[CVE-2026-89796][LOW] mm/damon/core: avoid infinite kdamond_merge_regions() internal loop

[CVE-2026-89796][LOW] mm/damon/core: avoid infinite kdamond_merge_regions() internal loop

[AL-KERNEL]

CVE: CVE-2026-89796
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: mm/damon/core: avoid infinite kdamond_merge_regions() internal loop
Commit: 8a96a06e04d210845be5d9581c0e9e338947e8f8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8a96a06e04d210845be5d9581c0e9e338947e8f8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89796
Analysis date: Wed, 16 Sep 2026 10:42:29 -0400
ActionableScore: 1
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

======================================================================
ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-89796 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89796

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-2026-89796	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89796 CHECK WITH IMPACT FROM ORIG NN 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-835;CWE-190;**CWE-400;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'DAMON kdamond_merge_regions can enter an infinite internal loop because the merge threshold is doubled before the loop termination condition is checked. If max_thres is larger than UINT_MAX / 2, the threshold multiplication can overflow and the exit condition can be bypassed while the number of regions still remains above max_nr_regions. This requires an unusual DAMON setup with very large aggregation intervals compared to the sampling interval and many non-contiguous regions that cannot be merged enough to satisfy the configured limit. For the CVSS the PR:L is used in the paranoid score only for environments where DAMON control is delegated to a local non admin service or reduced capability admin context. The realistic base case is PR:H because DAMON configuration is normally restricted to administrators. The issue is not network reachable. Impact is denial of service through CPU consumption or a stuck monitoring thread, with no evidence of memory corruption, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	NULLPTR RACE IMPROVEONLY LINUS TEST MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	NO	checked

======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=1
ActionableScoreLower=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 0
* Conservative score: 0
* Raw paranoid score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap was applied. Conservative arithmetic is +1 for DoS minus 1 for explicitly unrealistic trigger configuration. Paranoid arithmetic keeps the DoS signal but does not apply the unusual setup reduction, because delegated DAMON configuration cannot be fully ruled out from the supplied patch alone.

2. Signal breakdown

Positive signals:

* +1 Reliable DoS / CPU stall: the patched loop can become infinite when threshold doubling overflows and the number of DAMON regions remains above max_nr_regions.
* +0 Memory corruption: no UAF, OOB access, double free, refcount misuse, arbitrary write, or stale structured object use is established.
* +0 Confidentiality / integrity: no disclosure path or unauthorized modification primitive is shown.
* +0 Local unprivileged trigger: not established. DAMON control is commonly administrative, but the exact complete privilege boundary is not shown in the supplied patch.
* +0 Resource leak: not a persistent missing put/free/reference leak. The issue is an internal infinite loop.

Negative signals:

* -1 Conservative only: the commit explicitly says the issue requires max_thres higher than UINT_MAX / 2, unrealistically large aggregation intervals compared to the sampling interval, and an unrealistically large number of non-contiguous regions.

Important non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Remote reachability not established.
* RealUAF not established.
* Concrete repeated release/refcount operation not established.
* Structured stale-object leverage not awarded because there is no stale active object use.
* Extra Availability not awarded because the only demonstrated consequence is the same infinite loop / CPU consumption.
* ImportantCandidate not set because this is DoS-only arithmetic overflow behavior with no unresolved kernel corruption primitive.

3. Reachability analysis

The trigger requires DAMON parameters and region layout to reach an extreme state where max_thres exceeds UINT_MAX / 2 and regions cannot be merged below the configured upper limit. The supplied text describes this as an unreasonable user setup and unlikely in real world use.

DAMON configuration is typically controlled through kernel administrative interfaces, so a normal deployment likely requires host administrative control or a privileged monitoring service. However, the supplied patch does not show the complete access-control path, so HostAdminRequired is UNKNOWN rather than YES. No evidence shows that the full operation is namespace-scoped, so NamespaceScopedPrivilege is NO.

There is no network trigger. There is no special hardware requirement. This is not a race. The issue is deterministic once the unreasonable DAMON configuration and region state are reached.

