CVE-2025-21655: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-21655 affects the Linux kernel's iouring subsystem, specifically related to eventfd handling. The vulnerability was discovered in January 2025 and involves incorrect RCU (Read-Copy-Update) period deferral in the ioeventfd_signal() function. The issue affects Linux kernel versions in Ubuntu 24.10 and 24.04 LTS, while earlier versions are not affected (Ubuntu Security).

The vulnerability occurs when ioeventfddosignal() is invoked from an RCU callback. When dropping the reference to ioevfd, it directly calls ioeventfdfree() if the refcount drops to zero. This implementation is incorrect because any potential freeing of the ioevfd should be deferred for another RCU grace period. The proper fix involves using ioeventfd_put() instead of open-coding the decrement-and-test and free operations, ensuring correct deferral for another RCU grace period (Kernel Commit).

The vulnerability affects the memory management and synchronization mechanisms in the Linux kernel's io_uring subsystem. While the specific impact is not explicitly detailed in the available sources, issues with RCU period handling can potentially lead to memory corruption or system stability problems (Debian Security).

The issue has been fixed in the Linux kernel through a patch that modifies the ioeventfd handling code. The fix involves replacing the direct call to ioeventfdfree() with ioeventfd_put(), which properly defers the freeing operation for another RCU grace period. Users should update their systems to patched versions when available. For Ubuntu users, updates are required for versions 24.10 and 24.04 LTS, while earlier versions are not affected (Ubuntu Security).