From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-99000][LOW] drm/xe: Fix shrinker accounting double-subtraction on nested external pins Date: Mon, 26 Oct 2026 19:10:26 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-99000 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: 2 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 993c952e6832ad0aa4be82e11f0760b1f532090a List-Id: CVE: CVE-2026-99000 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: drm/xe: Fix shrinker accounting double-subtraction on nested external pins Commit: 993c952e6832ad0aa4be82e11f0760b1f532090a Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=993c952e6832ad0aa4be82e11f0760b1f532090a Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99000 Analysis date: Mon, 26 Oct 2026 19:10:26 -0400 ActionableScore: 2 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate Manual review required: NO Summary: Nested Xe buffer pins can underflow shrinker accounting and cause excessive reclaim and CPU consumption under memory pressure. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99000 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99000 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-2026-99000 LOW https://www.kernelcve.org/list/?q=CVE-2026-99000 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=LOW RESULT=DIFF TEMP_DISTANCE_MERGE_SKIP=AEGIS_NO_POSITIVE_SEVERITY_EVIDENCE (final Legacy and Aegis differ; FINAL: LOW, Aegis: MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-682;CWE-191;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'This is a shrinker resource accounting logic bug rather than a memory corruption issue. Nested external pins can subtract the same buffer pages more than once, while imported dma-bufs can be subtracted even though their pages were never added to the Xe shrinker counts. The resulting negative signed counters are interpreted as very large unsigned values by xe_shrinker_count, which can cause excessive reclaim activity and CPU consumption under memory pressure. For the CVSS the PR:L value reflects that a local user or process with access to GPU and dma-buf interfaces is needed, while reliable availability impact also depends on the pin path, participating importers, and memory pressure. There is no demonstrated UAF, OOB access, information leak, arbitrary write, or privilege escalation primitive. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 2 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 and the cross model evidence reconciliation rule was applied. 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_CIANNH and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 and KPANIC_UNSUPPORTED_REMOVED and V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED. The independent weaker Aegis shadow evaluated CVSS 3.1 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) MAYBE OOB LOCK LEAK HARDWARE TEST MEMORY DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 KPANIC_UNSUPPORTED_REMOVED V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=2 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=NO ManualReview=NO AutoDowngradeAllowed=YES RequiresHostUID0=NO AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw base score: 2. * Conservative score: 2; no floor or cap applied. * Raw paranoid score: 2. * Paranoid score: 2; stronger consequences would require evidence not supplied. * Final recommended bucket: **Ordinary Moderate**:: 2. Signal breakdown * +1 Local below-host-admin trigger: supplied evidence grounds GPU/dma-buf access by a local user or process; nested external pins can drive the accounting error. * +1 Strong availability concern: repeated subtraction or subtraction of never-accounted imported pages can produce huge unsigned shrinker counts and excessive reclaim/CPU activity under memory pressure. * No extra availability point: persistent host failure, panic, or a distinct availability effect is not shown. * No corruption point: the changed state is shrinker page/object accounting, not an OOB write, UAF, overwrite, or active pointer corruption. * NEW52/B5-R does not apply: there is no heap OOB write, retry-accumulated offset, or later consumption of corrupted descriptors. * NEW53/B3-PENDING does not apply: dma-buf involvement does not show a write into shared backing. * No confidentiality, integrity, authorization-bypass, or privilege-escalation signal is shown. 3. Reachability analysis A local process with access to Xe GPU and dma-buf interfaces can plausibly participate in multiple attachments/importers and nested pinning. This supports below-host-admin exposure, but the patch does not establish namespace-scoped capability semantics. Availability impact additionally depends on nested pins/imported objects and memory pressure; no special race or rare hardware penalty is applied beyond requiring an Intel Xe device. 4. Primitive and severity reading The primitive is signed shrinker-accounting underflow interpreted as a very large unsigned count. The downstream consumer is `xe_shrinker_count()`, leading to excessive reclaim activity rather than memory corruption. No stale lifetime use, repeated release, protected-state alteration, or useful non-crash exploitation primitive is shown. It is therefore CLASS_C, not an Important candidate, and automatic Moderate handling is permissible. 