CVE-2020-25704: Linux Kernel DOS Vulnerability

CVE-2020-25704 Overview

A memory leak vulnerability was discovered in the Linux kernel performance monitoring subsystem. The flaw exists in the way the kernel handles PERF_EVENT_IOC_SET_FILTER ioctl operations. A local user with access to performance monitoring facilities could exploit this vulnerability to repeatedly trigger memory allocations that are never freed, eventually exhausting system resources and causing a denial of service condition.

Critical Impact

Local users can starve system resources through repeated exploitation of the memory leak, causing denial of service and system instability.

Affected Products

• Linux Kernel (versions up to and including 5.10-rc2)
• Debian Linux 9.0
• StarWind Command Center
• StarWind Hyperconverged Appliance
• StarWind SAN & NAS v8r12
• StarWind Virtual SAN v8 build14398

Discovery Timeline

• 2020-11-09 - Vulnerability disclosed on Openwall OSS-Security mailing list
• 2020-12-02 - CVE-2020-25704 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-25704

Vulnerability Analysis

This vulnerability is classified as CWE-401 (Missing Release of Memory after Effective Lifetime). The flaw resides in the Linux kernel's performance monitoring subsystem, specifically in the code path that handles the PERF_EVENT_IOC_SET_FILTER ioctl call. When a user sets a filter on a performance event, memory is allocated to store the filter configuration. However, under certain conditions, this allocated memory is not properly freed when it should be, leading to a memory leak.

The vulnerability requires local access to exploit, meaning an attacker must have the ability to execute code on the target system. The attack complexity is low, as the exploitation mechanism is straightforward—repeatedly invoking the vulnerable ioctl to accumulate memory allocations. While the vulnerability does not impact confidentiality or integrity, it can severely affect system availability by exhausting kernel memory resources.

Root Cause

The root cause of this vulnerability is improper memory management in the performance monitoring subsystem. When the PERF_EVENT_IOC_SET_FILTER ioctl is invoked, the kernel allocates memory to store filter strings. The vulnerability occurs because under specific code paths, the previously allocated filter memory is not freed before new memory is allocated, or the memory is not released when the performance event is destroyed. This leads to accumulated memory that cannot be reclaimed, constituting a classic memory leak (CWE-401).

Attack Vector

The attack vector is local, requiring an authenticated user with access to the performance monitoring interfaces. The attacker can create a simple program that repeatedly opens performance events and sets filters using the PERF_EVENT_IOC_SET_FILTER ioctl. Each iteration leaks a small amount of kernel memory. Over time, or with aggressive iteration, the accumulated memory leak can exhaust available system memory, causing the kernel to enter an out-of-memory state.

The exploitation mechanism involves calling perf_event_open() syscall to create performance monitoring events, then using ioctl() with PERF_EVENT_IOC_SET_FILTER command to trigger the memory allocation that leaks. Since no special privileges beyond basic user access are typically required to use performance monitoring facilities, this vulnerability poses a risk in multi-user environments or systems where unprivileged users have shell access.

Detection Methods for CVE-2020-25704

Indicators of Compromise

• Gradual increase in kernel memory usage (slab memory) without corresponding workload increase
• System logs showing out-of-memory (OOM) killer activations
• Unusual number of perf_event_open syscalls from non-administrative users
• Processes repeatedly opening and closing performance monitoring events

Detection Strategies

• Monitor system memory metrics, particularly kernel slab allocations, for unexplained growth patterns
• Implement syscall auditing to track perf_event_open and related ioctl calls, especially from non-root users
• Deploy SentinelOne Singularity platform to detect anomalous system behavior patterns associated with resource exhaustion attacks
• Use kernel tracing tools (ftrace, perf) to monitor performance subsystem activity for suspicious patterns

Monitoring Recommendations

• Configure memory usage alerts to trigger when kernel memory consumption exceeds baseline thresholds
• Implement audit rules for performance monitoring syscalls: auditctl -a always,exit -F arch=b64 -S perf_event_open
• Review and restrict access to performance monitoring capabilities using perf_event_paranoid sysctl settings
• Deploy endpoint detection and response (EDR) solutions capable of correlating resource exhaustion with process behavior

How to Mitigate CVE-2020-25704

Immediate Actions Required

• Update the Linux kernel to a patched version that includes commit 7bdb157cdebbf95a1cd94ed2e01b338714075d00
• Restrict access to performance monitoring subsystem by setting /proc/sys/kernel/perf_event_paranoid to 3 or higher
• Apply vendor-specific patches from Debian, Red Hat, or StarWind as appropriate for your deployment
• Monitor systems for signs of exploitation while patches are being deployed

Patch Information

The vulnerability has been addressed in the Linux kernel through commit 7bdb157cdebbf95a1cd94ed2e01b338714075d00. This fix ensures proper cleanup of filter memory allocations in the performance monitoring subsystem. Debian has released advisories for affected versions—see the Debian LTS Advisory for package update information. For Red Hat systems, refer to Bug Report 1895961. StarWind customers should review Security Advisory SW-20220802-0003 for product-specific guidance.

Workarounds

• Set kernel.perf_event_paranoid=3 to completely disable performance events for unprivileged users
• Restrict shell access for untrusted users on multi-user systems
• Implement cgroups memory limits to contain the impact of memory exhaustion attacks
• Consider using Linux Security Modules (SELinux/AppArmor) to restrict access to performance monitoring capabilities

# Configuration example - Restrict perf_event access
# Add to /etc/sysctl.conf or /etc/sysctl.d/99-security.conf
echo "kernel.perf_event_paranoid = 3" >> /etc/sysctl.d/99-perf-security.conf
sysctl -p /etc/sysctl.d/99-perf-security.conf

# Verify the setting is applied
cat /proc/sys/kernel/perf_event_paranoid