 5.10.0-183.0.0

# Update kernel using yum (openEuler)
sudo yum update kernel

# Reboot to apply new kernel
sudo reboot