[CVE-2026-64458][MODERATE 7.0] mm/damon/ops-common: handle extreme intervals in damon_hot_score()

[CVE-2026-64458][MODERATE 7.0] mm/damon/ops-common: handle extreme intervals in damon_hot_score()

[AL-KERNEL]

CVE: CVE-2026-64458
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mm/damon/ops-common: handle extreme intervals in damon_hot_score()
Commit: 58321b4e6e4f0f412069ab27ccdd56292757343a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=58321b4e6e4f0f412069ab27ccdd56292757343a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64458
Analysis date: Sat, 25 Jul 2026 08:05:29 -0400
ActionableScore: 5
ActionableScore lower bound: 2
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum**:: Paranoid handling is Actionable Moderate. Conservative handling is lower because the normal trigger requires DAMON sysfs write permission.
Manual review required: YES

Summary:
Extreme DAMON monitoring intervals can cause divide-by-zero in `damon_hot_score()` and may also produce an out-of-range score used as a histogram index, leading to local kernel crash and a bounded OOB array-access concern when an attacker can write to DAMON sysfs controls.

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

The original Linux kernel CVE announcement for CVE-2026-64458 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64458

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: MODERATE 7.0
Manual review required: YES

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-2026-64458	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-369;CWE-129;*CWE-787;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'DAMON hot score calculation can mishandle extreme user supplied monitoring intervals. A zero sampling interval or zero aggregation interval can lead to divide by zero in damon_max_nr_accesses or damon_hot_score. A very large aggregation interval can make damon_hot_score return a value outside the valid 0 to DAMOS_MAX_SCORE range, and that value can later be used as an index into regions_score_histogram. For the CVSS the PR:L is used for the paranoid score because some deployments may delegate DAMON control to a service account, container root, or another reduced privilege local context rather than full host administration. The issue is not network reachable and requires local access to the DAMON sysfs control plane. Impact is at least local denial of service via kernel crash, and in the worst case may allow confidentiality or integrity impact due to out of bounds kernel array access.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 4)	YES	OOB UAF LINUS  DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7 DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: **2**
* Paranoid score: **5**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: Paranoid handling is Actionable Moderate. Conservative handling is lower because the normal trigger requires DAMON sysfs write permission.

## 2. Signal breakdown

### Conservative score: 2

Triggered signals:

* **Constrained kernel buffer overwrite / OOB array access candidate: +1**
  `damon_hot_score()` can return a value outside `[0, DAMOS_MAX_SCORE]`, and that value is used as an index into `regions_score_histogram`. The patch context supports OOB array access, but not a proven attacker-controlled arbitrary write.

* **Reliable kernel crash / strong DoS: +1**
  The commit gives a direct reproducer for divide-by-zero leading to kernel Oops in `damon_hot_score()`.

* **Requires admin/root/sysfs write permission in typical deployments: -2**
  The commit explicitly says sysfs write permission is required. DAMON admin sysfs control is normally restricted to privileged users.

* **Rare or configuration-dependent subsystem: -1**
  DAMON is not a universal default attack surface and requires the relevant DAMON/sysfs configuration and active use.

* **Availability impact realistic: +1**
  The divide-by-zero path is directly reproducible and can crash the kernel. This is counted because the Oops is explicit and host-wide, not merely a warning.

Conservative total: `+1 +1 -2 -1 +1 = 0`, raised to **2** for practical triage because the patch also includes an OOB array-access fix, not only divide-by-zero.

### Paranoid score: 5

Additional paranoid signals:

* **Local lower-privilege or delegated-control trigger: +1**
  Although typical access is root/admin, some environments may delegate DAMON control to a service account, container root with exposed host sysfs, or reduced-privilege automation. This supports a PR:L-style paranoid interpretation.

* **Weak LPE concern from OOB kernel array access: +1**
  The OOB index is not proven to be exploitable, but it is a concrete kernel memory safety concern beyond pure DoS.

* **Confidentiality impact plausible: +1**
  Only in the paranoid interpretation, because OOB kernel memory interaction may have side effects beyond crash, though no disclosure primitive is shown.

* **Integrity impact plausible: +1**
  Only in the paranoid interpretation, because an OOB histogram index may imply an OOB write/update depending on the caller path.

Paranoid total lands at **5** after keeping the admin/configuration limitations and avoiding escalation to Important without a demonstrated strong primitive.

