CVE-2021-3923: Linux Kernel Information Disclosure Flaw

CVE-2021-3923 Overview

A flaw was found in the Linux kernel's implementation of RDMA over InfiniBand. An attacker with a privileged local account can leak kernel stack information when issuing commands to the /dev/infiniband/rdma_cm device node. While this access is unlikely to leak sensitive user information, it can be further used to defeat existing kernel protection mechanisms.

Critical Impact

This vulnerability enables privileged local attackers to leak kernel stack information, potentially bypassing kernel security mechanisms like KASLR (Kernel Address Space Layout Randomization).

Affected Products

• Linux Kernel (all versions until patched)
• Red Hat Enterprise Linux 6.0, 7.0, 8.0
• Fedora 37

Discovery Timeline

• 2023-03-27 - CVE-2021-3923 published to NVD
• 2025-02-24 - Last updated in NVD database

Technical Details for CVE-2021-3923

Vulnerability Analysis

This vulnerability is classified as an Information Disclosure flaw (CWE-200) affecting the Linux kernel's RDMA (Remote Direct Memory Access) over InfiniBand implementation. The flaw exists in how the kernel handles commands issued to the /dev/infiniband/rdma_cm device node.

When a privileged local user interacts with this device node, uninitialized kernel stack memory can be leaked to the attacker. While the leaked information may not directly contain sensitive user data, it exposes kernel memory addresses and internal state information that can be leveraged to bypass kernel protection mechanisms.

The attack requires local access with high privileges (root or equivalent), limiting the attack surface. However, in multi-tenant environments or systems where privilege escalation chains are possible, this vulnerability serves as a useful primitive for further exploitation.

Root Cause

The root cause lies in improper handling of kernel stack memory during RDMA connection management operations. The kernel fails to properly initialize or clear memory buffers before returning data to userspace through the /dev/infiniband/rdma_cm interface, resulting in information leakage from the kernel stack.

Attack Vector

This vulnerability requires local access to the system with privileged credentials. The attacker must have the ability to issue commands to the /dev/infiniband/rdma_cm device node. The exploitation is straightforward with low attack complexity once the prerequisites are met.

The attack scenario typically involves:

1. A privileged local attacker gains access to a system with InfiniBand hardware or RDMA capabilities
2. The attacker sends crafted commands to the /dev/infiniband/rdma_cm device
3. Kernel stack information is leaked in the response
4. The attacker uses leaked addresses to defeat KASLR or other kernel protections
5. This information enables more sophisticated kernel exploitation techniques

Detection Methods for CVE-2021-3923

Indicators of Compromise

• Unusual access patterns to /dev/infiniband/rdma_cm device node
• Unexpected processes with elevated privileges interacting with InfiniBand devices
• Anomalous system calls related to RDMA/InfiniBand operations from non-standard applications

Detection Strategies

• Monitor access to /dev/infiniband/* device nodes for unexpected privileged processes
• Implement audit rules for file access to InfiniBand device nodes using auditd
• Review process execution logs for unusual RDMA-related activity
• Use kernel module integrity monitoring to detect exploitation attempts

Monitoring Recommendations

• Enable kernel auditing for device node access with auditctl -w /dev/infiniband/ -p rwxa
• Deploy endpoint detection solutions that monitor kernel-level interactions
• Implement baseline monitoring for InfiniBand device access patterns
• Review authentication logs for privilege escalation attempts that could precede exploitation

How to Mitigate CVE-2021-3923

Immediate Actions Required

• Apply the latest kernel security patches from your distribution vendor
• Restrict access to /dev/infiniband/rdma_cm device node to only essential administrative accounts
• Review and audit which processes require access to InfiniBand devices
• Consider disabling the RDMA/InfiniBand kernel modules if not required for operations

Patch Information

Red Hat has tracked this issue in their bug tracking system. For detailed patch information and updates, refer to the Red Hat Bug Report #2019643. Additional technical discussion regarding the fix can be found in the Kernel Archive Mail Thread.

System administrators should update their Linux kernel to the latest patched version available from their distribution. Check with your vendor for specific patch availability:

• Red Hat Enterprise Linux: Check RHEL security advisories
• Fedora: Update via dnf update kernel
• Upstream Linux Kernel: Apply patches from the kernel mailing list

Workarounds

• Restrict device permissions: chmod 600 /dev/infiniband/rdma_cm
• Unload InfiniBand kernel modules if not needed: modprobe -r rdma_cm ib_core
• Add the modules to a blocklist to prevent automatic loading: add blacklist rdma_cm to /etc/modprobe.d/blacklist.conf
• Implement mandatory access control policies (SELinux/AppArmor) to limit device access

# Configuration example
# Restrict access to InfiniBand device nodes
chmod 600 /dev/infiniband/rdma_cm

# Blacklist RDMA modules if not required
echo "blacklist rdma_cm" >> /etc/modprobe.d/blacklist-rdma.conf
echo "blacklist ib_core" >> /etc/modprobe.d/blacklist-rdma.conf

# Unload modules if currently loaded
modprobe -r rdma_cm
modprobe -r ib_core

# Add audit rule to monitor device access
auditctl -w /dev/infiniband/ -p rwxa -k infiniband_access