CVE-2021-47040 is a vulnerability identified in the Linux kernel's io_uring subsystem, specifically related to the function io_provide_buffers_prep(). The vulnerability concerns improper overflow and sign extension checks when providing buffers to io_uring, a high-performance asynchronous I/O interface introduced in recent Linux kernels. The issue stems from the incorrect handling of buffer length values, which were previously signed integers, leading to potential overflow and sign extension problems. This could allow an attacker to craft malicious input that bypasses buffer length validation, potentially causing memory corruption or other undefined behavior. The vulnerability was addressed by implementing proper overflow checks using helper functions (check_<op>_overflow) and changing the buffer length field from a signed to an unsigned type to prevent sign extension issues. Although no known exploits are currently reported in the wild, the vulnerability affects Linux kernel versions prior to the patch and could be exploited in environments where untrusted users have access to io_uring interfaces. The fix improves the robustness of buffer length validation, mitigating risks associated with buffer overflows in the kernel's asynchronous I/O subsystem.

For European organizations, this vulnerability poses a risk primarily to systems running vulnerable Linux kernel versions that utilize io_uring for asynchronous I/O operations. Potential impacts include unauthorized kernel memory corruption, which could lead to privilege escalation, denial of service (system crashes), or arbitrary code execution within the kernel context. Organizations relying on Linux servers for critical infrastructure, cloud services, or containerized environments could face service disruptions or compromise of sensitive data if exploited. Given that io_uring is increasingly adopted for performance optimization in modern Linux distributions, the attack surface is expanding. However, exploitation requires local access or the ability to submit crafted io_uring requests, limiting remote exploitation scenarios. Nonetheless, insider threats, compromised user accounts, or multi-tenant cloud environments could be vectors for attack. The absence of known exploits suggests limited immediate threat, but the vulnerability's nature warrants prompt patching to prevent future exploitation attempts.

European organizations should prioritize updating their Linux kernels to versions that include the patch for CVE-2021-47040. Specifically, they should: 1) Identify all Linux systems using io_uring, especially those running kernel versions prior to the fix date (February 28, 2024). 2) Apply vendor-provided kernel updates or backported patches that address the overflow and sign extension issues in io_provide_buffers_prep(). 3) Restrict access to io_uring interfaces by limiting permissions and using Linux security modules (e.g., SELinux, AppArmor) to confine processes that do not require asynchronous I/O capabilities. 4) Monitor system logs and audit io_uring usage for anomalous or unexpected buffer provisioning requests. 5) In containerized or multi-tenant environments, enforce strict namespace and capability isolation to reduce the risk of local privilege escalation via this vulnerability. 6) Engage with Linux distribution vendors for timely security advisories and patches. These steps go beyond generic advice by focusing on access control to io_uring and proactive monitoring specific to this subsystem.