[CVE-2026-99000][LOW] drm/xe: Fix shrinker accounting double-subtraction on nested external pins

[CVE-2026-99000][LOW] drm/xe: Fix shrinker accounting double-subtraction on nested external pins

[AL-KERNEL]

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.

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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.

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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

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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 <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: intel-xe@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v6.15+
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Assisted-by: LLM
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
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).