CVE-2025-71203: Linux Kernel Information Disclosure Flaw

CVE-2025-71203 Overview

A speculative execution vulnerability has been identified in the Linux kernel's RISC-V architecture implementation. The flaw exists in the syscall table indexing mechanism where user-controlled syscall numbers are used to index into the syscall table without proper speculation barriers. This allows potential speculative out-of-bounds access that could lead to data leakage via cache side-channel attacks.

The vulnerability stems from missing sanitization of array indexing operations under speculative execution conditions. When a user provides a syscall number, the kernel performs a bounds check before accessing the syscall table. However, speculative execution can bypass this bounds check, allowing the processor to speculatively access memory outside the intended array bounds. The resolution implements array_index_nospec() to clamp the syscall number after bounds checking, preventing speculative out-of-bounds access.

Critical Impact

Attackers with local access may exploit this vulnerability to leak sensitive kernel memory contents through cache timing side-channel attacks, potentially exposing cryptographic keys, credentials, or other sensitive data.

Affected Products

• Linux kernel (RISC-V architecture)
• Systems running RISC-V processors with speculative execution capabilities
• Linux kernel versions prior to the security patches

Discovery Timeline

• 2026-02-14 - CVE CVE-2025-71203 published to NVD
• 2026-02-18 - Last updated in NVD database

Technical Details for CVE-2025-71203

Vulnerability Analysis

This vulnerability belongs to the class of Spectre-like side-channel attacks that exploit speculative execution in modern processors. The RISC-V syscall handling path accepts a user-supplied syscall number that serves as an index into the kernel's syscall table. While the kernel performs a bounds check to ensure the syscall number is valid before proceeding, speculative execution allows the processor to execute instructions beyond the bounds check before the check completes.

During speculative execution, if an attacker provides an out-of-bounds syscall number, the processor may speculatively load data from arbitrary kernel memory addresses. Although the speculative results are eventually discarded when the bounds check fails, the memory access leaves traces in the CPU cache. An attacker can then use cache timing analysis to infer the contents of the speculatively accessed memory, effectively leaking sensitive kernel data.

The fix applies array_index_nospec(), a kernel helper function designed to prevent speculative out-of-bounds array access. This function ensures that the index value used in speculative execution is clamped to valid bounds, regardless of the actual bounds check result.

Root Cause

The root cause is the absence of speculative execution barriers in the RISC-V syscall dispatch path. The syscall number, being entirely user-controlled, is used directly as an array index after a standard bounds check. However, processors with speculative execution capabilities may execute the array access speculatively before the bounds check completes, creating a timing side-channel that can be exploited to leak kernel memory contents.

Attack Vector

The attack requires local access to the vulnerable system. An attacker can craft malicious syscall invocations with out-of-bounds syscall numbers to trigger speculative memory accesses. By carefully timing cache accesses and analyzing cache states, the attacker can reconstruct the contents of kernel memory that was speculatively accessed. This technique is similar to other Spectre-variant attacks and requires:

1. Local execution privileges on the target system
2. The ability to invoke syscalls with arbitrary numbers
3. High-precision timing capabilities to detect cache state changes
4. Repeated measurements to overcome noise and extract data reliably

The vulnerability mechanism involves the speculative execution of syscall table indexing operations without proper bounds clamping. When a user-controlled syscall number exceeds valid bounds, speculative execution may access kernel memory beyond the syscall table, leaving cache timing artifacts that can be measured by the attacker. For detailed technical implementation, refer to the kernel git commits.

Detection Methods for CVE-2025-71203

Indicators of Compromise

• Unusual patterns of invalid syscall invocations from user-space processes
• Processes exhibiting high-frequency syscall activity with invalid syscall numbers
• Anomalous cache timing measurements or cache contention patterns
• Processes attempting to perform high-precision timing operations repeatedly

Detection Strategies

• Monitor for processes making repeated syscalls with invalid or out-of-range syscall numbers
• Implement syscall auditing to detect unusual syscall patterns indicative of side-channel probing
• Deploy runtime integrity monitoring to detect unauthorized kernel memory access attempts
• Use hardware performance counters to identify abnormal cache miss patterns that may indicate side-channel exploitation

Monitoring Recommendations

• Enable kernel syscall auditing with focus on failed or invalid syscall attempts
• Configure alerting for processes with abnormal syscall failure rates
• Monitor system logs for patterns consistent with speculative execution attacks
• Consider deploying SentinelOne Singularity Platform for real-time kernel-level threat detection and response

How to Mitigate CVE-2025-71203

Immediate Actions Required

• Update the Linux kernel to a patched version that includes the array_index_nospec() fix for RISC-V syscall handling
• Prioritize patching on RISC-V systems processing sensitive data or in multi-tenant environments
• Review and restrict local access to systems where patching cannot be immediately applied
• Consider implementing additional access controls to limit potential attacker capabilities

Patch Information

The vulnerability has been addressed in the Linux kernel with patches that implement proper speculative execution barriers using array_index_nospec(). The following kernel git commits contain the fix:

• Commit 25fd7ee7bf58ac3ec7be3c9f82ceff153451946c
• Commit 8b44e753795107a22ba31495686e83f4aca48f36
• Commit c45848936ebdb4fcab92f8c39510db83c16d0239

System administrators should update to kernel versions containing these commits to remediate the vulnerability.

Workarounds

• Restrict local access to RISC-V systems until patches can be applied
• Implement strict process isolation and sandboxing to limit potential side-channel exploitation
• Disable or restrict access to high-precision timing sources where feasible
• Consider enabling additional kernel hardening options that may limit speculative execution attack surface

# Check current kernel version for RISC-V systems
uname -r

# Verify if kernel includes the fix (check git log for relevant commits)
# Update to the latest kernel version from your distribution
# For example, on Debian/Ubuntu-based systems:
sudo apt update && sudo apt upgrade linux-image-*

# Reboot to apply the new kernel
sudo reboot