CVE-2026-31464: Linux Kernel Information Disclosure Flaw

CVE-2026-31464 Overview

CVE-2026-31464 is an Out-of-Bounds Read vulnerability in the Linux kernel's SCSI ibmvfc driver. The vulnerability exists in the ibmvfc_discover_targets_done() function, where a malicious or compromised VIO (Virtual I/O) server can return a num_written value in the discover targets MAD (Management Application Datagram) response that exceeds max_targets. This value is stored directly in vhost->num_targets without validation and is subsequently used as the loop bound in ibmvfc_alloc_targets() to index into disc_buf[], which is only allocated for max_targets entries.

Critical Impact

A malicious VIO server can exploit this vulnerability to access kernel memory outside the DMA-coherent allocation. The out-of-bounds data is subsequently embedded in Implicit Logout and PLOGI MADs sent back to the VIO server, enabling kernel memory information disclosure.

Affected Products

• Linux kernel (ibmvfc SCSI driver)
• IBM Power Systems running PowerVM with affected kernel versions
• Systems utilizing Virtual I/O Server (VIOS) infrastructure

Discovery Timeline

• 2026-04-22 - CVE CVE-2026-31464 published to NVD
• 2026-04-23 - Last updated in NVD database

Technical Details for CVE-2026-31464

Vulnerability Analysis

The vulnerability is rooted in improper input validation within the IBM Virtual Fibre Channel (ibmvfc) SCSI driver. When the driver receives a discover targets MAD response from a VIO server, it stores the num_written field directly into vhost->num_targets without checking whether this value exceeds the expected maximum (max_targets).

This creates a classic out-of-bounds access condition. The disc_buf[] buffer is allocated based on max_targets, but the loop in ibmvfc_alloc_targets() iterates based on the attacker-controlled num_written value. When indices exceed max_targets, the driver accesses kernel memory beyond the DMA-coherent allocation boundary.

The information disclosure aspect is particularly concerning because the out-of-bounds data read from kernel memory is then incorporated into subsequent MAD messages (Implicit Logout and PLOGI) that are transmitted back to the VIO server. This allows an attacker controlling or compromising the VIO server to exfiltrate sensitive kernel memory contents.

Root Cause

The root cause is missing bounds validation on the num_written value received from the VIO server before storing it in vhost->num_targets. The driver trusted this externally-provided value without clamping it to the maximum allocated buffer size (max_targets). This violates the security principle of never trusting input from external sources, even from infrastructure components like VIO servers.

Attack Vector

A threat actor with control over or access to a compromised VIO server can craft a malicious discover targets MAD response containing a num_written value larger than the legitimate max_targets allocation. The attack flow is as follows:

1. The attacker positions themselves on a malicious or compromised VIO server
2. The VIO server responds to a discover targets request with a crafted num_written value exceeding max_targets
3. The kernel stores this unvalidated value and uses it as a loop boundary
4. The subsequent loop accesses memory beyond the disc_buf[] allocation
5. Out-of-bounds kernel memory contents are embedded in outgoing MAD messages
6. The attacker receives the MAD messages containing leaked kernel memory

The fix involves clamping num_written to max_targets before storing it, ensuring the loop boundary never exceeds the allocated buffer size.

Detection Methods for CVE-2026-31464

Indicators of Compromise

• Unusual VIO server behavior or unexpected MAD response patterns
• Anomalous memory access patterns in the ibmvfc driver
• Unexpected kernel memory contents appearing in network traffic to VIO servers
• System logs indicating ibmvfc driver anomalies or SCSI subsystem errors

Detection Strategies

• Monitor kernel logs for ibmvfc driver warnings or errors that may indicate exploitation attempts
• Implement integrity monitoring on VIO server communications
• Deploy kernel-level memory access monitoring for the ibmvfc driver module
• Review SCSI subsystem logs for unusual discover targets operations

Monitoring Recommendations

• Enable verbose logging for the ibmvfc SCSI driver to capture detailed operation logs
• Monitor network traffic between guests and VIO servers for anomalous MAD message sizes
• Implement runtime kernel memory protection mechanisms where available
• Consider deploying kernel live patching solutions for rapid vulnerability mitigation

How to Mitigate CVE-2026-31464

Immediate Actions Required

• Update the Linux kernel to a patched version containing the fix
• Review VIO server configurations and ensure they are running trusted, up-to-date software
• Restrict access to VIO server management interfaces to trusted administrators only
• Monitor systems for any signs of exploitation while awaiting patch deployment

Patch Information

The Linux kernel maintainers have released patches that fix this vulnerability by clamping num_written to max_targets before storing it. Multiple patches are available for different kernel versions:

• Kernel patch 394a1cac3c12
• Kernel patch 4ed727e35b0a
• Kernel patch 61d099ac4a7a
• Kernel patch 786f10b1966e
• Kernel patch a007246cb6c9
• Kernel patch bae4df0a643f
• Kernel patch d1466bf991b2
• Kernel patch d842348f8a00

Workarounds

• If immediate patching is not possible, consider disabling the ibmvfc driver if the functionality is not required
• Implement network segmentation to isolate VIO server communications
• Deploy additional monitoring on systems using the ibmvfc driver until patches can be applied
• Consider using kernel live patching solutions for zero-downtime mitigation

# Check if ibmvfc module is loaded
lsmod | grep ibmvfc

# If not required, temporarily blacklist the module (requires reboot)
echo "blacklist ibmvfc" >> /etc/modprobe.d/blacklist-ibmvfc.conf

# Verify current kernel version
uname -r

# Check for available kernel updates (RHEL/CentOS)
yum check-update kernel

# Check for available kernel updates (Debian/Ubuntu)
apt list --upgradable | grep linux-image