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CVE-2026-68138
net/sched: serialize qdisc_rtab_list against concurrent get/put
Published 2026-08-10 (2 months ago) by the Linux kernel CNA. Subsystem net/sched. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-68138 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixedDLA-4777-1 · 2026-09-08 · text | 6.1.187-1 | Update the kernel and reboot. DLA-4777-1 shipped the fix on 2026-09-08, in 6.1.187-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixedDSA-6466-1 · 2026-08-25 · text | 6.12.105-1 | Update the kernel and reboot. DSA-6466-1 shipped the fix on 2026-08-25, in 6.12.105-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.6-1 | Update the kernel and reboot. Fixed in 7.1.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.6-1 | Update the kernel and reboot. Fixed in 7.1.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4788-1 · 2026-09-19 · text | 6.12.107-1~deb12u1 | Update the kernel and reboot. DLA-4788-1 shipped the fix on 2026-09-19, in 6.12.107-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
qdisc_get_rtab() and qdisc_put_rtab() mutate the process-global singly linked list qdisc_rtab_list and a plain non-atomic 'int refcnt' with no lock. This was only safe because every caller historically held the RTNL mutex, which serialized all rate-table lookups, inserts and frees. That invariant no longer holds. cls_flower sets TCF_PROTO_OPS_DOIT_UNLOCKED, so tc_new_tfilter() keeps rtnl_held == false for it and sets TCA_ACT_FLAGS_NO_RTNL. That flag propagates through tcf_exts_validate_ex() -> tcf_action_init() -> tcf_action_init_1() -> tcf_police_init(), which calls qdisc_get_rtab()/qdisc_put_rtab() with the RTNL mutex NOT held. Two RTM_NEWTFILTER requests on different CPUs, each adding a flower filter with a police action carrying the same rate, then race on qdisc_rtab_list and on the non-atomic refcnt, leading to a use-after-free / double-free of the kmalloc-2k struct qdisc_rate_table. qdisc_rtab_list is a single global (not per-netns), so the corrupted object is shared system-wide. BUG: KASAN: slab-use-after-free in qdisc_put_rtab+0x12f/0x160 qdisc_put_rtab+0x12f/0x160 tcf_police_init+0xda9/0x1590 tcf_action_init_1+0x460/0x6b0 tcf_action_init+0x439/0xa40 tcf_exts_validate_ex+0x42d/0x550 fl_change+0xddd/0x7da0 tc_new_tfilter+0xaa7/0x2420 rtnetlink_rcv_msg+0x95e/0xe90 which belongs to the cache kmalloc-2k of size 2048 Protect qdisc_rtab_list and the refcount with a dedicated spinlock. The (sleeping, GFP_KERNEL) allocation in qdisc_get_rtab() is performed before taking the lock; if a concurrent inserter added an identical table in the meantime the freshly allocated one is freed under the lock, so no duplicate is leaked. qdisc_put_rtab() now decrements the refcount and unlinks under the same lock.
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/sch_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_SCHED /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, sid, bookworm · linux-6.12y
- y
- built into the kernel image on every flavour - only a patch removes it
- Build switch
- CONFIG_NET_SCHED
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
- 2026-08-10 (2 months ago)source
- CVSS
- 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.28%, higher than 19.4% of all CVEssource
- 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 | 5.10.266 | 1b050d09dd1a |
| 5.1 | 5.15.217 | 6e0241f6cbb1 |
| 5.1 | 6.1.184 | f93c89392bd3 |
| 5.1 | 6.6.153 | 4131dd0b6f67 |
| 5.1 | 6.12.105 | d981098b7675 |
| 5.1 | 6.18.46 | 8ddc2eb0d2da |
| 5.1 | 7.1.6 | fb29e1b41052 |
| 5.1 | 7.2 | f43ee0c0730d |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4788-1 | 2026-09-19 | bookworm 6.12.107-1~deb12u1 | tracker |
| DLA-4777-1 | 2026-09-08 | bookworm 6.1.187-1 | tracker |
| DSA-6466-1 | 2026-08-25 | trixie 6.12.105-1 | tracker |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- The bug is reached only via local RTM_NEWTFILTER rtnetlink messages (tc flower filter with police action) through rtnetlink_rcv_msg→tc_new_tfilter→fl_change→tcf_police_init→qdisc_get_rtab/qdisc_put_rtab; no remote packet or network protocol path exists.
- AC:LAttack complexity, low
- cls_flower is TCF_PROTO_OPS_DOIT_UNLOCKED, so concurrent RTM_NEWTFILTER requests run without RTNL; the attacker controls both racing threads/CPUs and can repeatedly install flower filters with identical police rate tables to deterministically hit the global qdisc_rtab_list/refcnt race.
- PR:LPrivileges required, low
- Non-GET rtnetlink handlers require netlink_net_capable(CAP_NET_ADMIN); this is namespace-scoped, so an unprivileged local user obtains CAP_NET_ADMIN via user namespaces (unshare -Urn), sets up clsact/ingress, and installs flower+police rules without init-namespace root.
- UI:NUser interaction, none
- Exploitation is fully self-contained: the attacker issues the concurrent tc/netlink commands from its own processes and needs no victim to mount filesystems, click links, or perform any other interactive action.
- S:UScope, unchanged
- Impact is heap corruption and UAF/double-free in kernel kmalloc-2k slabs within the same kernel security authority; although qdisc_rtab_list is global across netns, this is standard kernel memory corruption/privilege escalation, not a VM, IOMMU, or sandbox boundary escape.
- C:HConfidentiality impact, high
- Concurrent non-atomic refcnt and list updates cause slab use-after-free/double-free of struct qdisc_rate_table (KASAN-confirmed in qdisc_put_rtab); freed 2KB objects are attacker-influenceable via kmalloc timing and are classically weaponizable for arbitrary kernel memory disclosure.
- I:HIntegrity impact, high
- The race corrupts a process-global singly linked list and can double-free a kmalloc-2k qdisc_rate_table, yielding classic heap metadata/object corruption primitives that can be developed into arbitrary kernel writes and local privilege escalation/code execution.
- A:HAvailability impact, high
- Demonstrated KASAN slab-use-after-free in qdisc_put_rtab on the error/teardown path, and concurrent double-free of the same rate table can panic the kernel; the trigger is repeatable at will with concurrent netlink requests and causes task death or system-wide instability.