CVE-2024-42256: Linux Kernel Server Re-pick Vulnerability

CVE-2024-42256 Overview

A race condition vulnerability has been identified in the Linux kernel's CIFS (Common Internet File System) subsystem. The flaw occurs during subrequest retry operations where the server selection process is executed multiple times, leading to misaccounting of in-flight request counters. When a subrequest requires retry, cifs_prepare_write() repicks the server before renegotiating credits, then cifs_issue_write() invokes smb2_async_writev() which performs an additional server selection. If different servers are selected between these operations, the server->in_flight counter increments against one server record while decrements occur against another, causing accounting inconsistencies.

Critical Impact

This vulnerability can lead to system instability and kernel warnings, particularly affecting systems using Azure server connections in multichannel mode. The misaccounting of server requests can result in denial of service conditions.

Affected Products

• Linux Kernel (CIFS/SMB2 subsystem)
• Systems using netfslib-driven CIFS operations
• Azure server connections in multichannel mode

Discovery Timeline

• 2024-08-08 - CVE-2024-42256 published to NVD
• 2024-09-06 - Last updated in NVD database

Technical Details for CVE-2024-42256

Vulnerability Analysis

The vulnerability stems from a design issue in the CIFS write operation retry logic within the Linux kernel. The problem arises in the interaction between netfslib and the CIFS subsystem when handling write subrequests that require retry. The flow involves multiple function calls: cifs_prepare_write() initiates server selection and credit renegotiation, followed by cifs_issue_write() which calls smb2_async_writev(). The flaw exists because smb2_async_writev() contains redundant server repick code, creating a scenario where two different servers may be selected within the same operation context.

This results in the server->in_flight counter being incremented on one server structure while being decremented on another during callback processing. The immediate symptom is a kernel warning triggered in smb2_add_credits() at fs/smb/client/smb2ops.c:97. The issue is particularly reproducible during xfstests execution against Azure servers operating in multichannel mode, with tests such as generic/249, generic/215, and generic/308 demonstrating the condition.

Root Cause

The root cause is duplicate server selection logic present in smb2_async_writev() that was unnecessary given that cifs_prepare_write() should always execute first in netfslib-driven operations. This redundant code path allowed for potential server switching mid-operation, violating the assumption that server context should remain consistent throughout a single write operation lifecycle.

Attack Vector

The vulnerability is network-accessible and does not require authentication or user interaction. An attacker capable of manipulating network conditions between a vulnerable Linux system and its CIFS/SMB server could potentially trigger retry conditions that exploit this race condition. The multichannel mode configuration increases attack surface by providing multiple server endpoints that could be selected during the repick process.

The kernel warning manifests as follows:

WARNING: CPU: 4 PID: 72896 at fs/smb/client/smb2ops.c:97 smb2_add_credits+0x3f0/0x9e0 [cifs]
...
RIP: 0010:smb2_add_credits+0x3f0/0x9e0 [cifs]
...
 smb2_writev_callback+0x334/0x560 [cifs]
 cifs_demultiplex_thread+0x77a/0x11b0 [cifs]
 kthread+0x187/0x1d0
 ret_from_fork+0x34/0x60
 ret_from_fork_asm+0x1a/0x30

Detection Methods for CVE-2024-42256

Indicators of Compromise

• Kernel warning messages referencing smb2_add_credits at fs/smb/client/smb2ops.c:97
• Warning stack traces showing smb2_writev_callback and cifs_demultiplex_thread in the call chain
• Unexpected CIFS connection behavior during multichannel operations
• System instability when running workloads against Azure SMB servers

Detection Strategies

• Monitor kernel logs (dmesg, /var/log/kern.log) for CIFS-related warnings containing smb2_add_credits
• Implement alerting on CPU warnings originating from the cifs kernel module
• Deploy kernel tracing using ftrace or eBPF to monitor server->in_flight counter anomalies
• Use SentinelOne Singularity platform to detect kernel-level anomalies and unexpected module behavior

Monitoring Recommendations

• Configure centralized logging to capture kernel warnings across affected systems
• Enable CIFS debug logging temporarily during investigation with echo 1 > /proc/fs/cifs/cifsFYI
• Monitor for performance degradation in SMB/CIFS operations that may indicate retry loops
• Implement network monitoring for unusual SMB multichannel reconnection patterns

How to Mitigate CVE-2024-42256

Immediate Actions Required

• Apply the latest kernel security patches containing the fix for CVE-2024-42256
• Consider temporarily disabling CIFS multichannel mode if experiencing symptoms: echo 0 > /sys/module/cifs/parameters/enable_multichannel
• Monitor affected systems for kernel warnings and stability issues
• Schedule maintenance windows for kernel updates on production systems using CIFS

Patch Information

The fix removes the redundant server repick code in smb2_async_writev(). Since this function is only called from netfslib-driven code where cifs_prepare_write() should always execute first, the server should never be NULL and the preparatory step will be repeated automatically during retry operations.

Official kernel patches are available:

• Kernel.org Commit Reference 1
• Kernel.org Commit Reference 2

Workarounds

• Disable CIFS multichannel mode to reduce the likelihood of triggering the race condition
• Avoid running intensive SMB/CIFS workloads until the kernel is patched
• If using Azure file shares, consider temporary migration to single-channel mode
• Implement network quality improvements to reduce retry frequency

# Disable CIFS multichannel mode temporarily
echo 0 > /sys/module/cifs/parameters/enable_multichannel

# Verify multichannel is disabled
cat /sys/module/cifs/parameters/enable_multichannel

# Re-enable after patching (if desired)
echo 1 > /sys/module/cifs/parameters/enable_multichannel