CVE-2023-0459: Linux Kernel Information Disclosure Flaw

CVE-2023-0459 Overview

CVE-2023-0459 is an information disclosure vulnerability affecting 64-bit versions of the Linux kernel. The copy_from_user() function fails to implement the __uaccess_begin_nospec speculation barrier, allowing a local attacker to bypass the access_ok check and pass a kernel pointer to copy_from_user(). This vulnerability enables unauthorized disclosure of sensitive kernel memory contents to user-space processes.

Critical Impact

Local attackers can leak sensitive kernel memory information by exploiting the missing speculation barrier in the copy_from_user() function on 64-bit Linux systems, potentially exposing cryptographic keys, credentials, or other sensitive data stored in kernel memory.

Affected Products

• Linux Kernel (64-bit versions)
• Linux Kernel versions prior to commit 74e19ef0ff8061ef55957c3abd71614ef0f42f47
• Systems running vulnerable x86_64 Linux kernel builds

Discovery Timeline

• May 25, 2023 - CVE-2023-0459 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-0459

Vulnerability Analysis

This vulnerability stems from an incomplete implementation of Spectre variant 1 mitigations in the Linux kernel's user access copy functions. The copy_from_user() function on 64-bit architectures did not properly invoke __uaccess_begin_nospec, which is essential for inserting a speculation barrier before accessing user-space memory pointers.

Without this speculation barrier, an attacker can craft a scenario where the CPU speculatively executes memory accesses using a kernel pointer that would normally be rejected by the access_ok check. This speculative execution can leave observable side effects in the CPU cache, enabling Spectre-style attacks to extract kernel memory contents.

The vulnerability is particularly significant because copy_from_user() is a fundamental kernel primitive used extensively throughout the kernel codebase for copying data from user-space to kernel-space. The high confidentiality impact reflects the potential for attackers to read arbitrary kernel memory locations.

Root Cause

The root cause is the missing barrier_nospec() speculation barrier in the copy_from_user() code path on 64-bit Linux systems. The __uaccess_begin_nospec macro, which should insert this barrier to prevent speculative execution based on untrusted user pointers, was not consistently applied. This allowed the CPU to speculatively execute memory operations before the access_ok bounds check was architecturally resolved, creating a timing side channel for information disclosure.

Attack Vector

The attack requires local access to the system with low privileges. An attacker can exploit this vulnerability by:

1. Crafting a malicious user-space application that invokes system calls utilizing copy_from_user()
2. Manipulating the pointer passed to copy_from_user() to point to kernel memory addresses
3. Using cache-timing side channels to observe the speculative memory accesses
4. Extracting sensitive kernel memory contents through the observed cache behavior

The local attack vector and low privilege requirement mean this vulnerability could be exploited by any authenticated user on a vulnerable system.

// Security patch in lib/usercopy.c - Adding speculation barrier
// Source: https://github.com/torvalds/linux/commit/74e19ef0ff8061ef55957c3abd71614ef0f42f47

#include <linux/fault-inject-usercopy.h>
#include <linux/instrumented.h>
#include <linux/uaccess.h>
+#include <linux/nospec.h>
 
/* out-of-line parts */

Source: GitHub Linux Commit Fix

// Security patch in kernel/bpf/core.c - Removing architecture-specific barrier
// Source: https://github.com/torvalds/linux/commit/74e19ef0ff8061ef55957c3abd71614ef0f42f47

		 * reuse preexisting logic from Spectre v1 mitigation that
		 * happens to produce the required code on x86 for v4 as well.
		 */
-#ifdef CONFIG_X86
		barrier_nospec();
-#endif
		CONT;
#define LDST(SIZEOP, SIZE)						\
	STX_MEM_##SIZEOP:						\

Source: GitHub Linux Commit Fix

Detection Methods for CVE-2023-0459

Indicators of Compromise

• Unusual kernel memory access patterns from user-space processes
• Suspicious system calls with malformed user pointers attempting to read kernel addresses
• Abnormal cache timing behavior indicative of side-channel exploitation attempts
• Processes repeatedly invoking copy functions with addresses outside user-space ranges

Detection Strategies

• Monitor system logs for kernel warnings about invalid user pointer access attempts
• Deploy kernel integrity monitoring tools to detect anomalous memory access patterns
• Implement audit rules for system calls that heavily utilize copy_from_user()
• Use hardware performance counters to detect cache-timing attack signatures

Monitoring Recommendations

• Enable kernel auditing subsystem to log suspicious user-to-kernel copy operations
• Monitor for processes exhibiting high rates of failed memory access operations
• Deploy endpoint detection solutions capable of identifying Spectre-variant exploitation techniques
• Review system call audit logs for patterns consistent with memory probing activities

How to Mitigate CVE-2023-0459

Immediate Actions Required

• Update the Linux kernel to a version containing commit 74e19ef0ff8061ef55957c3abd71614ef0f42f47 or later
• Prioritize patching 64-bit Linux systems, particularly those with multi-user access
• Apply vendor-specific kernel security updates from your Linux distribution
• Consider restricting local access to sensitive systems until patching is complete

Patch Information

The vulnerability is addressed in the Linux kernel commits 74e19ef0ff8061ef55957c3abd71614ef0f42f47 and 4b842e4e25b12951fa10dedb4bc16bc47e3b850c. These commits add the necessary barrier_nospec() speculation barrier to copy_from_user() and remove the small constant size optimizations in raw_copy_{to,from}_user() that bypassed the speculation barrier. Organizations should update to kernel versions that include these fixes or apply backported patches from their distribution vendor.

Workarounds

• Limit local system access to only trusted users until the kernel can be patched
• Enable additional Spectre mitigations at the kernel level where available
• Deploy mandatory access controls (SELinux, AppArmor) to restrict untrusted process capabilities
• Consider disabling unprivileged BPF if applicable to reduce attack surface

# Check current kernel version
uname -r

# Verify if the fix is present (look for barrier_nospec in usercopy)
grep -r "barrier_nospec" /usr/src/linux/lib/usercopy.c 2>/dev/null || echo "Source not available, check kernel version"

# Update kernel using your distribution's package manager
# For Debian/Ubuntu:
sudo apt update && sudo apt upgrade linux-image-generic

# For RHEL/CentOS/Fedora:
sudo dnf update kernel

# Reboot to apply the new kernel
sudo reboot