CVE-2025-21680: Linux Kernel Buffer Overflow Vulnerability

CVE-2025-21680 Overview

CVE-2025-21680 is a high-severity out-of-bounds array access vulnerability in the Linux kernel's pktgen (packet generator) module. The vulnerability exists in the get_imix_entries function within net/core/pktgen.c, where an incorrect boundary check allows a local attacker to pass a sufficient number of imix entries that leads to invalid access beyond the bounds of the pkt_dev->imix_entries array. This flaw was discovered by the Linux Verification Center (linuxtesting.org) using the SVACE static analysis tool.

Critical Impact

Local attackers with low privileges can exploit this out-of-bounds access vulnerability to potentially achieve high impact on confidentiality, integrity, and availability of the affected system.

Affected Products

• Linux Kernel versions prior to patched releases
• Linux Kernel 6.13-rc1 through 6.13-rc7
• Systems running vulnerable kernel versions with the pktgen module enabled

Discovery Timeline

• 2025-01-31 - CVE-2025-21680 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-21680

Vulnerability Analysis

The vulnerability resides in the get_imix_entries function in the Linux kernel's packet generator (pktgen) networking subsystem. The pkt_dev->imix_entries array is defined as type imix_pkt [20], meaning it can hold a maximum of 20 entries. However, due to an incorrect boundary check in the code, it is possible to write beyond index 19 (the last valid index) when processing user-supplied imix entries.

When a user writes configuration data through the procfs interface (/proc/net/pktgen/), the pktgen_if_write function is invoked, which subsequently calls get_imix_entries. The UBSAN (Undefined Behavior Sanitizer) runtime detects when index 20 is accessed, which is out of range for the 20-element array. This out-of-bounds access can lead to memory corruption, potentially allowing local privilege escalation or denial of service conditions.

Root Cause

The root cause is an improper validation of array index (CWE-129) in the get_imix_entries function. The boundary check fails to properly limit the number of entries that can be processed, allowing the array index to exceed the maximum valid index of 19 for the 20-element imix_pkt array. The fix corrects this boundary check to allow the array to be filled completely while preventing any out-of-bounds access.

Attack Vector

The attack requires local access to the system with the ability to write to the pktgen procfs interface. An attacker would need to craft a malicious imix configuration with more than 20 entries and write it to the /proc/net/pktgen/ interface. The attack flow is as follows:

1. Attacker gains local access to a system with the pktgen module loaded
2. Attacker writes a crafted imix configuration through the procfs interface at /proc/net/pktgen/
3. The pktgen_if_write function processes the input and calls get_imix_entries
4. Due to incorrect boundary checking, the function accesses memory beyond the imix_entries array bounds
5. This out-of-bounds access can corrupt kernel memory, potentially leading to privilege escalation or system crash

The call trace from the UBSAN report shows the exploitation path through vfs_write → proc_reg_write → pde_write → pktgen_if_write → get_imix_entries, where the out-of-bounds access at line 874 of pktgen.c occurs.

Detection Methods for CVE-2025-21680

Indicators of Compromise

• UBSAN kernel log messages indicating array-index-out-of-bounds errors in net/core/pktgen.c
• Kernel oops or crashes originating from the pktgen module
• Unexpected writes to /proc/net/pktgen/ interfaces from unprivileged processes

Detection Strategies

• Monitor kernel logs for UBSAN warnings containing "array-index-out-of-bounds" and references to pktgen.c
• Implement auditd rules to track access to /proc/net/pktgen/ directories
• Deploy endpoint detection to identify anomalous procfs write patterns to pktgen interfaces

Monitoring Recommendations

• Enable UBSAN in kernel builds to detect exploitation attempts that trigger out-of-bounds access
• Monitor for unusual process activity involving pktgen module interactions
• Implement file integrity monitoring on critical kernel module configurations

How to Mitigate CVE-2025-21680

Immediate Actions Required

• Update the Linux kernel to a patched version that includes the boundary check fix
• If immediate patching is not possible, consider disabling or unloading the pktgen module if it is not required
• Restrict access to /proc/net/pktgen/ to only trusted administrative accounts
• Review and audit systems for signs of exploitation attempts

Patch Information

Multiple kernel patches have been released to address this vulnerability. The fix corrects the boundary check in get_imix_entries to properly validate array indices before access. Organizations should apply the appropriate patch for their kernel version:

• Kernel Commit 1a9b65c
• Kernel Commit 3450092c
• Kernel Commit 76201b59
• Kernel Commit 7cde21f5
• Kernel Commit e5d24a70

Debian users should refer to the Debian LTS Announcement for distribution-specific patch information.

Workarounds

• Unload the pktgen kernel module using rmmod pktgen if the packet generator functionality is not required
• Blacklist the pktgen module to prevent automatic loading by adding blacklist pktgen to /etc/modprobe.d/blacklist.conf
• Implement strict access controls on the /proc/net/pktgen/ directory to prevent unauthorized writes

# Configuration example - Blacklist pktgen module
echo "blacklist pktgen" >> /etc/modprobe.d/blacklist.conf

# Unload the module if currently loaded
rmmod pktgen 2>/dev/null || true

# Update initramfs to persist changes
update-initramfs -u