Cloudanix VulnDB

CVE-2026-93197

None

CVSS

—

EPSS

0.2%

Published

Sep 17, 2026

CISA due date

—

Cloudanix analysis

Schedule a fix SSVC: Track

A bookkeeping error in the Linux kernel's memory grouping code can make the system repeatedly scan empty lists belonging to already-deleted groups, wasting processor time and slowing the machine.

Exploitation
Local Privilege
Attacker needs
Low Privilege
Complexity
Moderate
Impact
Denial Of Service
Component
OS kernel
Class
Other
Remediation
Upgrade
Effort
Medium
Patch risk
Low

What it actually does

Stale per-zone LRU size counters remain on offlined memory cgroups after their folios are reparented; shrink_lruvec and MGLRU scanners repeatedly read phantom values through lruvec_lru_size, performing useless SWAP_CLUSTER_MAX walks over empty lists and over-budgeting shadow nodes.

Why it matters to the business

Excessive CPU consumption on hosts that create and destroy many memory cgroups, reducing overall system throughput and increasing latency for memory-pressure workloads.

How an attack unfolds

An attacker or workload repeatedly creates, populates, and destroys memory cgroups; on each offline the LRU sizes are copied rather than moved, leaving phantom counters that cause the kernel's reclamation and MGLRU scanners to burn cycles on dying cgroups until they are finally freed.

How to fix it

Apply the upstream fix that moves LRU size accounting (including LRU_UNEVICTABLE) during lruvec_reparent_lru and lru_gen_reparent_memcg instead of copying; the change must be taken after commit bf4ade7dbd76.

How to confirm the fix worked

After patching, confirm that dying cgroups (CSS_DYING set) show zeroed lru_zone_size counters and that mem_cgroup_iter no longer returns phantom non-zero sizes for empty lists.

Typical attack path

  1. Create and populate many child memory cgroups
  2. Offline or kill the cgroups to trigger reparenting
  3. Induce memory pressure so shrink_lruvec or count_shadow_nodes runs on the stale children
  4. Observe sustained CPU time spent walking empty LRU lists

How to detect it

Search kernel logs and traces for repeated calls to get_scan_count or count_shadow_nodes against cgroups with CSS_DYING set; inspect /proc/cgroups or cgroupfs for parent/child pairs reporting identical non-zero LRU sizes after child offline.

  • mem_cgroup_update_lru_size WARN_ONCE
  • lruvec_lru_size phantom counter on CSS_DYING cgroup
  • 380+ mz->lru_zone_size entries describing folios on no list

What your security team should do

  • Prioritize kernel upgrade on systems that frequently create and destroy memory cgroups
  • Monitor CPU usage of shrink_lruvec, count_shadow_nodes and mem_cgroup_iter under memory pressure
  • Limit unprivileged cgroup creation where feasible until patched kernels are deployed

The record

In the Linux kernel, the following vulnerability has been resolved: memcg: move LRU size accounting on reparenting instead of copying it When a memory cgroup is offlined its LRU folios are reparented to the parent. lruvec_reparent_lru() splices the child's lists into the parent's and credits the parent with the child's per-zone lru_zone_size[], but never clears the child's copy, so the size is copied rather than moved. lru_gen_reparent_memcg() does the same for MGLRU. The parent is left correct, credited with exactly the folios it took over. The stale value sits on the child and nothing will correct it: folio->memcg_data now resolves to the parent, so every later update_lru_size() for those folios goes there. Dying cgroups are not freed immediately and mem_cgroup_iter() still walks them, so shrink_lruvec() keeps being called on them. get_scan_count() reads the phantom counter through lruvec_lru_size() and the scan loop then grinds through nr[] in SWAP_CLUSTER_MAX steps against an empty list, for as long as the dead cgroup lives. Under MGLRU the MGLRU scanner runs instead, but count_shadow_nodes() sums all of NR_LRU_LISTS through lruvec_lru_size() and over-budgets the shadow node limit just the same. On one 251 GiB host a sweep of every mz->lru_zone_size[] found 380 counters describing folios on no list at all: 124777314 pages, 476 GiB, 1.89x the machine's RAM, across 57 cgroups. All were on memcgs with CSS_DYING set and CSS_ONLINE clear, and parent/child pairs reported byte-identical sizes. LRU_UNEVICTABLE needs its size moved too. Its list is deliberately not spliced because lruvec_init() poisons the head - the unevictable LRU is imaginary and folios are never threaded on it - but the size is kept by lruvec_add_folio()/lruvec_del_folio() and those folios account to the parent from here on. This depends on commit bf4ade7dbd76 ("memcg: keep folio's objcg same as its node") and must not be backported ahead of it. Without that invariant a folio's objcg can belong to another node, so a folio already spliced onto the parent's list can still resolve to the child's lruvec until the objcg's node is reparented in a later iteration of memcg_reparent_objcgs(); clearing the child's counter early then lets lruvec_del_folio() underflow it and trip the WARN_ONCE()/VM_BUG_ON() in mem_cgroup_update_lru_size().

Affected packages

Ecosystem Package Introduced Fixed