CVE-2020-29373: Linux Kernel Path Traversal Vulnerability

CVE-2020-29373 Overview

A path traversal vulnerability was discovered in fs/io_uring.c in the Linux kernel before version 5.6. The vulnerability involves unsafe handling of the root directory during path lookups, which allows a process inside a mount namespace to escape to unintended filesystem locations. This issue is tracked with commit ID ff002b30181d.

Critical Impact

A local attacker with low privileges can exploit this vulnerability to escape mount namespace restrictions and access files outside of intended filesystem boundaries, potentially leading to unauthorized disclosure of sensitive information.

Affected Products

• Linux Kernel versions prior to 5.6

Discovery Timeline

• 2020-11-28 - CVE-2020-29373 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-29373

Vulnerability Analysis

This vulnerability stems from improper handling of filesystem path lookups within the io_uring subsystem of the Linux kernel. The io_uring interface, introduced to provide high-performance asynchronous I/O capabilities, contains a flaw in how it processes path resolution when operating within mount namespaces.

When a process performs file operations through io_uring while confined within a mount namespace, the path lookup mechanism fails to properly respect the namespace boundaries. This allows the process to traverse beyond its intended filesystem root, effectively escaping the isolation provided by the mount namespace.

The vulnerability has local attack vector requirements, meaning an attacker needs local access to the system but does not require user interaction. The scope is changed, indicating that exploitation can affect resources beyond the vulnerable component's security scope. While this vulnerability allows high-impact confidentiality breaches through unauthorized file access, it does not directly enable integrity or availability impacts.

Root Cause

The root cause lies in the fs/io_uring.c implementation which unsafely handles the root directory reference during path lookups. When io_uring processes asynchronous file operations, it fails to properly validate that path resolutions remain within the confines of the current mount namespace's root filesystem. This oversight allows path components to escape the intended namespace boundary through directory traversal techniques.

Attack Vector

This is a local privilege escalation vulnerability that requires an attacker to have local access to a Linux system running a vulnerable kernel version. The attack scenario typically involves:

1. An attacker gains local access to a system as a low-privileged user
2. The attacker creates or operates within a mount namespace (commonly used in container environments)
3. Using io_uring syscalls, the attacker crafts file operation requests with path traversal sequences
4. Due to the unsafe root directory handling, the kernel resolves paths outside the mount namespace boundary
5. The attacker gains read access to files that should be inaccessible from within the namespace

This vulnerability is particularly concerning for containerized environments and sandboxed applications that rely on mount namespace isolation for security boundaries.

Detection Methods for CVE-2020-29373

Indicators of Compromise

• Unusual io_uring syscall activity from containerized or sandboxed processes
• File access attempts to paths outside expected mount namespace boundaries
• Anomalous open/read operations on sensitive system files from processes that should be namespace-restricted
• Audit logs showing file access events that cross namespace boundaries

Detection Strategies

• Monitor io_uring related syscalls (io_uring_setup, io_uring_enter, io_uring_register) for suspicious patterns
• Implement auditd rules to track file access from processes within mount namespaces
• Deploy kernel-level monitoring to detect path resolution anomalies
• Review container escape detection mechanisms for namespace boundary violations

Monitoring Recommendations

• Enable kernel audit logging for io_uring operations
• Implement file integrity monitoring on sensitive directories
• Monitor for unexpected file access patterns from containerized workloads
• Review kernel logs for path resolution errors or warnings

How to Mitigate CVE-2020-29373

Immediate Actions Required

• Upgrade the Linux kernel to version 5.6 or later which contains the security fix
• Review container and sandbox configurations for potential exposure
• Audit systems running vulnerable kernel versions for signs of exploitation
• Consider restricting io_uring syscall access where not required

Patch Information

The vulnerability has been addressed in Linux kernel version 5.6. The fix is contained in commit ff002b30181d30cdfbca316dadd099c3ca0d739c. System administrators should update their kernel to version 5.6 or later to remediate this vulnerability. For detailed patch information, refer to the Linux Kernel Changelog 5.6 and the Linux Kernel Commit ff002b3. Additional technical details can be found in the Chromium Project Zero Issue #2011.

Workarounds

• Disable io_uring functionality using seccomp filters if the feature is not required
• Implement additional access controls on sensitive filesystem paths
• Use AppArmor or SELinux policies to restrict file access from sandboxed processes
• Consider using container runtimes with enhanced isolation mechanisms until patches are applied

# Disable io_uring syscalls via seccomp (example using Docker)
# Add to container security profile
{
  "defaultAction": "SCMP_ACT_ALLOW",
  "syscalls": [
    {
      "names": ["io_uring_setup", "io_uring_enter", "io_uring_register"],
      "action": "SCMP_ACT_ERRNO"
    }
  ]
}