CVE-2020-29368: Linux Kernel Race Condition Vulnerability

CVE-2020-29368 Overview

A race condition vulnerability was discovered in the Linux kernel's Transparent Huge Pages (THP) implementation within the __split_huge_pmd function located in mm/huge_memory.c. The vulnerability exists in versions before 5.7.5 and can grant unintended write access due to a race condition in the THP mapcount check during copy-on-write (CoW) operations. This flaw is tracked as CID-c444eb564fb1.

Critical Impact

Local attackers with low privileges can exploit this race condition to gain unintended write access to memory pages, potentially leading to privilege escalation or data corruption on affected Linux systems.

Affected Products

• Linux Kernel (versions before 5.7.5)
• NetApp Cloud Backup
• NetApp Element Software
• NetApp HCI Management Node
• NetApp SolidFire
• NetApp HCI Bootstrap OS
• NetApp HCI Compute Node
• NetApp H410C Firmware

Discovery Timeline

• 2020-11-28 - CVE-2020-29368 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-29368

Vulnerability Analysis

This vulnerability (CWE-362: Race Condition) exists in the Linux kernel's memory management subsystem, specifically within the Transparent Huge Pages (THP) handling code. The copy-on-write mechanism, which is fundamental to Linux memory management, fails to properly handle concurrent access scenarios when splitting huge pages. When a process forks, the parent and child share memory pages until one of them attempts to write, at which point a private copy should be created. However, due to the race condition in the mapcount check within __split_huge_pmd, an attacker can manipulate timing to gain write access to memory pages that should remain read-only.

Root Cause

The root cause lies in the improper synchronization within the __split_huge_pmd function when checking the mapcount during THP splitting operations. The mapcount check, which determines whether a page is shared and requires copy-on-write protection, is not performed atomically with the subsequent page table modifications. This allows a small window where concurrent operations can bypass the copy-on-write protection, resulting in unintended write access to shared memory pages.

Attack Vector

The attack requires local access with low-level user privileges. An attacker must be able to execute code on the target system and exploit precise timing conditions to win the race. The exploitation involves:

1. Creating a shared memory mapping using Transparent Huge Pages
2. Triggering a fork operation to initiate copy-on-write semantics
3. Manipulating the timing of page splitting operations via concurrent memory access patterns
4. Exploiting the window between the mapcount check and page table modification to gain write access

This race condition allows the attacker to write to memory pages that should be protected, potentially modifying data belonging to other processes or the kernel itself.

Detection Methods for CVE-2020-29368

Indicators of Compromise

• Unusual memory access patterns or unexpected page fault handling in kernel logs
• Processes gaining write access to memory regions that should be read-only
• Anomalous behavior from applications after fork operations
• Kernel crash dumps indicating corruption in THP-related data structures

Detection Strategies

• Monitor kernel logs for unusual THP splitting activity or related warnings
• Deploy runtime kernel integrity monitoring to detect unexpected memory modifications
• Implement process behavior analysis to identify anomalous memory access patterns
• Use kernel tracing tools (ftrace, eBPF) to monitor __split_huge_pmd function calls and timing anomalies

Monitoring Recommendations

• Enable and review kernel audit logs for memory-related system calls
• Deploy endpoint detection solutions capable of monitoring kernel-level operations
• Implement file integrity monitoring for critical system files that could be modified via this vulnerability
• Monitor for privilege escalation attempts that may follow successful exploitation

How to Mitigate CVE-2020-29368

Immediate Actions Required

• Upgrade the Linux kernel to version 5.7.5 or later
• Apply vendor-specific patches for affected NetApp products
• Consider disabling Transparent Huge Pages as a temporary mitigation if patching is not immediately possible
• Restrict local access to systems running vulnerable kernel versions

Patch Information

The vulnerability is fixed in Linux kernel version 5.7.5 and later. The fix is documented in commit c444eb564fb16645c172d550359cb3d75fe8a040. For detailed changelog information, refer to the Linux Kernel ChangeLog 5.7.5. NetApp customers should consult the NetApp Security Advisory ntap-20210108-0002 for guidance on affected products. Additional technical details about the discovery can be found in the Chromium Project Zero Issue #2045.

Workarounds

• Disable Transparent Huge Pages (THP) as a temporary mitigation until patching is possible
• Limit local user access to affected systems
• Implement strict process isolation using containers or virtual machines
• Apply security hardening measures such as SELinux or AppArmor to limit potential damage from exploitation

# Disable Transparent Huge Pages temporarily
echo never > /sys/kernel/mm/transparent_hugepage/enabled
echo never > /sys/kernel/mm/transparent_hugepage/defrag

# Verify THP is disabled
cat /sys/kernel/mm/transparent_hugepage/enabled
# Should show: always madvise [never]

# Check current kernel version
uname -r
# Upgrade if below 5.7.5