CVE-2020-27786: Linux Kernel Use-After-Free Vulnerability

CVE-2020-27786 Overview

A use-after-free vulnerability was discovered in the Linux kernel's implementation of MIDI (Musical Instrument Digital Interface). An attacker with a local account and permissions to issue ioctl commands to MIDI devices can trigger this memory corruption flaw. A write to specific memory after it has been freed and before subsequent use causes the flow of execution to change, potentially allowing for memory corruption or privilege escalation.

Critical Impact

This vulnerability allows local attackers to potentially escalate privileges or corrupt memory, threatening system confidentiality, integrity, and availability.

Affected Products

• Linux Kernel (multiple versions)
• Red Hat OpenShift Container Platform 4.4, 4.5, 4.6
• Red Hat Enterprise Linux 7.0, 8.0
• Red Hat Enterprise MRG 2.0
• NetApp Cloud Backup
• NetApp SolidFire Baseboard Management Controller

Discovery Timeline

• 2020-12-03 - Vulnerability disclosed on Openwall OSS-Security Mailing List
• 2020-12-11 - CVE CVE-2020-27786 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-27786

Vulnerability Analysis

This use-after-free vulnerability (CWE-416) exists in the Linux kernel's MIDI subsystem. The flaw occurs when memory is freed but subsequently referenced and written to before being reallocated. This temporal memory safety violation can be triggered by a local attacker who has permissions to issue ioctl commands to MIDI devices on the system.

The vulnerability presents a significant risk because it affects the kernel itself, which operates at the highest privilege level in the system. Successful exploitation could allow an attacker to modify kernel memory, potentially leading to arbitrary code execution with kernel privileges.

Root Cause

The root cause is improper memory management in the kernel's MIDI implementation. Specifically, memory associated with MIDI operations is freed but the pointer to that memory is not properly invalidated. When the code subsequently attempts to write to this location, it accesses memory that may have been reallocated for other purposes, leading to corruption of unrelated data structures or enabling attacker-controlled memory manipulation.

Attack Vector

The attack requires local access to the system and permissions to interact with MIDI devices via ioctl commands. An attacker must:

1. Gain local access to a system with MIDI device support
2. Have sufficient privileges to issue ioctl commands to MIDI devices
3. Trigger specific sequences of operations that cause memory to be freed
4. Cause a write operation to the freed memory before reallocation

The local attack vector means remote exploitation is not directly possible, though this vulnerability could be chained with other flaws to achieve full system compromise. The exploitation does not require user interaction once the attacker has local access.

Detection Methods for CVE-2020-27786

Indicators of Compromise

• Unexpected kernel crashes or system instability related to MIDI subsystem operations
• Suspicious ioctl calls to MIDI device files (e.g., /dev/snd/midi*, /dev/midi*)
• Anomalous memory access patterns in kernel logs potentially indicating use-after-free conditions
• Privilege escalation attempts following MIDI device interactions

Detection Strategies

• Monitor for unusual ioctl system calls targeting MIDI devices using auditd or similar kernel auditing tools
• Deploy kernel-level integrity monitoring to detect unauthorized memory modifications
• Implement SentinelOne Singularity Platform for real-time behavioral detection of privilege escalation attempts
• Review kernel logs for signs of memory corruption such as kernel panics with MIDI-related stack traces

Monitoring Recommendations

• Enable kernel auditing for device-related system calls, particularly ioctl operations
• Configure alerts for unexpected MIDI device access by non-standard processes
• Monitor system stability metrics for signs of kernel memory corruption
• Use SentinelOne's kernel-level visibility to track process behavior and privilege changes

How to Mitigate CVE-2020-27786

Immediate Actions Required

• Apply kernel patches from your distribution vendor immediately
• Restrict access to MIDI devices by limiting permissions on device files
• Review system accounts with access to sound/MIDI devices and remove unnecessary privileges
• Consider disabling MIDI support if not required for system operations

Patch Information

The vulnerability has been addressed in the upstream Linux kernel. The fix is available in the Linux Kernel Commit c1f6e3c. Red Hat has tracked this issue in Bug Report #1900933. NetApp has released guidance in their Security Advisory ntap-20210122-0002.

Organizations should update their Linux kernel to the patched version provided by their distribution. For Red Hat Enterprise Linux and OpenShift Container Platform users, follow Red Hat's security advisory guidance for applying the appropriate updates.

Workarounds

• Restrict access to MIDI device files using file system permissions (e.g., chmod 600 /dev/snd/midi*)
• Use Linux Security Modules (SELinux, AppArmor) to confine access to MIDI devices
• Remove or blacklist MIDI-related kernel modules if MIDI functionality is not required (e.g., blacklist snd_rawmidi in /etc/modprobe.d/)
• Implement process sandboxing for applications that require MIDI access

# Restrict MIDI device permissions
chmod 600 /dev/snd/midi*
chmod 600 /dev/midi*

# Blacklist MIDI kernel module if not needed
echo "blacklist snd_rawmidi" >> /etc/modprobe.d/blacklist-midi.conf

# Verify module is not loaded
lsmod | grep midi