CVE-2020-27152: Linux Kernel DOS Vulnerability

CVE-2020-27152 Overview

An issue was discovered in the ioapic_lazy_update_eoi function in arch/x86/kvm/ioapic.c in the Linux kernel before version 5.9.2. The vulnerability causes an infinite loop due to improper interaction between a resampler and edge triggering, tracked as CID-77377064c3a9. This flaw can be exploited by a local attacker with low privileges to cause a denial of service condition on systems running vulnerable KVM hypervisor configurations.

Critical Impact

Local attackers can trigger an infinite loop in the KVM IOAPIC emulation code, causing system unresponsiveness and denial of service on virtualization hosts.

Affected Products

• Linux Kernel versions prior to 5.9.2
• Systems using KVM virtualization with IOAPIC emulation
• x86/x86_64 architectures with KVM-enabled kernels

Discovery Timeline

• 2020-11-06 - CVE CVE-2020-27152 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-27152

Vulnerability Analysis

The vulnerability resides in the KVM (Kernel-based Virtual Machine) subsystem's IOAPIC (I/O Advanced Programmable Interrupt Controller) emulation code. Specifically, the ioapic_lazy_update_eoi function in arch/x86/kvm/ioapic.c contains a logic flaw that leads to an infinite loop condition.

The issue stems from improper handling of the interaction between the interrupt resampler mechanism and edge-triggered interrupts. When certain conditions are met during the End-of-Interrupt (EOI) lazy update process, the code enters a loop that never terminates, consuming CPU resources indefinitely.

This vulnerability is classified under CWE-835 (Loop with Unreachable Exit Condition), indicating that the loop's termination logic contains a flaw that prevents normal exit under specific circumstances. The attack requires local access and low privileges, but no user interaction is necessary to trigger the condition.

Root Cause

The root cause lies in the improper coordination between the resampler component and edge-triggered interrupt handling within the IOAPIC emulation. The resampler is designed to resend interrupts that may have been missed, while edge triggering detects interrupt state transitions. When these two mechanisms interact incorrectly, the lazy EOI update function fails to properly update state, causing the loop to continue indefinitely without reaching a valid exit condition.

Attack Vector

The vulnerability requires local access to exploit. An attacker with low-privilege access to a KVM guest or to the host system could craft specific interrupt conditions that trigger the infinite loop in the IOAPIC emulation code. This would cause the KVM host to become unresponsive, affecting all virtual machines running on that host.

The attack does not require user interaction and can be triggered programmatically by manipulating interrupt handling in a way that exposes the faulty code path in ioapic_lazy_update_eoi.

Detection Methods for CVE-2020-27152

Indicators of Compromise

• Unexplained high CPU utilization on KVM host systems, particularly in kernel space
• KVM processes or kernel threads becoming unresponsive or stuck
• Virtual machines experiencing sudden performance degradation or hangs
• System logs showing repeated interrupt-related messages from KVM subsystem

Detection Strategies

• Monitor for kernel hang or soft lockup warnings related to KVM or IOAPIC operations
• Implement watchdog monitoring for KVM host CPU utilization anomalies
• Review kernel logs for references to ioapic_lazy_update_eoi or interrupt handling errors
• Deploy system monitoring that alerts on prolonged kernel execution without user-space scheduling

Monitoring Recommendations

• Enable KVM tracing and monitoring to detect abnormal interrupt patterns
• Configure system watchdogs to detect and respond to kernel soft lockups
• Monitor /proc/interrupts for unusual interrupt counts on IOAPIC lines
• Set up alerts for extended periods of 100% CPU utilization in kernel mode

How to Mitigate CVE-2020-27152

Immediate Actions Required

• Upgrade Linux kernel to version 5.9.2 or later immediately
• Assess all KVM virtualization hosts running vulnerable kernel versions
• Schedule maintenance windows for kernel updates on production virtualization infrastructure
• Consider temporarily disabling or restricting access to vulnerable KVM hosts until patched

Patch Information

The vulnerability has been patched in Linux kernel version 5.9.2. The fix is tracked by commit ID 77377064c3a94911339f13ce113b3abf265e06da in the Linux kernel git repository. Administrators should update to kernel version 5.9.2 or later to remediate this vulnerability.

For detailed patch information, refer to the Linux Kernel Changelog 5.9.2 and the Linux Git Commit 77377064.

Additional details are available in the Openwall OSS Security Discussion and Kernel Bugzilla Report #208767.

Workarounds

• Limit local access to KVM hosts to trusted administrators only
• Implement strict access controls on virtualization management interfaces
• Consider using hardware-assisted interrupt handling where available to reduce exposure
• Monitor and restrict capabilities of guest VMs that could trigger the vulnerable code path

# Check current kernel version
uname -r

# Verify if KVM modules are loaded
lsmod | grep kvm

# Update kernel on Debian/Ubuntu systems
sudo apt update && sudo apt install linux-image-5.9.2 linux-headers-5.9.2

# Update kernel on RHEL/CentOS systems (check vendor backports)
sudo yum update kernel