CVE-2020-36557 Overview
A race condition vulnerability exists in the Linux kernel before version 5.6.2, occurring between the VT_DISALLOCATE ioctl and the closing/opening of ttys. This timing-dependent flaw can result in a use-after-free condition, potentially leading to denial of service on affected systems.
Critical Impact
Local attackers can exploit this race condition to cause system instability or denial of service by triggering a use-after-free condition in the kernel's virtual terminal subsystem.
Affected Products
- Linux Kernel versions prior to 5.6.2
- linux linux_kernel
Discovery Timeline
- 2022-07-21 - CVE CVE-2020-36557 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2020-36557
Vulnerability Analysis
This vulnerability is classified as CWE-362 (Race Condition), specifically a Time-of-Check Time-of-Use (TOCTOU) issue within the Linux kernel's virtual terminal (VT) subsystem. The race condition occurs when the VT_DISALLOCATE ioctl is invoked concurrently with tty open or close operations.
The vulnerability requires local access to exploit, as an attacker must be able to make ioctl system calls to the virtual terminal subsystem. While the attack complexity is high due to the timing-sensitive nature of race conditions, successful exploitation does not require any privileges or user interaction.
When exploited, this vulnerability can cause a use-after-free condition where the kernel attempts to access memory that has already been freed. This type of memory corruption can lead to system crashes and denial of service conditions, impacting system availability.
Root Cause
The root cause lies in improper synchronization between the VT_DISALLOCATE ioctl handler and the tty open/close operations. When these operations execute concurrently without proper locking mechanisms, a timing window exists where one operation can free memory structures that another operation is still referencing. This lack of proper mutual exclusion between the ioctl and tty operations creates the race condition that leads to the use-after-free vulnerability.
Attack Vector
The attack vector is local, requiring the attacker to have access to the system to execute code that interacts with the virtual terminal subsystem. The exploitation involves:
- An attacker initiates the VT_DISALLOCATE ioctl to deallocate a virtual terminal
- Simultaneously, the attacker triggers tty open or close operations on the same terminal
- Due to missing synchronization, the kernel may access freed memory structures
- This results in a use-after-free condition that can crash the system
The vulnerability mechanism involves concurrent access to shared kernel data structures without proper locking. The fix implemented in commit ca4463bf8438b403596edd0ec961ca0d4fbe0220 addresses this by ensuring proper synchronization between these operations. For technical details, refer to the Linux Commit ca4463b Analysis.
Detection Methods for CVE-2020-36557
Indicators of Compromise
- Unexpected kernel panics or oops messages related to the VT subsystem or tty drivers
- System crashes occurring during virtual terminal operations or when switching between terminals
- Kernel log entries indicating use-after-free or invalid memory access in tty-related code paths
Detection Strategies
- Monitor kernel logs for panic traces involving vt_disallocate, tty operations, or console drivers
- Deploy kernel debugging tools such as KASAN (Kernel Address Sanitizer) to detect use-after-free conditions in development or testing environments
- Implement system monitoring to track unusual patterns of virtual terminal allocation and deallocation
Monitoring Recommendations
- Configure kernel crash dump collection (kdump) to capture detailed information when system crashes occur
- Enable audit logging for ioctl system calls to detect potential exploitation attempts
- Monitor system stability metrics and track any increase in unexpected system reboots or crashes
How to Mitigate CVE-2020-36557
Immediate Actions Required
- Upgrade the Linux kernel to version 5.6.2 or later to receive the security fix
- Review and apply all available security patches for your Linux distribution
- If immediate patching is not possible, evaluate restricting access to virtual terminal operations for non-essential users
Patch Information
The vulnerability has been addressed in Linux kernel version 5.6.2. The fix is documented in the Linux Kernel ChangeLog 5.6.2. The specific commit that resolves this issue is ca4463bf8438b403596edd0ec961ca0d4fbe0220, which implements proper synchronization between the VT_DISALLOCATE ioctl and tty operations.
Organizations should prioritize updating to a patched kernel version. Most major Linux distributions have backported this fix to their supported kernel versions.
Workarounds
- Restrict local access to the system to trusted users only, as exploitation requires local access
- Limit capabilities for users who do not require direct access to virtual terminal operations
- Monitor systems for unusual activity related to virtual terminal operations until patching can be completed
# Check current kernel version
uname -r
# Verify if running a patched kernel (5.6.2 or later)
# Compare output against vulnerable versions
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
