From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-98414][LOW] memcg: avoid charging the root memcg from obj_cgroup_charge_pages() Date: Sun, 25 Oct 2026 21:05:50 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-98414 X-AL-KERNEL-Priority: LOW X-AL-KERNEL-Severity: LOW X-AL-KERNEL-Base-Severity: LOW X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 0 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: d5cdff14c385bf42d03c0dd4c3e88ddf4c28b8a9 List-Id: CVE: CVE-2026-98414 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: memcg: avoid charging the root memcg from obj_cgroup_charge_pages() Commit: d5cdff14c385bf42d03c0dd4c3e88ddf4c28b8a9 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d5cdff14c385bf42d03c0dd4c3e88ddf4c28b8a9 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98414 Analysis date: Sun, 25 Oct 2026 21:05:50 -0400 ActionableScore: 0 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate Manual review required: NO Summary: A host-admin-triggered objcg reparenting path leaks root-memcg page accounting and emits a warning, without demonstrated memory corruption, panic, or practical exhaustion. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98414 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98414 The original announcement does not normally provide a security severity estimate, CVSS assessment, or enough information to determine whether the reported kernel bug represents a practically relevant security issue. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. It combines an autonomous classifier with LLM-assisted technical analysis and a separate ActionableScore mechanism. The purpose of this report is to prioritize Linux kernel CVEs before manual review, identify cases that require prompt investigation, and support automatic closure of issues that are unlikely to have meaningful security impact. Published priority for this report: LOW Manual review required: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-98414 LOW https://www.kernelcve.org/list/?q=CVE-2026-98414 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=1 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both LOW) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';*CWE-772;CWE-682;**CWE-400;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:N';BEST CVSS score: '3.3';DESCR 'A logic and accounting asymmetry can charge reparented objcg pages to the root memcg while the matching uncharge path skips the root memcg. This causes the internal root page counter to grow without being decremented, but root memory current is not exposed and no enforced root limit, kernel crash, memory corruption, or practical host wide exhaustion is demonstrated. For the CVSS the PR:H value reflects that reliable reproduction requires local control of cgroups and a write to the root memory reclaim interface. The paranoid vector uses PR:L only for deployments that delegate equivalent memory cgroup management through a privileged service or reduced capability administrator, and it assigns only low availability impact because the demonstrated primitive is accounting retention. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 3.3 and the production ActionableScore returned 0 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE to LOW because the ActionableScore threshold rule was applied. final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 0.0 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0) MAYBE LEAK LINUS TEST MEMORY DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 KPANIC_UNSUPPORTED_REMOVED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=0 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=YES NamespaceScopedPrivilege=NO ImportantCandidate=NO ManualReview=NO AutoDowngradeAllowed=YES RequiresHostUID0=UNKNOWN AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw base score: -1. * Conservative score: 0 after clamping the negative raw score to the scoring range. * Raw paranoid score: -1. * Paranoid score: 0 after the same clamp; delegated lower-privilege access is not established by the supplied evidence. * Final recommended bucket: **Ordinary Moderate**:: 2. Signal breakdown * +1 — Concrete persistent accounting leak: pages charged through a reparented objcg increment the root memcg page counter, while the root-specific uncharge path never decrements it. * -2 — Host-administrator-equivalent trigger: the demonstrated sequence manages child cgroups and writes the host root cgroup’s `memory.reclaim`; no below-host-admin complete path is shown. * +0 — The warning is not shown to panic or cause a host-wide failure. * +0 — No OOB write, UAF, shared-backing write, authorization bypass, practical exhaustion, or protected-state integrity impact is shown. * B5-R does not apply: there is no retry-accumulated heap OOB write or later consumption of corrupted structured state. * No availability point: the hidden root counter grows, but root `memory.current` is not exposed, no root limit is enforced, and practical exhaustion is not demonstrated. 