CVE-2025-21927: Linux Kernel Buffer Overflow Vulnerability

CVE-2025-21927 Overview

CVE-2025-21927 is an Out-of-Bounds Write vulnerability in the Linux kernel's NVMe-TCP subsystem. The vulnerability exists in the nvme_tcp_recv_pdu() function, which fails to validate the header length field in incoming packets. When header digests are enabled, a malicious target can send a crafted packet with an invalid header length value (such as 255), causing the nvme_tcp_verify_hdgst() function to access memory outside the allocated buffer and corrupt adjacent memory regions by overwriting them with a calculated digest value.

Critical Impact

This memory corruption vulnerability allows a local attacker with low privileges to potentially achieve high impact on confidentiality, integrity, and availability of affected Linux systems running NVMe over TCP.

Affected Products

• Linux Kernel versions prior to the security fix
• Linux Kernel 6.14-rc1 through 6.14-rc5
• Systems using NVMe-TCP with header digest validation enabled

Discovery Timeline

• 2025-04-01 - CVE-2025-21927 published to NVD
• 2025-10-01 - Last updated in NVD database

Technical Details for CVE-2025-21927

Vulnerability Analysis

The vulnerability is classified as CWE-787 (Out-of-Bounds Write), representing a critical memory safety issue in the kernel's NVMe-TCP implementation. The flaw stems from missing input validation on the Protocol Data Unit (PDU) header length field received from NVMe-TCP targets.

When an NVMe-TCP connection operates with header digests enabled, the kernel calculates and verifies a digest value for each received header. The nvme_tcp_recv_pdu() function processes incoming PDUs but does not enforce bounds checking on the header length value extracted from the packet. A malicious or compromised NVMe-TCP target can exploit this by sending a packet containing an oversized header length value.

When nvme_tcp_verify_hdgst() is subsequently called, it operates on a buffer region determined by the untrusted header length value. If the header length exceeds the actual allocated buffer size (e.g., a value of 255 when a much smaller buffer was allocated), the digest calculation will read beyond the legitimate buffer boundary. The resulting digest value is then written to a location outside the intended memory region, corrupting kernel memory.

Root Cause

The root cause is the absence of validation logic in nvme_tcp_recv_pdu() to verify that the received header length matches the expected size for the specific PDU type. The function trusts the header length value provided in the incoming network packet without comparing it against known valid header lengths or the size of the allocated receive buffer.

Attack Vector

The attack requires local access to the system and low privileges. An attacker who can control or influence an NVMe-TCP target (or position themselves as a man-in-the-middle on the storage network) can craft malicious PDUs with invalid header length values. When the vulnerable kernel receives and processes such packets:

1. The nvme_tcp_recv_pdu() function accepts the PDU with the oversized header length field
2. The kernel proceeds to verify the header digest using the untrusted length value
3. nvme_tcp_verify_hdgst() calculates a digest over memory extending beyond the allocated buffer
4. The calculated digest is written to memory outside the intended buffer region
5. This out-of-bounds write corrupts adjacent kernel memory structures

The memory corruption could potentially be leveraged to achieve privilege escalation, denial of service, or arbitrary code execution depending on what data structures are adjacent to the corrupted buffer.

Detection Methods for CVE-2025-21927

Indicators of Compromise

• Kernel panics or oops messages referencing nvme_tcp_recv_pdu or nvme_tcp_verify_hdgst functions
• Unexpected system crashes on systems utilizing NVMe-TCP storage connections
• Memory corruption artifacts detected in kernel logs or crash dumps
• Abnormal NVMe-TCP traffic patterns with malformed PDU header lengths

Detection Strategies

• Deploy kernel-level monitoring for abnormal memory access patterns in the NVMe-TCP subsystem
• Implement network traffic analysis to identify NVMe-TCP packets with invalid header length values
• Configure crash dump analysis tools to flag kernel panics involving NVMe-TCP functions
• Use SentinelOne's kernel behavior monitoring to detect exploitation attempts

Monitoring Recommendations

• Enable kernel tracing on NVMe-TCP related functions for systems connected to untrusted storage networks
• Monitor system stability metrics for hosts using NVMe over TCP
• Implement alerting on kernel log messages containing nvme_tcp error patterns
• Review network flow data for anomalies in NVMe-TCP traffic characteristics

How to Mitigate CVE-2025-21927

Immediate Actions Required

• Update the Linux kernel to a patched version containing the security fix
• Consider temporarily disabling NVMe-TCP header digest verification if patching is not immediately possible
• Restrict network access between hosts and NVMe-TCP targets to trusted connections only
• Monitor affected systems for signs of exploitation until patches are applied

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability by implementing proper validation of header length values in nvme_tcp_recv_pdu(). Packets with unexpected header lengths are now rejected before digest verification occurs.

The following kernel commits contain the fix:

• Kernel Git Commit 22b06c89
• Kernel Git Commit 9fbc953d
• Kernel Git Commit ad95bab0

Apply the appropriate patch for your kernel version from the stable kernel tree.

Workarounds

• Disable header digest verification on NVMe-TCP connections if the storage network is trusted (reduces security but eliminates the vulnerable code path)
• Implement network-level access controls to prevent untrusted entities from communicating with NVMe-TCP targets
• Use network segmentation to isolate NVMe-TCP traffic from potentially hostile network segments
• Consider migrating critical storage connections to NVMe over RDMA (NVMe-oF RDMA) if feasible as an alternative transport

# Verify if NVMe-TCP is in use on your system
lsmod | grep nvme_tcp

# Check active NVMe-TCP connections
nvme list-subsys | grep tcp

# Review kernel version for patch status
uname -r