CVE-2021-4028: Linux Kernel Use-After-Free Vulnerability

CVE-2021-4028 Overview

A use-after-free vulnerability exists in the Linux kernel's implementation of the RDMA (Remote Direct Memory Access) communications manager listener code. This flaw allows an attacker with local access to set up a socket to listen on a high port, causing a list element to be used after it has been freed. Given the ability to execute code, a local attacker could leverage this use-after-free condition to crash the system or potentially escalate privileges.

Critical Impact

Local attackers can exploit this use-after-free vulnerability to cause system crashes (denial of service) or escalate their privileges to gain elevated access on affected Linux systems.

Affected Products

• Linux Kernel (multiple versions)
• SUSE Linux Enterprise 15.0 SP3
• SUSE Linux Enterprise 15.0 SP4

Discovery Timeline

• 2022-08-24 - CVE CVE-2021-4028 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-4028

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption flaw that occurs when a program continues to use a pointer after the memory it references has been freed. In the context of the Linux kernel's RDMA communications manager, the vulnerable code fails to properly manage the lifecycle of list elements associated with listener sockets.

When a local attacker sets up a socket to listen on a high port through the RDMA communications manager interface, a race condition or improper synchronization can lead to a list element being freed while still being referenced elsewhere in the code. Subsequent access to this freed memory can result in undefined behavior, including system crashes or, more critically, the execution of attacker-controlled code in kernel context.

The local attack vector requires the attacker to have existing access to the system, but does not require elevated privileges to trigger the vulnerability. Successful exploitation can result in complete compromise of confidentiality, integrity, and availability of the affected system.

Root Cause

The root cause of this vulnerability lies in improper memory management within the RDMA communications manager listener code. Specifically, the kernel fails to properly synchronize access to list elements when handling listener socket operations. This results in a use-after-free condition where memory is freed prematurely while other parts of the code still hold references to it.

The vulnerability exists because the code path that frees the list element does not properly coordinate with other code paths that may still be accessing or iterating over the same list structure. This type of flaw is common in complex kernel subsystems where multiple execution contexts may access shared data structures.

Attack Vector

The attack requires local access to the affected system. An attacker with unprivileged local access can trigger the vulnerability by:

1. Setting up a socket to listen on a high port using the RDMA communications manager interface
2. Manipulating socket operations to trigger the race condition that leads to the use-after-free
3. Exploiting the freed memory to either crash the system or execute arbitrary code with kernel privileges

The vulnerability is exploited through the RDMA subsystem's socket handling mechanisms. Detailed technical information can be found in the Linux Kernel Mailing List Discussion and the Linux Kernel Commit Update.

Detection Methods for CVE-2021-4028

Indicators of Compromise

• Unexpected kernel panics or system crashes related to RDMA subsystem operations
• Unusual socket activity on high-numbered ports involving RDMA communications
• Kernel log messages indicating memory corruption or use-after-free conditions in the RDMA subsystem
• Signs of privilege escalation or unauthorized kernel-level access following RDMA-related activity

Detection Strategies

• Monitor kernel logs (dmesg, /var/log/kern.log) for RDMA subsystem errors, memory corruption warnings, or use-after-free messages
• Implement kernel integrity monitoring to detect unexpected changes to kernel memory or loaded modules
• Use kernel debugging tools such as KASAN (Kernel Address Sanitizer) in development environments to detect memory safety violations
• Deploy endpoint detection solutions capable of monitoring kernel-level activity and anomalous process behavior

Monitoring Recommendations

• Enable enhanced auditing for RDMA subsystem operations and socket creation events
• Configure alerting for repeated kernel crashes or panics, particularly those involving the RDMA communications manager
• Monitor for privilege escalation attempts following any system instability
• Implement SentinelOne Singularity platform for real-time kernel-level threat detection and behavioral analysis

How to Mitigate CVE-2021-4028

Immediate Actions Required

• Update the Linux kernel to a patched version that addresses this vulnerability
• Review system access controls to minimize the number of users with local access to affected systems
• Monitor affected systems for signs of exploitation attempts
• Consider disabling RDMA functionality if not required for business operations

Patch Information

The vulnerability has been addressed in upstream Linux kernel commits. The specific fix is available in the Linux Kernel Commit Update. System administrators should apply kernel updates from their distribution vendors:

• SUSE: Refer to the SUSE Bug Report #1193167 for patched package versions
• Red Hat: Consult the Red Hat CVE-2021-4028 Advisory for affected products and remediation guidance
• NetApp: Review the NetApp Security Advisory ntap-20221228-0002 for affected NetApp products

Workarounds

• Restrict local access to affected systems to trusted users only
• Disable or unload the RDMA kernel modules (rdma_cm, ib_core) if RDMA functionality is not required
• Implement mandatory access control policies (SELinux, AppArmor) to limit the ability of unprivileged users to interact with RDMA subsystems
• Apply network segmentation to isolate systems that require RDMA functionality

# Disable RDMA kernel modules if not required
sudo modprobe -r rdma_cm
sudo modprobe -r ib_core

# Blacklist RDMA modules to prevent automatic loading
echo "blacklist rdma_cm" | sudo tee -a /etc/modprobe.d/blacklist-rdma.conf
echo "blacklist ib_core" | sudo tee -a /etc/modprobe.d/blacklist-rdma.conf