CVE-2025-68763: Linux Kernel Buffer Overflow Vulnerability

CVE-2025-68763 Overview

A vulnerability has been identified in the Linux kernel's StarFive cryptographic driver where improper handling of the sg_nents_for_len return value can lead to integer conversion errors. The function's return value was incorrectly assigned to an unsigned long variable in starfive_hash_digest, causing negative error codes to be converted to large positive integers, potentially leading to buffer overflow conditions.

Critical Impact

Improper integer type handling in the StarFive crypto driver can convert error codes to large positive values, potentially causing buffer overflows during cryptographic operations.

Affected Products

• Linux Kernel (StarFive crypto driver)
• Systems using StarFive hardware cryptographic acceleration
• RISC-V platforms with StarFive SoCs

Discovery Timeline

• 2026-01-05 - CVE CVE-2025-68763 published to NVD
• 2026-01-08 - Last updated in NVD database

Technical Details for CVE-2025-68763

Vulnerability Analysis

This vulnerability represents a classic integer conversion error in the Linux kernel's cryptographic subsystem. The sg_nents_for_len function returns a signed integer that can be negative when an error occurs. In the vulnerable code path within starfive_hash_digest, this return value was assigned to an unsigned long variable without proper error checking.

When sg_nents_for_len returns a negative error code (such as -EINVAL or -ENOMEM), the assignment to an unsigned type causes the value to wrap around to a very large positive integer. This corrupted value is then used to calculate buffer sizes or iteration counts in scatter-gather list processing, potentially leading to out-of-bounds memory access.

The impact of this vulnerability is primarily on systems using StarFive RISC-V SoCs with hardware cryptographic acceleration enabled. An attacker with the ability to trigger cryptographic operations with malformed input could potentially exploit this to cause memory corruption or denial of service.

Root Cause

The root cause is an improper variable type declaration combined with missing error checking. The sg_nents_for_len function is designed to return either a positive count of scatter-gather entries or a negative error code. By storing this value in an unsigned long variable, the code path lost the ability to distinguish between successful returns and error conditions, leading to undefined behavior when errors occurred.

Attack Vector

The attack vector requires local access to trigger cryptographic operations through the StarFive driver. An attacker would need to craft input that causes sg_nents_for_len to fail (return a negative value), which when interpreted as a large unsigned value, could cause the driver to process memory beyond valid buffer boundaries.

The fix adds proper error checking for sg_nents_for_len and returns immediately on failure, preventing the integer conversion issue from being exploited. The patches available in the kernel git repository correct the variable type and add appropriate validation logic.

Detection Methods for CVE-2025-68763

Indicators of Compromise

• Unexpected kernel panics or crashes related to the StarFive crypto driver
• Memory corruption errors in kernel logs referencing starfive_hash_digest or related functions
• System instability during cryptographic operations on StarFive-based platforms

Detection Strategies

• Monitor kernel logs for oops or panic messages involving the starfive-cryp driver module
• Implement kernel module integrity monitoring to detect tampering with crypto drivers
• Use kernel address sanitizer (KASAN) during development/testing to catch out-of-bounds access

Monitoring Recommendations

• Enable kernel crash dump collection to capture diagnostic information if exploitation is attempted
• Monitor for unusual patterns in cryptographic API usage that could indicate fuzzing or exploitation attempts
• Deploy endpoint detection solutions capable of monitoring kernel-level anomalies

How to Mitigate CVE-2025-68763

Immediate Actions Required

• Update the Linux kernel to a patched version containing the fix for sg_nents_for_len error handling
• If updates are not immediately available, consider disabling the StarFive crypto driver if not required for system operation
• Monitor affected systems for signs of exploitation or unexpected behavior

Patch Information

The vulnerability has been addressed through multiple kernel commits that add proper error checking for sg_nents_for_len return values. The patches are available through the official kernel git repository:

• Kernel Commit 0c3854d65cc4
• Kernel Commit 1af5c973dd74
• Kernel Commit 9b3f71cf02e0
• Kernel Commit e9eb52037a52

System administrators should apply the appropriate patch based on their kernel version.

Workarounds

• Disable the StarFive crypto hardware driver by blacklisting the module: add blacklist starfive-cryp to /etc/modprobe.d/blacklist.conf
• Use software-based cryptographic implementations as an alternative until patching is complete
• Restrict access to cryptographic operations on affected systems to trusted users only

# Blacklist the StarFive crypto driver as a temporary workaround
echo "blacklist starfive-cryp" | sudo tee /etc/modprobe.d/blacklist-starfive-cryp.conf
sudo update-initramfs -u