## 3. Reachability analysis

The bug is triggered through DAMON configuration under sysfs, specifically by setting extreme monitoring intervals such as zero `sample_us`, zero `aggr_us`, or very large aggregation intervals. In typical deployments this requires privileged write access to DAMON sysfs controls, so it is not remotely reachable and not generally available to an unprivileged local user.

Namespaces and containers matter only if the host exposes DAMON sysfs controls or delegates DAMON management to a less privileged context. A normal unprivileged container should not be able to trigger this. A reduced-capability service account or container root with host sysfs access could make the paranoid PR:L interpretation reasonable.

Call-site confidence: **medium-high**. The patch and commit identify the direct caller relationship for the divide-by-zero and state that the returned score is used as an index into `regions_score_histogram`, but the exact histogram write/read call site is not fully shown in the provided diff.

## 4. Severity interpretation

This is stronger than an ordinary low-risk validation bug because it has a demonstrated kernel Oops and a separate OOB array-access condition. Realistically, the most likely impact is local privileged DoS. The OOB index makes manual review justified, but the patch does not show a strong corruption primitive such as attacker-controlled overwrite target, UAF reclaim, type confusion, callback control, or arbitrary write.

Therefore, conservative handling is **ordinary Moderate / Low-like due to privileged reachability**, while paranoid handling is **Actionable Moderate** because of the concrete OOB array-access path.

## 5. One-sentence report phrase

Extreme DAMON monitoring intervals can cause divide-by-zero in `damon_hot_score()` and may also produce an out-of-range score used as a histogram index, leading to local kernel crash and a bounded OOB array-access concern when an attacker can write to DAMON sysfs controls.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: the divide-by-zero path is clear DoS, but the OOB `regions_score_histogram` indexing makes this more than a pure crash bug and should be manually checked for whether the access is read-only, increment/write, or otherwise capable of corrupting adjacent kernel state.

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

Patch: mm/damon/ops-common: handle extreme intervals in damon_hot_score()
Commit: 58321b4e6e4f0f412069ab27ccdd56292757343a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=58321b4e6e4f0f412069ab27ccdd56292757343a

Commit description:

Fix three issues in damon_hot_score() that comes from wrong handling of
extreme (zero or too high) monitoring intervals user setup.

When the user sets sampling interval zero, damon_max_nr_accesses(), which
is called from damon_hot_score(), causes a divide-by-zero.  Needless to
say, it is a problem.

When the user sets the aggregation interval zero, the function returns
zero.  It is wrong, since the real maximum nr_acceses in the setup should
be one.  Worse yet, it can cause another divide-by-zero from its caller,
damon_hot_score(), since it uses damon_max_nr_accesses() return value as a
denominator.

When the user sets the aggregation interval very high, damon_hot_score()
could return a value out of [0, DAMOS_MAX_SCORE] range.  Since the return
value is used as an index to the regions_score_histogram array, which is
DAMOS_MAX_SCORE+1 size, it causes out of bounds array access.

The issues can be relatively easily reproduced like below.  The sysfs
write permission is required, though.

    # ./damo start --damos_action lru_prio --damos_quota_space 100M \
            --damos_quota_interval 1s
    # cd /sys/kernel/mm/damon/admin/kdamonds/0
    # echo 0 > contexts/0/monitoring_attrs/intervals/sample_us
    # echo 0 > contexts/0/monitoring_attrs/intervals/aggr_us
    # echo commit > state
    # dmesg
    [...]
    [  131.329762] Oops: divide error: 0000 [#1] SMP NOPTI
    [...]
    [  131.336089] RIP: 0010:damon_hot_score+0x27/0xd0
    [...]

Fix the divide-by-zero intervals problems by explicitly handling the zero
intervals in damon_max_nr_accesses().  Fix the out-of-bound array access
by applying [0, DAMOS_MAX_SCORE] bounds before returning from
damon_hot_score().

The issue was discovered [1] by Sashiko.

Link: https://lore.kernel.org/20260623135834.67189-1-sj@kernel.org
Link: https://lore.kernel.org/20260619202459.145010-1-sj@kernel.org [1]
Fixes: 198f0f4 ("mm/damon/vaddr,paddr: support pageout prioritization")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 5.16.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  include/linux/damon.h
  mm/damon/ops-common.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=58321b4e6e4f0f412069ab27ccdd56292757343a

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

The original Linux kernel CVE announcement for CVE-2026-64458 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64458

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 2
  Paranoid ActionableScore: 5

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: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE 7.0

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).