CVE-2020-29661: Linux Kernel Use-After-Free Vulnerability

CVE-2020-29661 Overview

A use-after-free vulnerability was discovered in the TTY subsystem of the Linux kernel through version 5.9.13. The flaw exists in drivers/tty/tty_jobctrl.c and is related to improper locking when handling the TIOCSPGRP ioctl command. This vulnerability allows a local attacker with low privileges to potentially execute arbitrary code or cause system instability by exploiting the race condition in terminal job control handling.

Critical Impact

Local attackers can exploit this use-after-free vulnerability to achieve privilege escalation, execute arbitrary code with kernel privileges, or cause denial of service conditions on affected Linux systems.

Affected Products

• Linux Kernel (versions through 5.9.13)
• Fedora 32 and 33
• Debian Linux 9.0 and 10.0
• NetApp Active IQ Unified Manager for VMware vSphere
• Broadcom Fabric Operating System
• NetApp SolidFire Baseboard Management Controller
• NetApp H410C, A700S, 8300, 8700, A400 Storage Systems
• Oracle Tekelec Platform Distribution

Discovery Timeline

• December 9, 2020 - CVE-2020-29661 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-29661

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption class that occurs when a program continues to reference memory after it has been freed. In the context of CVE-2020-29661, the Linux kernel's TTY job control subsystem fails to properly synchronize access to TTY structures during TIOCSPGRP ioctl operations.

The TIOCSPGRP ioctl is used to set the foreground process group of a terminal. When this operation is performed concurrently with other TTY operations, a race condition can occur where the TTY structure is freed while still being accessed. This creates an opportunity for attackers to manipulate the freed memory region and potentially gain control of kernel execution flow.

Successful exploitation requires local access to the system and the ability to interact with terminal devices. An attacker could leverage this vulnerability to escalate privileges from a low-privileged user account to root, potentially compromising the entire system.

Root Cause

The root cause of this vulnerability lies in inadequate locking mechanisms within the tty_jobctrl.c file. Specifically, the code path handling TIOCSPGRP did not properly acquire necessary locks before accessing TTY data structures. This allowed concurrent operations to free the TTY structure while the TIOCSPGRP handler was still using it, resulting in a use-after-free condition.

The kernel commit 54ffccbf053b5b6ca4f6e45094b942fab92a25fc addresses this issue by implementing proper locking to prevent the race condition.

Attack Vector

The attack requires local access to the system and the ability to open and interact with TTY devices. The exploitation scenario involves:

1. A local attacker opens a pseudo-terminal or accesses a TTY device
2. The attacker triggers concurrent operations on the TTY, including TIOCSPGRP ioctl calls
3. By carefully timing these operations, the attacker can trigger the race condition
4. Once the use-after-free condition is achieved, the attacker can manipulate freed memory to gain kernel code execution

The vulnerability can be exploited through crafted ioctl calls that race against TTY cleanup operations. Technical details about the broken locking mechanism are available in the Packet Storm Security advisory.

Detection Methods for CVE-2020-29661

Indicators of Compromise

• Unusual kernel crashes or system instability related to TTY operations
• Unexpected privilege escalation events from low-privileged user accounts
• Kernel oops messages referencing tty_jobctrl.c or related TTY subsystem functions
• Anomalous process creation with elevated privileges following TTY interactions

Detection Strategies

• Monitor kernel logs for use-after-free warnings or memory corruption indicators in TTY-related code paths
• Deploy kernel auditing to track TIOCSPGRP ioctl calls on systems where exploitation is suspected
• Implement runtime kernel integrity monitoring solutions to detect unauthorized code execution
• Use Linux Audit Framework to log all ioctl operations on TTY devices

Monitoring Recommendations

• Enable kernel crash dump collection (kdump) to capture forensic evidence of exploitation attempts
• Configure auditd rules to monitor /dev/tty* and /dev/pts/* device access patterns
• Implement process behavior monitoring to detect unexpected privilege transitions
• Review system logs for suspicious terminal-related operations, particularly from non-interactive sessions

How to Mitigate CVE-2020-29661

Immediate Actions Required

• Update Linux kernel to a patched version (post-5.9.13) that includes the fix
• Apply vendor-specific security patches from Debian, Fedora, NetApp, Oracle, or Broadcom as applicable
• Restrict access to TTY devices for untrusted users where possible
• Consider implementing kernel live patching solutions for systems that cannot be immediately rebooted

Patch Information

The vulnerability has been addressed in the Linux kernel through commit 54ffccbf053b5b6ca4f6e45094b942fab92a25fc. This patch implements proper locking in the tty_jobctrl.c file to prevent the race condition that enables the use-after-free attack.

Vendor-specific patches are available:

• Debian Security Advisory DSA-4843
• Fedora Package Announcements
• NetApp Security Advisory
• Oracle Security Alert October 2021

For systems using kernel live patching, refer to the Kernel Live Patch Security Notice LSN-0082-1.

Workarounds

• Limit local user access to systems running vulnerable kernel versions
• Implement strict user account controls and principle of least privilege
• Use SELinux or AppArmor policies to restrict TTY device access for non-essential processes
• Monitor and audit all local user activity on systems that cannot be immediately patched

# Check current kernel version
uname -r

# For Debian/Ubuntu systems, update kernel
sudo apt update && sudo apt upgrade linux-image-$(uname -r)

# For RHEL/CentOS/Fedora systems
sudo dnf update kernel

# Enable audit logging for TTY ioctl operations
sudo auditctl -a always,exit -F arch=b64 -S ioctl -F path=/dev/tty -k tty_monitoring