CVE-2021-31916: Linux Kernel Privilege Escalation Flaw

CVE-2021-31916 Overview

An out-of-bounds (OOB) memory write vulnerability exists in the list_devices function within drivers/md/dm-ioctl.c in the Multi-device driver module in the Linux kernel before version 5.12. This flaw is caused by a missing bound check that allows an attacker with special user privileges (CAP_SYS_ADMIN) to write to out-of-bounds memory locations, potentially leading to a system crash or leakage of internal kernel information.

Critical Impact

Successful exploitation requires CAP_SYS_ADMIN capability but can result in system crashes (denial of service) or disclosure of sensitive kernel memory contents, threatening system availability and potentially confidentiality.

Affected Products

• Linux Kernel (versions before 5.12)
• Red Hat Enterprise Linux 7.0
• Red Hat Enterprise Linux 8.0
• Debian Linux 9.0

Discovery Timeline

• 2021-05-06 - CVE-2021-31916 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-31916

Vulnerability Analysis

The vulnerability resides in the list_devices function of the Linux kernel's device-mapper ioctl handler (dm-ioctl.c). The device-mapper subsystem provides a generic framework for mapping physical block devices to higher-level virtual block devices, commonly used for LVM, disk encryption, and software RAID implementations.

The flaw occurs due to an insufficient bound check when processing device enumeration requests. The original code only verified that the output buffer length was greater than or equal to the needed size, but failed to ensure the buffer was large enough to hold at least one device entry structure (sizeof(nl->dev)). This oversight allows an attacker with CAP_SYS_ADMIN privileges to trigger an out-of-bounds memory write when the system has no mapped devices configured.

When exploited, this vulnerability can corrupt adjacent kernel memory structures, leading to kernel panics, system instability, or potential information leakage through error conditions that expose kernel memory contents.

Root Cause

The root cause is classified as CWE-787 (Out-of-bounds Write). The vulnerability stems from an incomplete boundary validation in the list_devices function. The code failed to verify that the result buffer was large enough to accommodate the minimum required device structure before attempting to write device information. When no devices are present, the function could still attempt to access or initialize memory beyond the allocated buffer boundaries.

Attack Vector

This is a local attack vector requiring the attacker to have CAP_SYS_ADMIN capability on the target system. An attacker with administrative privileges can invoke device-mapper ioctl calls to trigger the vulnerable code path in the list_devices function. While the privilege requirement limits the attack surface, the vulnerability poses a significant risk in multi-tenant environments, containerized deployments with elevated capabilities, or scenarios where an attacker has already gained partial administrative access.

// Security patch in drivers/md/dm-ioctl.c - dm ioctl: fix out of bounds array access when no devices
// Source: https://github.com/torvalds/linux/commit/4edbe1d7bcffcd6269f3b5eb63f710393ff2ec7a
 	 * Grab our output buffer.
 	 */
 	nl = orig_nl = get_result_buffer(param, param_size, &len);
-	if (len < needed) {
+	if (len < needed || len < sizeof(nl->dev)) {
 		param->flags |= DM_BUFFER_FULL_FLAG;
 		goto out;
 	}

The patch adds an additional check (len < sizeof(nl->dev)) to ensure the buffer is large enough to hold at least one device entry structure, preventing the out-of-bounds access condition.

Detection Methods for CVE-2021-31916

Indicators of Compromise

• Unexpected kernel panics or system crashes originating from the device-mapper subsystem
• Kernel oops messages referencing dm-ioctl.c or list_devices in stack traces
• Unusual ioctl system calls targeting device-mapper interfaces from processes with elevated privileges

Detection Strategies

• Monitor for kernel crash dumps containing references to drivers/md/dm-ioctl.c or the list_devices function
• Implement audit rules for ioctl calls to /dev/mapper/control device nodes
• Use kernel live patching detection tools to verify patch status on running systems

Monitoring Recommendations

• Enable kernel crash dump collection and analysis for forensic investigation
• Monitor system logs for device-mapper related warnings or errors
• Deploy endpoint detection solutions capable of identifying kernel exploitation attempts

How to Mitigate CVE-2021-31916

Immediate Actions Required

• Update the Linux kernel to version 5.12 or later which contains the security fix
• Apply vendor-specific security patches from Red Hat, Debian, or other distribution maintainers
• Review and minimize the number of processes running with CAP_SYS_ADMIN capability

Patch Information

The vulnerability has been addressed in Linux kernel version 5.12 and backported to supported distributions. The fix is available through commit 4edbe1d7bcffcd6269f3b5eb63f710393ff2ec7a in the upstream Linux kernel repository. Distribution-specific patches are available through Red Hat Bug Report #1946965 and Debian security advisories.

Workarounds

• Restrict CAP_SYS_ADMIN capability to only trusted processes and users
• Use kernel security modules (SELinux, AppArmor) to limit access to device-mapper interfaces
• Consider using seccomp filters to restrict ioctl calls in containerized environments

# Configuration example
# Audit device-mapper ioctl access
auditctl -a always,exit -F arch=b64 -S ioctl -F path=/dev/mapper/control -k dm_ioctl_access

# Check current kernel version
uname -r

# Verify if patch is applied (look for the commit in kernel changelog)
grep -r "dm ioctl: fix out of bounds" /usr/share/doc/linux-*/changelog* 2>/dev/null