CVE-2023-3640: Linux Kernel Privilege Escalation Flaw

CVE-2023-3640 Overview

A possible unauthorized memory access flaw was found in the Linux kernel's cpu_entry_area mapping of X86 CPU data to memory, where a user may guess the location of exception stacks or other important data. Based on the previous CVE-2023-0597, the 'Randomize per-cpu entry area' feature was implemented in /arch/x86/mm/cpu_entry_area.c, which works through the init_cea_offsets() function when KASLR is enabled. However, despite this feature, there is still a risk of per-cpu entry area leaks. This issue could allow a local user to gain access to some important data with memory in an expected location and potentially escalate their privileges on the system.

Critical Impact

Local attackers can bypass KASLR protections by predicting the location of exception stacks and other sensitive kernel data structures, potentially leading to privilege escalation on affected Linux systems.

Affected Products

• Linux Kernel (all versions prior to patch)
• Red Hat Enterprise Linux 8.0
• Red Hat Enterprise Linux 9.0

Discovery Timeline

• 2023-07-24 - CVE-2023-3640 published to NVD
• 2025-04-15 - Last updated in NVD database

Technical Details for CVE-2023-3640

Vulnerability Analysis

This vulnerability represents an Information Disclosure flaw (CWE-203: Observable Discrepancy) that undermines the effectiveness of Kernel Address Space Layout Randomization (KASLR) on x86 Linux systems. The issue resides in how the Linux kernel maps CPU-specific data structures to memory through the cpu_entry_area mechanism.

The cpu_entry_area is a per-CPU memory region that contains critical kernel data structures including exception stacks, GDT (Global Descriptor Table), and TSS (Task State Segment). When KASLR is enabled, this area should be randomized to prevent attackers from predicting the locations of these sensitive structures.

Following CVE-2023-0597, the kernel developers implemented the 'Randomize per-cpu entry area' feature through the init_cea_offsets() function in /arch/x86/mm/cpu_entry_area.c. However, this implementation contains weaknesses that allow local users to infer or guess the location of these memory mappings, effectively defeating the KASLR protection.

Root Cause

The root cause lies in the incomplete implementation of per-cpu entry area randomization. While the init_cea_offsets() function attempts to randomize the cpu_entry_area locations when KASLR is enabled, the implementation leaves observable patterns or side-channels that can be exploited. This allows attackers to deduce the memory layout through timing attacks, cache-based side channels, or other observable behaviors, ultimately revealing the locations of exception stacks and other critical kernel data structures.

Attack Vector

The attack requires local access to the target system. An attacker with unprivileged local access can exploit this vulnerability through the following attack chain:

1. Information Gathering: The attacker executes code to probe memory access patterns or timing characteristics to identify cpu_entry_area locations
2. KASLR Bypass: Using the leaked information about exception stack locations, the attacker defeats KASLR protections
3. Privilege Escalation: With knowledge of kernel memory layout, the attacker can leverage additional kernel vulnerabilities or craft memory corruption attacks with precise targeting to escalate privileges

The attack complexity is low as it does not require sophisticated exploitation techniques once the information leakage vector is identified. No user interaction is required, and the attacker needs only low-level privileges (any local user account) to execute the attack.

Detection Methods for CVE-2023-3640

Indicators of Compromise

• Unusual patterns of memory access attempts targeting kernel address spaces from userland processes
• Processes performing repeated timing measurements or cache probing operations indicative of side-channel attacks
• Unexpected privilege escalation events following suspicious memory probing activity
• System logs showing anomalous kernel memory access patterns

Detection Strategies

• Monitor for processes executing unusual sequences of memory probing operations that could indicate KASLR bypass attempts
• Implement audit logging for processes attempting to access kernel memory regions or exhibiting side-channel attack patterns
• Deploy kernel runtime integrity monitoring to detect unauthorized access to cpu_entry_area structures
• Use SentinelOne's kernel-level behavioral analysis to identify privilege escalation attempts following information disclosure

Monitoring Recommendations

• Enable kernel audit subsystem to log suspicious memory access patterns and privilege changes
• Monitor system call patterns for anomalous behavior consistent with kernel address space probing
• Implement continuous monitoring of user sessions for privilege escalation indicators
• Review security logs for failed access attempts to kernel memory regions followed by successful exploitation attempts

How to Mitigate CVE-2023-3640

Immediate Actions Required

• Apply the latest kernel security updates from your Linux distribution vendor immediately
• For Red Hat Enterprise Linux systems, apply RHSA-2023:6583 security errata
• Restrict local system access to trusted users only until patches are applied
• Monitor affected systems for signs of exploitation attempts

Patch Information

Red Hat has released security updates to address this vulnerability. Affected users should apply the following patches:

• Red Hat Enterprise Linux 8.0 and 9.0: Apply the security errata RHSA-2023:6583
• Additional technical details available at the Red Hat CVE-2023-3640 Details page
• Bug tracking and patch progress can be monitored via Bugzilla Report #2217523

For other Linux distributions, check with your vendor for specific patch availability and instructions.

Workarounds

• Limit local system access to only trusted users until patches can be applied
• Consider enabling additional kernel hardening options such as KPTI (Kernel Page Table Isolation) if not already enabled
• Monitor and restrict access to debugging interfaces that could facilitate exploitation
• Implement strict process isolation using containers or virtual machines for untrusted workloads

# Check current kernel version for patch status
uname -r

# On Red Hat-based systems, check for available security updates
yum check-update --security

# Apply security updates on Red Hat Enterprise Linux
sudo yum update kernel --security

# Verify KASLR is enabled (additional hardening)
cat /proc/sys/kernel/randomize_va_space
# Output should be 2 for full randomization