CVE-2023-1513: Linux Kernel Information Disclosure Flaw

CVE-2023-1513 Overview

A memory information disclosure vulnerability was discovered in the Linux Kernel's KVM (Kernel-based Virtual Machine) subsystem. When calling the KVM_GET_DEBUGREGS ioctl on 32-bit systems, uninitialized portions of the kvm_debugregs structure may be copied to userspace, resulting in an information leak that could expose sensitive kernel memory contents to unprivileged users.

Critical Impact

Local attackers with low privileges can exploit uninitialized memory in KVM's debug register handling to leak kernel memory information on 32-bit Linux systems.

Affected Products

• Linux Kernel (all versions prior to patch)
• Fedora 37
• Red Hat Enterprise Linux 7.0, 8.0, 9.0

Discovery Timeline

• 2023-03-23 - CVE-2023-1513 published to NVD
• 2025-02-25 - Last updated in NVD database

Technical Details for CVE-2023-1513

Vulnerability Analysis

This vulnerability (CWE-665: Improper Initialization) stems from incomplete memory initialization in the KVM hypervisor's handling of debug register queries. When a userspace application invokes the KVM_GET_DEBUGREGS ioctl system call on 32-bit architectures, the kernel allocates a kvm_debugregs structure to return debug register information. However, certain portions of this structure are not properly initialized before being copied back to userspace, potentially leaking kernel stack or heap memory contents.

The vulnerability is architecture-specific, affecting only 32-bit systems where structure padding and alignment differences can leave uninitialized gaps in memory. On 64-bit systems, the structure layout typically does not exhibit this behavior.

Root Cause

The root cause is improper initialization of the kvm_debugregs structure in the KVM subsystem. Before copying structure data to userspace via the KVM_GET_DEBUGREGS ioctl handler, the kernel fails to zero out all fields, leaving padding bytes or unused structure members containing stale kernel memory. This violates the principle that kernel-to-userspace data transfers should never expose uninitialized memory.

Attack Vector

Exploitation requires local access to the system with the ability to create and interact with KVM virtual machines. An attacker needs:

1. Local unprivileged access to a 32-bit Linux system with KVM enabled
2. Permission to open /dev/kvm and create VM file descriptors
3. The ability to invoke the KVM_GET_DEBUGREGS ioctl on a vCPU

The attacker would repeatedly call KVM_GET_DEBUGREGS to harvest uninitialized memory contents, potentially revealing kernel pointers, cryptographic material, or other sensitive data that could assist in further exploitation such as defeating KASLR (Kernel Address Space Layout Randomization).

The fix involves properly initializing the kvm_debugregs structure using memset() before populating it with valid data, ensuring no uninitialized bytes are copied to userspace. See the GitHub Linux Commit for the patch details.

Detection Methods for CVE-2023-1513

Indicators of Compromise

• Unusual or repeated calls to KVM_GET_DEBUGREGS ioctl from non-virtualization processes
• Processes without legitimate virtualization purposes accessing /dev/kvm
• Unexpected memory scanning patterns from local user accounts
• Anomalous system call activity targeting KVM interfaces on 32-bit systems

Detection Strategies

• Monitor auditd logs for ioctl system calls targeting KVM device files with suspicious frequency
• Implement seccomp filters to restrict KVM ioctl access to authorized virtualization applications
• Use kernel tracing (ftrace, eBPF) to monitor KVM_GET_DEBUGREGS invocations
• Deploy host-based intrusion detection to identify unauthorized KVM device access

Monitoring Recommendations

• Enable audit rules for /dev/kvm access: auditctl -w /dev/kvm -p rwxa -k kvm_access
• Review KVM usage patterns and whitelist legitimate virtualization workloads
• Monitor for KASLR bypass attempts that may follow information disclosure exploitation
• Implement process isolation for virtualization workloads using SELinux or AppArmor

How to Mitigate CVE-2023-1513

Immediate Actions Required

• Update the Linux kernel to a patched version that includes commit 2c10b61421a28e95a46ab489fd56c0f442ff6952
• Apply vendor-specific security patches from Red Hat, Fedora, or Debian as applicable
• Restrict access to /dev/kvm to only trusted users and applications
• Consider migrating 32-bit virtualization workloads to 64-bit systems where feasible

Patch Information

The vulnerability has been addressed in the upstream Linux kernel. The fix ensures proper initialization of the kvm_debugregs structure before copying to userspace. Organizations should apply the following vendor patches:

• Linux Kernel: Apply commit 2c10b61421a28e95a46ab489fd56c0f442ff6952
• Red Hat Enterprise Linux: Refer to Red Hat Bug Report #2179892 for RHEL-specific patches
• Debian: Apply updates from the Debian LTS Security Announcements
• Fedora: Update to the latest kernel package containing the security fix

Workarounds

• Restrict /dev/kvm access using file permissions: chmod 600 /dev/kvm
• Use group-based access control to limit KVM usage to authorized accounts
• Disable KVM module on systems that do not require virtualization: modprobe -r kvm_intel kvm_amd kvm
• Implement mandatory access control policies to confine KVM access

# Configuration example
# Restrict KVM access to the kvm group only
chown root:kvm /dev/kvm
chmod 660 /dev/kvm

# Add only authorized users to the kvm group
usermod -aG kvm trusted_user

# Optionally disable KVM if not needed
echo "blacklist kvm" >> /etc/modprobe.d/blacklist-kvm.conf
echo "blacklist kvm_intel" >> /etc/modprobe.d/blacklist-kvm.conf
echo "blacklist kvm_amd" >> /etc/modprobe.d/blacklist-kvm.conf