CVE-2025-39999: 
Linux Debian vulnerability analysis and mitigation

In the Linux kernel, a vulnerability (CVE-2025-39999) has been identified and resolved related to a double free issue in the block multiqueue (blk-mq) subsystem. The vulnerability was discovered and disclosed on October 15, 2025. The issue specifically affects the Linux kernel's block layer implementation (NVD).

The vulnerability occurs when users trigger tags growth using the queue sysfs attribute nrrequests. During this process, hctx->schedtags is freed directly and replaced with newly allocated tags through the blkmqtagupdatedepth() function. The issue arises because hctx->sched_tags is derived from elevator->et->tags, while et->tags still references the freed tags, leading to a double-free condition when the elevator exits (NVD).

When exploited, this vulnerability can cause a kernel panic due to the double-free condition, potentially leading to system instability or denial of service. The issue affects the block layer's tag management system, which is crucial for I/O scheduling and queue management (NVD).

The issue has been fixed by ensuring that et->tags is also replaced with the newly allocated tags when tag growth occurs. However, it's noted that there are still some long-term problems that will require thorough refactoring to be completely resolved (NVD).