CVE-2025-40216: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-40216 is a vulnerability in the Linux kernel's io_uring resource management subsystem (io_uring/rsrc), caused by incorrect handling of user virtual address alignment. The flaw involves faulty bitmask logic used to calculate the offset of the first page into a folio after coalescing, which does not account for the fact that user pointers carry no guaranteed alignment. It was published on December 4, 2025, and affects Linux kernel versions from commit a8edbb424b13 up to the patched commits in stable branches. The CVSS estimate is Medium severity, with an EPSS score of approximately 0.017% (Red Hat CVE, ENISA EUVD).

The root cause is improper handling of unaligned user virtual addresses (vaddr) in the io_uring/rsrc code path responsible for resource registration. When coalescing pages into folios, the code incorrectly uses bitmask operations to derive the page offset within a folio, assuming user pointers are aligned — an assumption that is not guaranteed by the kernel ABI. This logic error can produce incorrect offset calculations, potentially leading to memory corruption or incorrect memory access. The fix removes the flawed bitmask logic and replaces it with a correct offset calculation. Patches are available in the stable kernel tree at commits 3a3c6d61577d (6.12.x), f167692415 (6.15.x), and 50998b0ae7d9 (ENISA EUVD, Red Hat CVE).

The vulnerability could result in incorrect memory access within the kernel's io_uring subsystem when processing resource registrations with unaligned user pointers. Depending on the nature of the miscalculation, this could lead to data corruption, unexpected kernel behavior, or potentially a system crash affecting availability. The scope is limited to systems using io_uring with resource registration operations; confidentiality and integrity impacts are considered low to moderate given the medium severity estimate (Red Hat CVE, ENISA EUVD).

No public proof-of-concept exploit code or in-the-wild exploitation has been reported for CVE-2025-40216. The EPSS score is approximately 0.017%, indicating a very low probability of exploitation in the near term. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. It is detectable via Nessus plugin 277593 (Tenable, ENISA EUVD).

The fix has been backported to stable Linux kernel branches: version 6.12.36 (commit 3a3c6d61577d), version 6.15.5 (commit f16769241594), and the 6.16 development branch (commit 50998b0ae7d9). Users should update to the patched kernel versions as soon as they are available through their Linux distribution's package management system. No configuration-based workaround is documented; upgrading the kernel is the recommended remediation (ENISA EUVD, Red Hat CVE).