CVE-2025-40364: Linux Kernel io_uring Vulnerability

CVE-2025-40364 Overview

CVE-2025-40364 is a Linux kernel vulnerability in the io_uring subsystem. The flaw resides in io_req_prep_async(), which can import provided buffers without properly committing the ring state before doing so. The kernel maintainers resolved the issue by giving up on the pending import operation first, allowing it to be reimported later if needed. The vulnerability affects local attackers with low privileges and can lead to compromise of confidentiality, integrity, and availability on affected systems.

Critical Impact

Local attackers with low privileges can exploit incorrect ring state handling in io_uring provided buffer preparation to corrupt kernel state, potentially leading to privilege escalation or system compromise.

Affected Products

• Linux Kernel (multiple stable branches prior to the fix commits)
• Distributions shipping vulnerable io_uring implementations
• Systems with io_uring enabled and accessible to unprivileged users

Discovery Timeline

• 2025-04-18 - CVE-2025-40364 published to the National Vulnerability Database (NVD)
• 2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-40364

Vulnerability Analysis

The vulnerability exists in the Linux kernel's io_uring asynchronous I/O interface, specifically within the io_req_prep_async() function. This function prepares I/O requests for asynchronous execution. When a request uses provided buffers, io_req_prep_async() can trigger an import of those buffers.

The root problem is state management. The function performs buffer import operations without first committing the ring state associated with the previously selected buffer. This creates inconsistency between the request's view of the buffer and the ring's tracking state. The io_uring provided buffer mechanism relies on strict accounting between userspace-shared ring buffers and kernel-side request state.

An attacker with local access and the ability to submit io_uring operations can trigger this code path to induce inconsistent state. The impact spans confidentiality, integrity, and availability at the kernel level [CWE-noinfo].

Root Cause

The root cause is missing ring state commitment before buffer reimport in io_req_prep_async(). The fix commits the ring state by relinquishing the buffer before the async preparation path executes an import. If a buffer is required later, the request path reimports it cleanly, avoiding double-accounting or stale buffer references.

Attack Vector

Exploitation requires local access to a system exposing io_uring to the attacker's user context. The attacker crafts a sequence of io_uring submissions that use provided buffers and force asynchronous preparation. By racing or repeatedly triggering io_req_prep_async(), the attacker manipulates ring state inconsistencies to corrupt kernel memory tracking. The vulnerability requires no user interaction and no elevated privileges beyond the ability to invoke io_uring syscalls.

No public proof-of-concept exploit code has been published for CVE-2025-40364. Technical details are available in the upstream kernel commits referenced in the Linux stable tree.

Detection Methods for CVE-2025-40364

Indicators of Compromise

• Unexpected kernel oops or panics referencing io_uring functions such as io_req_prep_async or provided buffer handlers
• Kernel log entries indicating buffer accounting errors or use-after-free warnings in io_uring paths
• Unprivileged processes issuing high volumes of io_uring_setup, io_uring_enter, or IORING_OP_PROVIDE_BUFFERS operations

Detection Strategies

• Enable kernel address sanitizer (KASAN) in test environments to surface memory corruption originating in io_uring
• Monitor dmesg and syslog for warnings and BUG messages tied to io_uring subsystem paths
• Audit which local user contexts have permission to invoke io_uring syscalls, particularly on multi-tenant hosts

Monitoring Recommendations

• Deploy eBPF or auditd rules to flag anomalous io_uring_setup and io_uring_enter syscall patterns from unprivileged users
• Track kernel version inventory across the fleet to identify hosts running unpatched builds
• Correlate crash telemetry with the presence of io_uring-using workloads to prioritize investigation

How to Mitigate CVE-2025-40364

Immediate Actions Required

• Apply the upstream kernel patches referenced in the vendor advisories and reboot affected hosts
• Update to a distribution kernel package that incorporates the io_uring fix commits
• Inventory systems that expose io_uring to unprivileged users and prioritize them for patching

Patch Information

The Linux kernel maintainers resolved this issue across multiple stable branches. Fix commits include 233b210, 35ae791, a1b1771, a94592e, b86f1d5, d63b0e8, and f0ef945. Apply the vendor-supplied kernel package that includes these commits for your distribution.

Workarounds

• Restrict access to io_uring by setting kernel.io_uring_disabled=2 via sysctl to disable it system-wide where operationally acceptable
• Use seccomp policies to block io_uring_setup, io_uring_enter, and io_uring_register syscalls for untrusted workloads
• Limit container and sandbox permissions to prevent unprivileged code from invoking io_uring interfaces

# Configuration example
# Disable io_uring system-wide (requires kernel >= 6.6)
sysctl -w kernel.io_uring_disabled=2
echo 'kernel.io_uring_disabled=2' >> /etc/sysctl.d/99-io-uring.conf

# Verify current kernel version and applied patches
uname -r