4. Primitive and severity interpretation

The exact primitive is an unsigned integer overflow in the merge threshold update logic, which can bypass the intended loop termination condition. The resulting behavior is an infinite internal loop in kdamond_merge_regions.

This is not an active structured UAF, fixed stale storage UAF, RCU lifetime bug, double release, refcount corruption, ownership violation, or authorization bypass. It is best classified as CLASS_C because the demonstrated consequence is DoS / invalid progress behavior only.

Demonstrated consequence: CPU consumption or a stuck DAMON monitoring thread.

Speculative consequence: broader system responsiveness degradation if the looping kernel thread consumes CPU for a long time. No privilege escalation, memory corruption, information disclosure, or integrity impact is supported.

ImportantCandidate=NO because there is no Important-preserving primitive. AutoDowngradeAllowed=YES because the issue is constrained DoS-only behavior with no unresolved corruption or security-boundary primitive.

5. One-sentence report phrase

DAMON kdamond_merge_regions can enter an infinite loop due to threshold overflow under unrealistic parameter and region configurations, causing local denial of service through CPU consumption without evidence of memory corruption or privilege escalation.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: DoS-only infinite loop, no UAF or corruption primitive, no network reachability, and explicitly unrealistic trigger conditions.

7. Call-site confidence

Call-site confidence: high

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: mm/damon/core: avoid infinite kdamond_merge_regions() internal loop
Commit: 8a96a06e04d210845be5d9581c0e9e338947e8f8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8a96a06e04d210845be5d9581c0e9e338947e8f8

Commit description:

Patch series "mm/damon: unurgent fixes for infinite loop, NULL de-ref and
races", v1.1.

Sashiko found a few issues in DAMON that could cause infinite loop, NULL
dereference and monitoring results degradation.  The first two sounds
scary but the infinite loop happens only under unreasonable user setup.
The NULL dereference is only in a unit test.  Monitoring results
degradation is trivial since it is only best-effort, and those happens
from only unlikely races.  Still those are bugs that better to fix if
possible.  Fix those.

This patch (of 6):

Due to online parameter update like events, the number of DAMON regions
could be higher than the user-set upper limit.  kdamond_merge_regions()
repeats merge regions until the number meets the limit, while doubling the
merge threshold up to the theoretical maximum threshold.  It is tried only
up to the theoretical maximum threshold because even the aggressive
merging can fail from reducing the number of regions under the
user-defined upper limit.  For example, there could be many user-defined
non-contiguous regions that cannot be merged.

The threshold based loop break condition is evaluated by comparing the
threshold for the next merging try against the theoretical maximum
threshold.  If max_thres is larger than UINT_MAX / 2, doubling the
threshold could make it overflow, and bypass the loop break condition.  In
the case, if the number of regions cannot be reduced under the upper limit
like explained above, the loop will run infinitely.

Prevent the case by doing the break condition check before doubling the
threshold.  Also, prevent the threshold exceeding the maximum threshold,
as it could overflow and apply the wrong merge threshold.

This issue is unlikely to occur in real world, since having the max_thres
higher than UINT_MAX / 2 require unrealistically large aggregation
intervals compared to the sampling interval.  Also, it requires an
unrealistically large number of uncontiguous regions setup.  Nonetheless,
the consequence is bad and the fix is simple.

The issue was discovered [1] by Sashiko.

Link: https://lore.kernel.org/20260715031002.108504-1-sj@kernel.org
Link: https://lore.kernel.org/20260715031002.108504-2-sj@kernel.org
Link: https://lore.kernel.org/20260709145425.96247-1-sj@kernel.org [1]
Fixes: 310d6c1 ("mm/damon/core: merge regions aggressively when max_nr_regions is unmet")
Signed-off-by: SJ Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 6.10.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  mm/damon/core.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=8a96a06e04d210845be5d9581c0e9e338947e8f8

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-89796 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89796

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:H/UI:N/S:U/C:N/I:N/A:H
  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: 0
  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
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).