CVE-2025-21686: 
Linux Debian vulnerability analysis and mitigation

CVE-2025-21686 is a vulnerability in the Linux kernel's iouring subsystem, discovered and disclosed on February 10, 2025. The vulnerability affects the buffer cloning functionality when using IORINGREGISTERCLONEBUFFERS between two uring instances with different memory management contexts (Red Hat XML, Debian Tracker).

The vulnerability occurs when IORINGREGISTERCLONE_BUFFERS is used to clone buffers from uring instance A to uring instance B, where A and B use different MMs for accounting. If uring instance A is closed before uring instance B, the pinned memory counters for uring instance B will be decremented incorrectly, even though the pinned memory was originally accounted through uring instance A. This can result in the MM of uring instance B ending up with negative locked memory. The vulnerability has been assigned a CVSS v3.1 score of 5.5 (AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H) (Red Hat XML).

The vulnerability can lead to incorrect memory accounting in the Linux kernel, potentially resulting in memory management issues. The impact is primarily on system stability and availability, with no direct impact on confidentiality or integrity (Red Hat XML).

The vulnerability has been fixed in the Linux kernel through a patch that adds additional checks to ensure both rings share the same accounting context before allowing buffer cloning. The fix has been incorporated into various Linux distributions, including Debian Bullseye (5.10.234-1), Bookworm (6.1.128-1), and Trixie (6.12.12-1) (Debian Tracker, Kernel Commit).