CVE-2023-3640: Linux Kernel Privilege Escalation Flaw

CVE-2023-3640 Overview

CVE-2023-3640 is an unauthorized memory access flaw in the Linux kernel's cpu_entry_area mapping of X86 CPU data to memory. A local user can guess the location of exception stacks or other important kernel data structures, bypassing protections introduced after CVE-2023-0597. The Randomize per-cpu entry area feature implemented in /arch/x86/mm/cpu_entry_area.c via the init_cea_offsets() function still leaks per-cpu entry area locations when Kernel Address Space Layout Randomization (KASLR) is enabled. Successful exploitation can lead to local privilege escalation on affected systems.

Critical Impact
A local, authenticated attacker can infer the location of sensitive kernel structures and use that information as a stepping stone toward privilege escalation to root.

Affected Products

Linux Kernel (X86 architecture with KASLR enabled)
Red Hat Enterprise Linux 8.0
Red Hat Enterprise Linux 9.0

Discovery Timeline

2023-07-24 - CVE-2023-3640 published to the National Vulnerability Database
2023-11-14 - Red Hat releases security advisory RHSA-2023:6583
2025-04-15 - Last updated in NVD database

Technical Details for CVE-2023-3640

Vulnerability Analysis

The flaw is classified under [CWE-203] Observable Discrepancy. The Linux kernel maps per-CPU data structures, including exception stacks, through the cpu_entry_area region on X86 systems. To mitigate predictable layouts exposed by CVE-2023-0597, kernel developers added per-CPU entry area randomization in arch/x86/mm/cpu_entry_area.c. The init_cea_offsets() function assigns randomized offsets to each CPU's entry area at boot when KASLR is enabled.

Despite this mitigation, the implementation still allows observable side effects that let a local process infer where per-CPU entry areas are located in virtual memory. Once the attacker recovers these offsets, exception stacks and other privileged data sit at known addresses, undermining the randomization assumption that other kernel exploit mitigations depend on.

Root Cause

The randomization performed by init_cea_offsets() does not produce sufficient entropy or isolation to prevent information disclosure. A local user can use timing, cache, or other observable behavior to derive the randomized offsets, defeating the protection added in response to CVE-2023-0597.

Attack Vector

Exploitation requires local access with low privileges and no user interaction. The attacker runs unprivileged code on the target host, leaks the per-CPU entry area layout, and chains the disclosure with a separate memory corruption primitive to escalate privileges. The vulnerability cannot be triggered remotely. Detailed exploitation specifics are described in the Red Hat Bug Report #2217523.

Detection Methods for CVE-2023-3640

Indicators of Compromise

Unprivileged processes performing repeated reads or timing measurements against kernel-related interfaces such as /proc/self/stat, /proc/kallsyms, or perf subsystems.
Unexpected execution of locally compiled binaries by service accounts or non-administrative users on Linux hosts.
Kernel oops or page-fault entries in dmesg correlated with subsequent privilege-escalation activity.

Detection Strategies

Monitor for execution of kernel information-gathering tools and side-channel measurement programs by non-root users.
Track loading of unsigned or out-of-tree kernel modules following suspicious local process activity.
Correlate sudden UID transitions from unprivileged to root on Linux endpoints with preceding anomalous syscall patterns.

Monitoring Recommendations

Enable auditd rules for execve, setuid, and init_module syscalls and ship logs to a centralized SIEM or data lake.
Baseline normal user behavior on multi-tenant Linux hosts so that local enumeration of kernel structures stands out.
Review kernel version inventory across the fleet and flag any RHEL 8.x or 9.x systems that have not received the RHSA-2023:6583 update.

How to Mitigate CVE-2023-3640

Immediate Actions Required

Apply the kernel updates from Red Hat Security Advisory RHSA-2023:6583 to all affected RHEL 8 and 9 systems.
Update upstream Linux kernels to a version that includes the cpu_entry_area randomization fix referenced in the Red Hat CVE-2023-3640 Details.
Restrict local shell access on multi-user systems and enforce least privilege for service accounts.

Patch Information

Red Hat addressed the issue in RHSA-2023:6583, which ships updated kernel packages for Red Hat Enterprise Linux 8 and 9. Customers running upstream Linux should rebase to a kernel that includes the corresponding fix to arch/x86/mm/cpu_entry_area.c. Reboot is required after installing the new kernel for the changes to take effect.

Workarounds

Limit local logon and code execution to trusted administrators until the patched kernel is installed.
Enforce mandatory access control with SELinux in enforcing mode to constrain unprivileged processes.
Disable unprivileged user namespaces where business workflows do not require them, reducing the local attack surface.

bash

# Verify the running kernel and apply the RHEL patch
uname -r
sudo dnf update --advisory=RHSA-2023:6583 -y
sudo reboot