CVE-2020-25220: Linux Kernel Use-After-Free Vulnerability

CVE-2020-25220 Overview

CVE-2020-25220 is a use-after-free vulnerability affecting multiple versions of the Linux kernel. The flaw exists because the skcd->no_refcnt field was not properly considered during a backport of a CVE-2020-14356 patch, leading to potential memory corruption conditions. This vulnerability is related to the cgroups (control groups) feature, which is a fundamental kernel mechanism used for resource management and containerization.

Critical Impact

A local attacker with low privileges could exploit this use-after-free condition to achieve arbitrary code execution, escalate privileges, or cause system instability through memory corruption in kernel space.

Affected Products

• Linux Kernel 4.9.x before 4.9.233
• Linux Kernel 4.14.x before 4.14.194
• Linux Kernel 4.19.x before 4.19.140

Discovery Timeline

• September 10, 2020 - CVE-2020-25220 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-25220

Vulnerability Analysis

This use-after-free vulnerability originated from an incomplete backport of a security patch for CVE-2020-14356 to the stable Linux kernel branches. The root issue lies in how the cgroups subsystem handles socket reference counting. During the backport process, the handling of the skcd->no_refcnt flag was not properly addressed, creating a scenario where memory could be accessed after it has been freed.

The cgroups feature in Linux provides a mechanism for organizing processes into hierarchical groups, enabling resource limiting, prioritization, and accounting. Socket handling within cgroups requires careful reference counting to prevent premature deallocation of socket-related data structures. When no_refcnt is improperly handled, the kernel may free a socket control data structure while references to it still exist, leading to a classic use-after-free condition.

Root Cause

The vulnerability stems from an incomplete security backport where the skcd->no_refcnt field check was omitted. This field indicates whether reference counting should be bypassed for certain socket operations within the cgroup context. Without this check, the backported code path could incorrectly handle socket lifecycle management, allowing a data structure to be freed while still being referenced elsewhere in the kernel.

Attack Vector

This is a local attack vector requiring the attacker to have low-privileged access to the target system. Exploitation involves manipulating cgroup socket operations to trigger the use-after-free condition. An attacker could:

1. Create a cgroup and associate socket operations with it
2. Trigger the vulnerable code path where reference counting is mishandled
3. Cause the kernel to free the socket control data prematurely
4. Access the freed memory to corrupt kernel data structures or achieve code execution

The attack mechanism exploits the timing between socket operations and cgroup cleanup. By carefully orchestrating these operations, an attacker can control the contents of the freed memory region when it is subsequently accessed, potentially redirecting kernel execution flow or modifying sensitive kernel data.

Detection Methods for CVE-2020-25220

Indicators of Compromise

• Unexpected kernel crashes or panics related to cgroup or socket subsystems
• Abnormal behavior in containerized environments utilizing cgroups for resource management
• Kernel log messages indicating memory corruption or invalid memory access in socket-related code paths
• Unusual privilege escalation attempts from low-privileged accounts

Detection Strategies

• Monitor kernel logs for use-after-free related KASAN (Kernel Address Sanitizer) warnings if enabled
• Deploy runtime kernel integrity monitoring solutions to detect unexpected code execution patterns
• Implement system call auditing for cgroup-related operations from untrusted processes
• Utilize SentinelOne's behavioral AI to detect anomalous kernel-level activity patterns indicative of exploitation attempts

Monitoring Recommendations

• Enable kernel auditing for cgroup namespace operations and socket system calls
• Monitor for processes attempting to rapidly create and destroy cgroups with associated sockets
• Implement real-time alerting on kernel oops or panic events related to memory subsystems
• Review container orchestration logs for unusual cgroup manipulation patterns

How to Mitigate CVE-2020-25220

Immediate Actions Required

• Update Linux kernel to patched versions: 4.9.233, 4.14.194, or 4.19.140 or later for respective branches
• If immediate patching is not possible, restrict access to cgroup creation for untrusted users
• Review and limit local user access on systems running vulnerable kernel versions
• Consider using kernel live patching solutions to apply fixes without system restart

Patch Information

Official patches have been released for all affected kernel branches. The fix ensures proper handling of the skcd->no_refcnt flag during socket operations within cgroups. Organizations should apply the appropriate patch for their kernel version:

• For kernel 4.9.x: Update to version 4.9.233 or later - see Linux Kernel Changelog 4.9.233
• For kernel 4.14.x: Update to version 4.14.194 or later - see Linux Kernel Changelog 4.14.194
• For kernel 4.19.x: Update to version 4.19.140 or later - see Linux Kernel Changelog 4.19.140

The specific kernel commit addressing this issue can be reviewed at the Linux Kernel Commit 82fd213. Additional advisories are available from Red Hat Bug Report and NetApp Security Advisory ntap-20201001-0004.

Workarounds

• Restrict cgroup creation permissions to trusted users only using appropriate system configuration
• Consider disabling cgroups v1 if not required and migrating to cgroups v2 where applicable
• Implement mandatory access control policies (SELinux, AppArmor) to limit cgroup operations
• Use containerization security features to isolate workloads from untrusted local users

# Restrict cgroup access - example for limiting cgroup creation
# Add to /etc/security/limits.conf or appropriate PAM configuration
# Limit cgroup creation to root and specific trusted groups
chmod 700 /sys/fs/cgroup
chown root:root /sys/fs/cgroup

# Alternatively, use systemd to restrict cgroup controller access
# In /etc/systemd/system.conf, add:
# DefaultCPUAccounting=yes
# DefaultMemoryAccounting=yes