* [CVE-2026-98939][LOW] x86/mm: Drop unnecessary PMD page copy when freeing
@ 2026-10-27 0:23 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-10-27 0:23 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98939
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: x86/mm: Drop unnecessary PMD page copy when freeing
Commit: d3c2d2692bdf4af94bb27bc7a3aa0a348a567606
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d3c2d2692bdf4af94bb27bc7a3aa0a348a567606
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98939
Analysis date: Mon, 26 Oct 2026 20:23:31 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
A reclaim-capable PMD-copy allocation under `init_mm.mmap_lock` creates a TTM shrinker lock inversion that may deadlock the host without showing memory corruption or authorization bypass.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98939 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98939
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-2026-98939 LOW https://www.kernelcve.org/list/?q=CVE-2026-98939 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE AEGIS_AS=1 LEGACY=LOW RESULT=DIFF (final Legacy and Aegis differ; FINAL: LOW, Aegis: MODERATE) 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-667;*CWE-833;BEST CVSS score: '4.7';DESCR 'A lock ordering flaw can occur because pud_free_pmd_page performs a GFP_KERNEL allocation while the huge vmap path holds init_mm mmap_lock. Reclaim can enter the TTM shrinker while pool_shrink_rwsem and init_mm mmap_lock are acquired in conflicting orders, creating a credible kernel deadlock and availability loss. For the CVSS the PR:L value is selected because practical triggering requires a local user or process able to generate memory pressure and exercise an accessible GPU or mapping workload. AC:H reflects the required reclaim timing and subsystem interaction. The demonstrated primitive is a lock inversion with no invalid memory access, UAF, OOB access, information leak, or write primitive, so C:N and I:N are retained while A:H represents a potentially host wide deadlock. The issue is not directly network reachable. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 1 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 2.5 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE. 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 1) MAYBE READ OOB LOCK DEADLOCK INIT UAF HARDWARE 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=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 1.
* Conservative score: 1; no floor or cap applied.
* Raw paranoid score: 1.
* Paranoid score: 1; unsupported reachability and corruption effects were not added.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
* +1 Reliable kernel deadlock/strong DoS: `pud_free_pmd_page()` performs a reclaim-capable `GFP_KERNEL` allocation while holding `init_mm.mmap_lock`; reclaim can enter the TTM shrinker and form the documented circular lock dependency.
* No timing penalty: although concurrent reclaim and mapping activity are required, the report says lockdep occurs as soon as kswapd shrinks the pool; a narrow, uncontrollable race is not established.
* No local-unprivileged bonus: the legacy summary proposes an accessible GPU workload, but the patch does not establish that one below-host-admin actor can drive the complete huge-vmap, reclaim, and TTM sequence.
* No memory-corruption, UAF, OOB-write, confidentiality, integrity, or authorization-bypass signal is shown.
* No extra availability point: the possible deadlock is the sole availability effect.
3. Reachability analysis
The demonstrated chain involves huge kernel vmap replacement, memory reclaim, and a discrete-GPU TTM shrinker. A local workload may influence pressure and GPU allocation, but the supplied source does not resolve who can invoke the complete sequence or whether prior privileged GPU/mapping setup is required. No namespace capability, remote path, special retry mechanism, or attacker-controlled race is shown. Host-admin and UID-0 requirements therefore remain unknown.
4. Primitive and severity reading
The primitive is a lock-order inversion caused by reclaim-capable allocation under `init_mm.mmap_lock`. It can plausibly deadlock kernel reclaim and cause host-wide unavailability, but it does not corrupt active structured state or perform a stale lifetime operation. This is CLASS_C availability behavior. NEW52/B5-R, B3-PENDING, and B4-PENDING do not apply. ImportantCandidate is NO because no non-crash corruption or boundary primitive remains unresolved; automatic downgrade is allowed.
5. One-sentence report phrase
A reclaim-capable PMD-copy allocation under `init_mm.mmap_lock` creates a TTM shrinker lock inversion that may deadlock the host without showing memory corruption or authorization bypass.
6. Manual review advice
NO MANUAL CHECK NEEDED
The patch and lock chain support ordinary availability triage without an Important-preserving primitive.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: x86/mm: Drop unnecessary PMD page copy when freeing
Commit: d3c2d2692bdf4af94bb27bc7a3aa0a348a567606
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d3c2d2692bdf4af94bb27bc7a3aa0a348a567606
Commit description:
On a box with a discrete GPU, lockdep reports a possible deadlock as
soon as kswapd shrinks the TTM page pool. The immediate cause is an
x86 commit that added an mmap_read_lock() to kernel page protection
munging code.
The huge vmap code holds the same lock over a GFP_KERNEL allocation,
which is a no-no now that reclaim can take it. That allocation is in a
page table *free* path and ends up being for dubious purposes[1].
Basically, it tries to avoid hardware setting Accessed=1 in page table
entries that are unreachable by the hardware, a non-issue.
Remove the PMD copy. Detach the original PMD page at the PUD, flush
the mid-level caches, and free the PTE tables straight from the
detached PMD page. With no allocation left, the locking issue is gone.
Lockdep splat/analysis:
WARNING: possible circular locking dependency detected
7.3.0-rc3-f6e7b42bf05b+ #183 Tainted: G U
------------------------------------------------------
kswapd0/269 is trying to acquire lock:
((init_mm).mmap_lock){++++}-{4:4}, at: change_page_attr_set_clr+0x29a/0x4a0
but task is already holding lock:
(pool_shrink_rwsem){.+.+}-{4:4}, at: ttm_pool_shrink+0xb2/0x330 [ttm]
Chain exists of:
(init_mm).mmap_lock --> fs_reclaim --> pool_shrink_rwsem
The cycle is built from three edges:
1) pool_shrink_rwsem -> (init_mm).mmap_lock
The TTM shrinker restores the caching attribute of every page it
frees, while holding pool_shrink_rwsem:
ttm_pool_shrink()
-> ttm_pool_dispose_list()
-> ttm_pool_free_page()
-> set_pages_wb()
-> change_page_attr_set_clr() [ init_mm mmap read lock ]
2) fs_reclaim -> pool_shrink_rwsem
The same shrinker, called from reclaim.
3) (init_mm).mmap_lock -> fs_reclaim
ioremap() installing a huge PUD mapping over an existing PMD table:
ioremap_page_range()
-> vmap_range_noflush()
-> vmap_try_huge_pud() [ init_mm mmap read lock ]
-> pud_free_pmd_page()
-> __get_free_page(GFP_KERNEL) [ enters reclaim ]
[ dhansen: Lots of changelog munging/trimming and merged comments from my
version of the fix. ]
Fixes: d5d8b86 ("x86/mm/pat: Acquire init_mm read lock on attribute changes to avoid UAF")
Suggested-by: Pedro Falcato <[email protected]>
Signed-off-by: Mikhail Gavrilov <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Pedro Falcato <[email protected]>
Link: https://lore.kernel.org/[email protected]
Link: https://lore.kernel.org/all/[email protected]/ [1]
Link: https://patch.msgid.link/[email protected]
Cc: [email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
arch/x86/mm/pgtable.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=d3c2d2692bdf4af94bb27bc7a3aa0a348a567606
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98939 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98939
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: Not available
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: 1
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
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