3. Reachability analysis The confirmed reproducer requires zswap and swap, control over cgroup creation, task migration and removal, plus access to the root memcg reclaim interface. This supports a host-management actor, not an ordinary user or attacker-created namespace. Numeric UID 0 is not explicitly proven as mandatory, but effective host-administrator control is. No hard race or special hardware is required. 4. Primitive and severity reading The defect is asymmetric root-memcg accounting: a charge is retained without its matching uncharge. This is passive bookkeeping/resource leakage, classified CLASS_C, not memory corruption or a lifetime UAF. The demonstrated effect is a debug warning and invisible counter growth. Host-wide exhaustion and security-boundary impact remain unproven. Therefore it is not an Important candidate and automatic downgrade is allowed. 5. One-sentence report phrase A host-admin-triggered objcg reparenting path leaks root-memcg page accounting and emits a warning, without demonstrated memory corruption, panic, or practical exhaustion. 6. Manual review advice NO MANUAL CHECK NEEDED The patch and call chain establish only constrained internal accounting retention under host-level cgroup control. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: memcg: avoid charging the root memcg from obj_cgroup_charge_pages() Commit: d5cdff14c385bf42d03c0dd4c3e88ddf4c28b8a9 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d5cdff14c385bf42d03c0dd4c3e88ddf4c28b8a9 Commit description: obj_cgroup_charge_pages() resolves the objcg to its memcg and calls try_charge_memcg(), which does not short circuit the root memcg. That memcg can be the root memcg: obj_cgroup_is_root() reflects the memcg the objcg was created for and is never updated, while memcg_reparent_objcgs() does redirect objcg->memcg to the parent on rmdir. An objcg of a dying child of root therefore passes every obj_cgroup_is_root() filter but resolves to the root memcg. Folios keep the objcg they were charged with, so this is easy to reach through zswap: allocate anon memory in a cgroup, move the task out, remove the cgroup, then write to the root cgroup's memory.reclaim. The reclaimed folios are charged through the reparented objcg and end up in refill_stock() with the root memcg: WARNING: mm/memcontrol.c:2198 at refill_stock+0x644/0x940 refill_stock+0x644/0x940 try_charge_memcg+0x12d6/0x1570 __obj_cgroup_charge+0x35/0xf0 obj_cgroup_charge+0x1de/0x210 obj_cgroup_charge_zswap+0x83/0x270 zswap_store+0x1620/0x2000 swap_writeout+0x94c/0x14c0 shrink_folio_list+0x3388/0x52b0 [...] try_to_free_mem_cgroup_pages+0x30d/0x830 user_proactive_reclaim+0x504/0x840 memory_reclaim+0x1f/0x30 Beyond the warning, the charge is asymmetric: obj_cgroup_uncharge_pages() skips refill_stock() for the root memcg, so the root's page counter grows and is never uncharged. It is not user visible, since memory.current is not exposed on the root, but it is a leak. Use try_charge(), which returns early for the root memcg, restoring the symmetry with obj_cgroup_uncharge_pages(). The above sequence was scripted into a standalone reproducer (zswap on, swap on a virtio disk, 512MB of anon memory faulted in inside a child of the root cgroup, the task then migrated to the root cgroup, the child removed, followed by "echo 600M swappiness=max > memory.reclaim" on the root) and run in a CONFIG_DEBUG_VM=y VM. It reproduces the splat on the first zswap store of a reparented folio, with the same call chain as the report. With this patch applied the splat is gone while the zswap store count over the run is unchanged, so the same path is still exercised. cgroup selftests test_zswap, test_kmem and test_memcontrol show no new failures. Link: https://lore.kernel.org/20260829023251.474083-1-shakeel.butt@linux.dev Fixes: 20d6c17 ("memcg: avoid refill_stock for root memcg") Signed-off-by: Shakeel Butt Reported-by: Farhad Alemi Closes: https://lore.kernel.org/all/CA+0ovCgWzUMK+nNbbtH7eV65Ca=fDN4Ozu7iASgryjvv8Tk8zQ@mail.gmail.com/ Reviewed-by: Muchun Song Reviewed-by: Johannes Weiner Cc: Michal Hocko Cc: Roman Gushchin Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman Changed files: mm/memcontrol.c Diff excerpt: Not included in this email. See the upstream URL for the full patch. Full patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d5cdff14c385bf42d03c0dd4c3e88ddf4c28b8a9 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98414 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98414 The original CVE announcement normally does not include a security-level estimate. In particular, it may not contain a CVSS assessment, an impact level, or enough information to determine whether the reported bug is a practically relevant security issue. One purpose of this parallel CVE list is to provide that missing technical and prioritization information. The original goal of the AL-KERNEL project was to prioritize Linux kernel CVE analysis automatically before manual review. The system can also help identify non-security issues that may be suitable for automatic closure. