CVE-2021-3492: Ubuntu Linux Shiftfs DOS Vulnerability

CVE-2021-3492 Overview

CVE-2021-3492 is a memory handling vulnerability in Shiftfs, an out-of-tree stacking file system included in Ubuntu Linux kernels. The vulnerability stems from improper handling of faults occurring during copy_from_user() operations, which can lead to either a double-free condition or memory not being freed at all. An attacker with local access could exploit this vulnerability to cause a denial of service through kernel memory exhaustion or gain elevated privileges by executing arbitrary code.

Critical Impact

Local attackers can achieve privilege escalation or cause denial of service through memory corruption in the Ubuntu Linux kernel's Shiftfs filesystem component.

Affected Products

• Canonical Ubuntu Linux (LTS versions)
• Canonical Ubuntu Linux (standard releases)
• Systems running Ubuntu kernels with Shiftfs enabled

Discovery Timeline

• 2021-04-17 - CVE-2021-3492 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-3492

Vulnerability Analysis

This vulnerability exists in the Shiftfs stacking file system, which is an Ubuntu-specific kernel extension designed to provide UID/GID shifting capabilities for container workloads. The flaw occurs when the copy_from_user() function encounters a fault during data transfer from user space to kernel space. Under normal conditions, the kernel should handle such faults gracefully by properly managing allocated memory. However, the Shiftfs implementation failed to correctly handle these error conditions.

When a fault occurs during copy_from_user(), the code path can result in two distinct memory corruption scenarios: either memory is freed twice (double-free) or memory that should be freed is never released (memory leak). Both conditions can be triggered by a local attacker with low privileges.

The double-free condition is particularly dangerous as it can lead to use-after-free scenarios, potentially allowing an attacker to corrupt kernel memory structures and execute arbitrary code with kernel privileges. The memory leak scenario, while less immediately severe, can be repeatedly triggered to exhaust kernel memory resources, resulting in system instability or denial of service.

This vulnerability is tracked by the Zero Day Initiative as ZDI-CAN-13562.

Root Cause

The root cause of CVE-2021-3492 lies in improper error handling within the Shiftfs file system code. When copy_from_user() fails to complete successfully due to a page fault or invalid memory access, the error handling logic does not properly track the state of allocated memory buffers. This results in either attempting to free the same memory region multiple times or losing references to allocated memory entirely.

The vulnerability is classified under CWE-401 (Missing Release of Memory after Effective Lifetime), though the double-free aspect could also be categorized as CWE-415.

Attack Vector

The attack vector is local, requiring an attacker to have low-privileged access to a vulnerable Ubuntu Linux system. The attacker can exploit this vulnerability by:

1. Creating conditions that trigger the vulnerable Shiftfs code path
2. Causing copy_from_user() to fault during data transfer operations
3. Leveraging the resulting memory corruption for privilege escalation or denial of service

The exploitation mechanism involves manipulating memory states to either trigger the double-free for code execution or repeatedly cause memory leaks to exhaust system resources.

The vulnerability can be exploited by crafting specific operations against the Shiftfs file system that cause faults during user-to-kernel memory copy operations. When copy_from_user() fails, the improper error handling leads to memory corruption conditions that an attacker can leverage for privilege escalation or system denial of service. For detailed technical analysis, refer to the Zero Day Initiative Advisory and the OpenWall security discussion.

Detection Methods for CVE-2021-3492

Indicators of Compromise

• Unexpected kernel memory consumption or system memory exhaustion
• Kernel panic or system crashes related to memory allocation failures
• Suspicious activity involving Shiftfs mount operations or container workloads
• Kernel oops messages referencing Shiftfs or copy_from_user() failures

Detection Strategies

• Monitor kernel logs for Shiftfs-related errors or memory corruption warnings
• Implement auditd rules to track Shiftfs mount and unmount operations
• Deploy kernel memory integrity monitoring to detect double-free exploitation attempts
• Use SentinelOne's behavioral AI to identify privilege escalation attempts following Shiftfs operations

Monitoring Recommendations

• Enable kernel memory debugging options such as KASAN (Kernel Address Sanitizer) on development systems
• Configure memory usage alerts to detect gradual memory exhaustion attacks
• Monitor for unusual process privilege changes that may indicate successful exploitation
• Implement container workload monitoring for abnormal Shiftfs usage patterns

How to Mitigate CVE-2021-3492

Immediate Actions Required

• Apply the security updates provided in Ubuntu Security Notice USN-4917-1
• If immediate patching is not possible, consider disabling Shiftfs if not required for operations
• Restrict local system access to trusted users only
• Monitor systems for signs of exploitation attempts

Patch Information

Ubuntu has released security patches to address this vulnerability. The fixes are available through the following kernel commits:

• Kernel commit 25c891a949bf918b59cbc6e4932015ba4c35c333
• Kernel commit 8fee52ab9da87d82bc6de9ebb3480fff9b4d53e6

For systems using Canonical's Livepatch service, a kernel live patch is available as documented in the Kernel Live Patch Security Notice LSN-0077-1.

Workarounds

• Disable Shiftfs kernel module if not required: sudo modprobe -r shiftfs
• Blacklist the Shiftfs module to prevent automatic loading
• Implement strict container runtime configurations that do not rely on Shiftfs
• Limit local user access and enforce principle of least privilege

# Disable and blacklist Shiftfs module
sudo modprobe -r shiftfs
echo "blacklist shiftfs" | sudo tee /etc/modprobe.d/blacklist-shiftfs.conf
sudo update-initramfs -u

# Verify module is not loaded
lsmod | grep shiftfs