5. One-sentence report phrase Nested Xe buffer pins can underflow shrinker accounting and cause excessive reclaim and CPU consumption under memory pressure. 6. Manual review advice NO MANUAL CHECK NEEDED The patch supports a constrained resource-accounting availability bug without a memory-corruption or security-boundary primitive. 7. Call-site confidence Call-site confidence: medium ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: drm/xe: Fix shrinker accounting double-subtraction on nested external pins Commit: 993c952e6832ad0aa4be82e11f0760b1f532090a Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=993c952e6832ad0aa4be82e11f0760b1f532090a Commit description: xe_bo_pin_external() and xe_bo_unpin_external() support nested pins, since the underlying ttm_bo_pin()/ttm_bo_unpin() maintain a pin_count refcount rather than a boolean. However, the shrinker accounting calls xe_ttm_tt_account_subtract() and xe_ttm_tt_account_add() are invoked unconditionally on every pin_external()/unpin_external() call, instead of only on the transition into and out of the pinned state. An external BO (dma-buf) can be pinned more than once while already pinned, for example when it has multiple attachments/importers, each independently calling xe_bo_pin_external(). Each such nested pin subtracts the BO's pages from the shrinker's accounting again, even though the BO's pages were already removed from consideration by the first pin. Symmetrically, each nested unpin adds them back again before the BO is actually unpinned. This desynchronizes the shrinker's page and object counts from reality, and since they are declared as signed long but interpreted as unsigned long in xe_shrinker_count(), the underflow can turn into an enormous shrinkable/purgeable page count, causing the shrinker to be invoked excessively under memory pressure. Guard the accounting calls with the same pin-count transition checks already used to guard the pinned_link list maintenance, so accounting is only updated on the outermost pin and the final unpin. While centralizing this logic into xe_bo_account_pin()/ xe_bo_account_unpin(), also close a second, related accounting gap affecting imported dma-bufs. For an imported bo's ttm_tt, xe's ttm_tt_populate hook (xe_ttm_tt_populate()) early-returns without calling xe_ttm_tt_account_add(), since TTM_TT_FLAG_EXTERNAL is set without TTM_TT_FLAG_EXTERNAL_MAPPABLE. However, TTM core's ttm_tt_populate() wrapper unconditionally marks the tt as populated on driver-hook success, so ttm_tt_is_populated() still reports true for it. xe_bo_account_pin()/xe_bo_account_unpin() trusted ttm_tt_is_populated() as a proxy for "these pages are accounted for by the shrinker", and would subtract/add pages for such a tt that were never added in the first place, again underflowing the shrinker's counts. This is reachable today via xe_bo_pin_external(), and would have become newly reachable via the fb-pin path once this patch routes __xe_pin_fb_vma() through xe_bo_pin_account() below, so fix it here rather than in a later patch, to avoid a commit in this series that introduces the regression before fixing it. Skip the accounting for imported bos, using the same EXTERNAL && !EXTERNAL_MAPPABLE test xe_ttm_tt_populate()/xe_ttm_tt_unpopulate() use to decide whether to act, via a new xe_ttm_bo_is_imported() helper (hoisted up from its only prior use site). v2: - Fix the critical issue caught by Sashiko AI review: the pin-count transition check introduced here was still bypassed by the fb-pin path (__xe_pin_fb_vma()/__xe_unpin_fb_vma()), which could leave a stale pinned_link entry and a permanent shrinker accounting leak when the fb-unpin performed the true last unpin. Addressed in a separate follow-up patch in this series rather than folded in here, since it is a distinct bug with its own Fixes: tag. - Drop the unneeded local "last_unpin" variable in xe_bo_unpin_external(), restoring the original inline bo->ttm.pin_count == 1 check; it was a no-op rename not used by any of the new accounting helpers, and was going to be deleted again by the follow-up patch anyway. v3: - Exclude imported dma-bufs from the pin/unpin shrinker accounting, fixing another underflow caught by Sashiko AI review. - Add a short comment on the backup-object pin site noting it's already populated/accounted at pin time, addressing (as a false-positive clarification, no code fix needed) a separate Medium-severity accounting-leak concern also caught by Sashiko AI review. Fixes: 00c8efc ("drm/xe: Add a shrinker for xe bos") Cc: Thomas Hellström Cc: Matthew Brost Cc: Rodrigo Vivi Cc: intel-xe@lists.freedesktop.org Cc: # v6.15+ Reviewed-by: Matthew Auld Reviewed-by: Himal Prasad Ghimiray Assisted-by: LLM Signed-off-by: Thomas Hellström Link: https://patch.msgid.link/20261002100830.12297-2-thomas.hellstrom@linux.intel.com [Commit description truncated; see the upstream URL below] Changed files: drivers/gpu/drm/xe/display/xe_fb_pin.c drivers/gpu/drm/xe/xe_bo.c drivers/gpu/drm/xe/xe_bo.h 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=993c952e6832ad0aa4be82e11f0760b1f532090a ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99000 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99000 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:H/PR:L/UI:N/S:U/C:N/I:N/A:L The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 4.7 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: 2 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).