CVE-2020-11565: Linux Kernel Privilege Escalation Flaw

CVE-2020-11565 Overview

CVE-2020-11565 is a stack-based out-of-bounds write vulnerability discovered in the Linux kernel through version 5.6.2. The flaw exists in the mpol_parse_str function within mm/mempolicy.c, where an empty nodelist is mishandled during mount option parsing. This vulnerability is tracked as CID-aa9f7d5172fa in the kernel source.

It is important to note that there is debate within the security community regarding the severity of this vulnerability. Some researchers argue that because the bug is triggered through mount options that can only be specified by a privileged user, exploiting the vulnerability does not grant any powers not already held by the attacker. Nevertheless, memory corruption vulnerabilities in the kernel space remain a concern for defense-in-depth security postures.

Critical Impact

A privileged local attacker could exploit this stack-based out-of-bounds write vulnerability to potentially corrupt kernel memory, leading to system instability, denial of service, or potentially arbitrary code execution in kernel context.

Affected Products

• Linux Kernel through version 5.6.2
• Canonical Ubuntu Linux 14.04 ESM
• Canonical Ubuntu Linux 16.04 LTS
• Canonical Ubuntu Linux 18.04 LTS
• Canonical Ubuntu Linux 20.04 LTS

Discovery Timeline

• April 6, 2020 - CVE-2020-11565 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-11565

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-bounds Write), a memory corruption flaw that occurs when data is written beyond the boundaries of an allocated buffer. In this case, the mpol_parse_str function in the Linux kernel's memory policy subsystem fails to properly handle empty nodelists when parsing NUMA memory policy mount options.

The vulnerability requires local access and high privileges to exploit. When a privileged user specifies malformed mount options containing an empty nodelist, the parsing logic in mpol_parse_str can write data beyond the allocated stack buffer. This can corrupt adjacent stack variables, return addresses, or other critical data structures.

While the attack requires elevated privileges, the integrity and availability impact is significant—successful exploitation could allow an attacker to corrupt kernel memory structures or cause system crashes. The vulnerability does not directly expose confidential information.

Root Cause

The root cause of CVE-2020-11565 lies in insufficient input validation within the mpol_parse_str function. When parsing NUMA policy mount options, the function expects a valid nodelist specification. However, when an empty nodelist string is provided, the parsing logic fails to properly validate this edge case before performing memory operations.

The missing bounds check allows the function to write data to stack memory locations that were not intended to be modified, resulting in a classic stack-based out-of-bounds write condition. The fix (commit aa9f7d5172fac9bf1f09e678c35e287a40a7b7dd) adds proper validation to handle empty nodelists gracefully.

Attack Vector

The attack vector for CVE-2020-11565 is local, requiring an attacker to have privileged access to the system. The exploitation path involves:

1. The attacker must have the ability to mount filesystems with custom mount options (typically requiring root or CAP_SYS_ADMIN capabilities)
2. The attacker crafts a mount command with malformed NUMA memory policy options containing an empty nodelist
3. When the kernel parses these mount options through mpol_parse_str, the empty nodelist triggers the out-of-bounds write
4. The corrupted stack memory could potentially be leveraged for further exploitation or cause a kernel panic

The vulnerability manifests during mount option parsing when an empty nodelist is encountered. The mpol_parse_str function attempts to process the nodelist without properly validating that it contains valid data, leading to memory corruption on the stack. For detailed technical analysis of the vulnerable code path and the applied fix, see the Linux Kernel Commit.

Detection Methods for CVE-2020-11565

Indicators of Compromise

• Unexpected kernel panics or oops messages referencing mpol_parse_str or memory policy functions
• Suspicious mount operations with unusual or malformed NUMA policy options in system logs
• Memory corruption artifacts in kernel space detected through crash analysis tools
• Anomalous privileged user activity involving filesystem mount operations with non-standard options

Detection Strategies

• Monitor kernel logs (dmesg, /var/log/kern.log) for stack corruption warnings or crashes related to memory policy code paths
• Implement audit rules to track mount system calls with custom options, especially those involving NUMA policies
• Deploy kernel crash analysis tools to identify exploitation attempts that result in system instability
• Use SentinelOne's behavioral AI engine to detect anomalous privileged operations that may indicate exploitation attempts

Monitoring Recommendations

• Enable kernel auditing for mount operations: auditctl -a always,exit -F arch=b64 -S mount -k numa_mount
• Configure alerting for kernel oops or panic events that reference memory management subsystems
• Implement file integrity monitoring on critical kernel modules and boot configurations
• Review system logs for patterns of unusual mount activity from privileged processes

How to Mitigate CVE-2020-11565

Immediate Actions Required

• Update the Linux kernel to a patched version that includes commit aa9f7d5172fac9bf1f09e678c35e287a40a7b7dd
• Apply vendor-specific security updates from Canonical, Debian, or your Linux distribution
• Audit privileged user accounts and ensure principle of least privilege is enforced
• Review and restrict access to filesystem mounting capabilities where possible

Patch Information

The vulnerability has been addressed in the upstream Linux kernel. The fix is available in commit aa9f7d5172fac9bf1f09e678c35e287a40a7b7dd. Multiple Linux distributions have released security updates:

• Ubuntu: Security notices USN-4363-1, USN-4364-1, USN-4367-1, USN-4368-1, USN-4369-1
• Debian: Security advisories DSA-4667, DSA-4698, and LTS announcements

Workarounds

• Restrict mount capabilities to only essential privileged users and processes
• Implement mandatory access control (SELinux, AppArmor) to limit which processes can invoke mount operations
• Monitor and alert on any mount operations using NUMA memory policy options
• Consider containerization to isolate workloads and limit kernel attack surface exposure

# Configuration example
# Audit mount operations with NUMA policy options
auditctl -a always,exit -F arch=b64 -S mount -k numa_policy_mount

# Restrict mount capabilities using Linux capabilities
# Remove CAP_SYS_ADMIN from non-essential processes
setcap -r /path/to/binary

# Check current kernel version for patch status
uname -r
# Compare against patched versions in your distribution's security advisories