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. The first analysis stage combines an autonomous classifier with additional LLM-based analysis. The autonomous classifier runs locally on a CPU and is based on a backpropagation neural network. Together, these mechanisms produce a technical vulnerability description, identify likely weakness types, estimate CVSS severity, and provide input for ActionableScore. Two CVSS estimates are retained because incomplete kernel vulnerability information often permits more than one defensible interpretation: Conservative CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:N The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L The Best / paranoid CVSS score: 3.3 The conservative vector represents a lower-impact interpretation. The Best/paranoid vector intentionally represents a plausible upper-bound interpretation and should not automatically be treated as demonstrated real-world impact. CVSS may also need to be adjusted for a particular Linux deployment, because actual reachability, privileges, enabled kernel configuration, hardware, namespaces, exposed device nodes, and other environmental conditions can differ significantly between systems. A separate ActionableScore mechanism evaluates practical remediation urgency. Its analysis may include reachability, attack prerequisites, subsystem exposure, memory-corruption characteristics, denial-of-service reliability, and possible confidentiality, integrity, or privilege-escalation impact. Conservative ActionableScore: unknown Paranoid ActionableScore: 0 The final base severity is taken directly from the second tab-separated field of the AL-KERNEL classification result. ActionableScore does not replace or independently override that final AL-KERNEL decision, and if ActionableScore adjusted impact level of ALKERNEL, then you would see self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7. For an AL-KERNEL result of MODERATE, this report uses the following additional presentation split: ActionableScore below 5 -> MODERATE REGULAR ActionableScore 5 or more -> MODERATE 7.0 The distinction between MODERATE REGULAR and MODERATE 7.0 makes it possible to identify Moderate issues that should receive manual analysis and fixes before lower-priority MODERATE REGULAR issues. In many cases, MODERATE REGULAR fixes may wait for a later rebase or routine update. There is one override in which MODERATE REGULAR becomes MODERATE 7.0 even when the ActionableScore is below 5. When the AL-KERNEL result contains the KPANIC flag, a MODERATE result is always presented as MODERATE 7.0. The KPANIC flag selected with few regexps without usage of AI at all, so it helps to detect cases when Kernel Crash happens and similar (to filter False-Negative results from the LLM usage). KPANIC indicates that a reliable kernel crash, kernel panic, or similarly serious kernel availability impact was identified by the classification workflow. AL-KERNEL base severity for this report: LOW KPANIC detected for this report: NO Published priority for this report (same as in Subject): LOW These results are intended to support engineering triage. They are machine-generated estimates, and cases marked for manual review should be validated by a human security engineer before final disposition. For more info read docs linked from here: https://kernelcve.org/ (and you can submit you own patch there to generate such a report for non-existant CVE-id yet). Note that in many cases this AI tool selects higher severity, than real is (means you can expect Importants instead of Moderate 7.0 or Moderates 7.0 instead of regular Moderates). If you see such cases, please use reply email interface to add additional manual analyses info to this particular CVE. And please, please, let me know when you see Lows instead of Importants or Important instead of Low (because particular for such cases I need to tune this AI tool to make it better for this one and next similar). My contact email for such notifications is alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).