CVE-2026-98811

What the bug is

Two racing tc filter add requests on the same chain/prio of an unlocked classifier both run change() on the shared proto and both can fail: the winner's tcf_chain_tp_delete_empty() attempt gives up because the loser's handle is still in the idr, and the loser's error path drops only its own reference without a second reclamation attempt. The empty proto stays linked in the chain, holding the chain reference, a block reference and the classifier module reference until the chain or block is torn down. Reclaim the proto on the error path of any failed request that holds a proto reference. The reclamation is emptiness-gated: tcf_chain_tp_delete_empty() unlinks the proto only when delete_empty() admits it is empty, so a live shared proto is never unlinked. A proto the request created is reclaimed unconditionally - it is the only owner, so marking it for deletion is safe even without a delete_empty callback. Classifiers without one (the check marks the proto unconditionally) are rtnl-serialized, so the raced window this guard closes cannot arise for them. This is a follow-up to commit d4e359b3608a ("net/sched: cls_api: fix teardown of an adopted proto on insert-race loss"), which stopped the loser of the insert race from unlinking the winner's live proto but left the empty-proto residual in place. Conditions to recreate: - CONFIG_NET_CLS_FLOWER=y; veth pair - tc qdisc add dev veth0 ingress - two concurrent `tc filter add dev veth0 ingress protocol ip pref 1 flower skip_sw ... action drop` (both fail in fl_hw_replace_filter after publishing their handle in the idr); repeat in a loop - an empty flower tp stays linked after both requests fail; visible as a bare `filter protocol ip pref 1 flower chain 0` header in `tc filter show` with no filter entries - CAP_NET_ADMIN (namespace-local via unshare -Urn suffices)

Where it lives

The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.

net/sched/cls_api.c

What this is

TC subsystemMAINTAINERS section matched on net/sched/ · netdev@vger.kernel.org · Maintained

Check it on the machine

grep -w CONFIG_NET_CLS /boot/config-$(uname -r)

Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.

Show whether Debian builds this, per release

Debian builds this code. Every release below compiles the affected file into the kernels it ships.

• bookworm, trixie, forky, sidy

y

built into the kernel image on every flavour - only a patch removes it

Build switch

CONFIG_NET_CLS

From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages

Signals

Published

2026-10-18 (today)source

CVSS

no vector published, unrated

Attack vector

not published

EPSS

not scored

CISA KEV

not listedthe KEV catalogue

Debian urgency

not yet assignedsource

Subsystem

net/sched

Upstream fixes

The versions the CNA records as introducing and fixing this, and the commit for each. source

Introduced	Fixed in	Commit
5.1	6.12.113	315e4fe65cf0
5.1	6.18.56	ebc9611ab154
5.1	7.2.10	570141625d21
5.1	7.3	0bb8eab29b55