CVE-2025-21971

Name	CVE-2025-21971
Description	In the Linux kernel, the following vulnerability has been resolved: net_sched: Prevent creation of classes with TC_H_ROOT The function qdisc_tree_reduce_backlog() uses TC_H_ROOT as a termination condition when traversing up the qdisc tree to update parent backlog counters. However, if a class is created with classid TC_H_ROOT, the traversal terminates prematurely at this class instead of reaching the actual root qdisc, causing parent statistics to be incorrectly maintained. In case of DRR, this could lead to a crash as reported by Mingi Cho. Prevent the creation of any Qdisc class with classid TC_H_ROOT (0xFFFFFFFF) across all qdisc types, as suggested by Jamal.
Source	CVE (at NVD; CERT, LWN, oss-sec, fulldisc, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, GitHub advisories/code/issues, web search, more)
References	DLA-4178-1, DLA-4193-1, DSA-5900-1

Vulnerable and fixed packages

The table below lists information on source packages.

Source Package	Release	Version	Status
linux (PTS)	bullseye	5.10.223-1	vulnerable
	bullseye (security)	5.10.237-1	fixed
	bookworm	6.1.137-1	fixed
	bookworm (security)	6.1.140-1	fixed
	trixie (security)	6.12.31-1	fixed
	sid, trixie	6.12.33-1	fixed
linux-6.1 (PTS)	bullseye (security)	6.1.137-1~deb11u1	fixed

The information below is based on the following data on fixed versions.

Package	Type	Release	Fixed Version	Origin
linux	source	bullseye	5.10.237-1	DLA-4178-1
linux	source	bookworm	6.1.133-1	DSA-5900-1
linux	source	(unstable)	6.12.20-1
linux-6.1	source	bullseye	6.1.137-1~deb11u1	DLA-4193-1

https://git.kernel.org/linus/0c3057a5a04d07120b3d0ec9c79568fceb9c921e (6.14-rc7)