CVE-2025-37953: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-37953 is a vulnerability discovered in the Linux kernel related to the HTB (Hierarchical Token Bucket) scheduler component. The vulnerability was initially reported and disclosed on May 20, 2025, affecting the Linux kernel's network scheduling functionality (NVD, Wiz Report).

The vulnerability manifests in a specific call sequence within the Linux kernel: htbdequeuetree() -> fqcodeldequeue() -> qdisctreereducebacklog() -> htbqlennotify() -> htbdeactivate() -> htbnextrbnode() -> htbdeactivate(). After the first htbdeactivate() call, clprio[prio]->ptr could be set to NULL, making htbnextrbnode() vulnerable. The issue is compounded when htbdeactivate() may be called again when qlen==0 after qdisctreereducebacklog(). The vulnerability has received a CVSS 3.1 score of 7.0 with vector string AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H (Red Hat).

The vulnerability can lead to a NULL pointer dereference in the Linux kernel's network scheduling component, potentially causing system instability or denial of service (Wiz Report).

The issue has been resolved by implementing two key fixes: 1) Making htbdeactivate() idempotent by having it return if already called before, and 2) Making htbnextrbnode() safe against ptr==NULL conditions. For the Debian stable distribution (bookworm), these problems have been fixed in version 6.1.140-1 (Debian Security).