[CVE-2026-63950][IMPORTANT] mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one

[CVE-2026-63950][IMPORTANT] mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one

[AL-KERNEL]

CVE: CVE-2026-63950
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one
Commit: 0fcc34d0d8fefca4fea349e45c10e3a3d90350eb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0fcc34d0d8fefca4fea349e45c10e3a3d90350eb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63950
Analysis date: Sun, 19 Jul 2026 15:32:23 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale nr_pages value in try_to_unmap_one can be reused across rmap walk iterations and corrupt folio refcount/mapcount accounting, causing a local kernel crash and creating a plausible memory lifetime corruption surface that should be manually reviewed.

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

The original Linux kernel CVE announcement for CVE-2026-63950 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63950

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: IMPORTANT
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
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CVE-2026-63950	IMPORTANT	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:H/I:H/A:H';CWE-911;CWE-665;CWE-682;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A stale nr_pages value in try_to_unmap_one() can be reused across page_vma_mapped_walk loop iterations when a previous batched unmap computed a larger range and a later iteration does not recompute it. Under specific memory management conditions involving lazyfree large anonymous folios and device exclusive mappings, this can corrupt folio refcount/mapcount accounting and lead to a kernel crash. For the CVSS the PR:L value is used because a local user process can set up relevant mappings and memory advice, although reliable triggering depends on specialized HMM or device exclusive behavior and reclaim timing. The issue is not network reachable. Impact is at least local denial of service via kernel crash. For the paranoid score, the refcount/mapcount corruption is treated as a memory management corruption primitive that may justify confidentiality and integrity impact in addition to availability impact.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-63950 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 354dffd29575cdf13154e8fb787322354aa9efc4 YES  NO NO unknown 	MAYBE	READ DANGER HARDWARE KPANIC MEMORY  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local userspace program can shape the relevant mappings with mmap and madvise. The exact device-exclusive and reclaim setup may depend on HMM capable hardware or drivers.
* Generic memory corruption: +2. The commit explicitly describes folio refcount/mapcount corruption in mm/rmap.
* Real lifetime corruption: +1. Refcount/mapcount corruption can affect folio lifetime and rmap accounting.
* Reliable kernel crash / strong DoS: +1. The commit states the userspace visible effect is kernel crashing somewhere due to refcount/mapcount corruption.
* Broad/default/common subsystem: +1. try_to_unmap_one and rmap/reclaim are core MM paths.
* Hard or special conditions: -1. Reliable triggering depends on lazyfree large anonymous folios, device-exclusive mappings, and reclaim timing or sysfs-triggered reclaim.

Paranoid additional interpretation:

* Privilege escalation plausible: +2. Not demonstrated, but folio refcount/mapcount corruption in core MM is a plausible UAF or lifetime-corruption review surface. This is stronger than a pure NULL dereference but still not evidence of arbitrary write or reliable LPE.

## 3. Reachability analysis

The bug is local, not network reachable. A local process can create anonymous large folio mappings and use MADV_FREE, but the full reproducer also requires device-exclusive PTE behavior through HMM and reclaim activity. Triggering node reclaim through sysfs may require privileges, but similar reclaim paths may occur under memory pressure. Containers or namespaces do not obviously make the core primitive remote, but access to HMM-capable devices or delegated device interfaces could lower practical privilege requirements in some deployments. Call-site confidence: high, because the affected loop and downstream folio_remove_rmap_ptes context are directly described.

## 4. Severity interpretation

This is more than an ordinary Moderate crash because the failure mode is refcount/mapcount corruption in core memory management, not just a warning or NULL dereference. Realistically demonstrated impact is local DoS via kernel crash under specialized conditions. Theoretical impact includes possible lifetime corruption or UAF-like behavior, but the provided patch does not demonstrate attacker-controlled reclaim, arbitrary write, object replacement, or a working privilege-escalation primitive. Therefore the conservative result is Actionable Moderate, while the paranoid result is a Strong Important candidate for manual review.

## 5. One-sentence report phrase

A stale nr_pages value in try_to_unmap_one can be reused across rmap walk iterations and corrupt folio refcount/mapcount accounting, causing a local kernel crash and creating a plausible memory lifetime corruption surface that should be manually reviewed.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

Reason: the demonstrated effect is DoS, but refcount/mapcount corruption in core MM is a plausible UAF or privilege-escalation review surface even though a concrete exploit primitive is not shown.

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

Patch: mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one
Commit: 0fcc34d0d8fefca4fea349e45c10e3a3d90350eb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0fcc34d0d8fefca4fea349e45c10e3a3d90350eb

Commit description:

Initialize nr_pages to 1 at the start of each loop iteration, like
folio_referenced_one() does.

Without this, nr_pages computed by a previous folio_unmap_pte_batch() call
can be reused on a later iteration that does not run
folio_unmap_pte_batch() again.

mmap a 64K large folio with MAP_ANONYMOUS | MAP_DROPPABLE, then call
madvise(MADV_FREE), then make the last page device-exclusive via
HMM_DMIRROR_EXCLUSIVE.

Trigger node reclaim through sysfs.  Now, in try_to_unmap_one(), we will
first clear the first 15 out of 16 entries mapping the lazyfree folio.
This will set nr_pages to 15.  In the next pvmw walk, this nr_pages gets
reused on a device-exclusive pte, thus potentially corrupting folio
refcount/mapcount.

At the moment, I have a userspace program which can make the kernel spit
out a trace, but the blow up is in folio_referenced_one(), because there
are existing bugs in the interaction between device-private and rmap
(which too I am investigating).  I did a one liner kernel change to avoid
going into folio_referenced_one(), and the kernel blows up at
folio_remove_rmap_ptes in try_to_unmap_one which is what I wanted.

Note that the bug is there not since file folio batching but lazyfree
folio batching, since device-exclusive only works for anonymous folios.

Userspace visible effect is simply kernel crashing somewhere due to
refcount/mapcount corruption.

Link: https://lore.kernel.org/20260518063656.3721056-1-dev.jain@arm.com
Fixes: 354dffd ("mm: support batched unmap for lazyfree large folios during reclamation")
Signed-off-by: Dev Jain <dev.jain@arm.com>
Acked-by: Barry Song <baohua@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Harry Yoo <harry@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  mm/rmap.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=0fcc34d0d8fefca4fea349e45c10e3a3d90350eb

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

The original Linux kernel CVE announcement for CVE-2026-63950 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63950

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:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 5
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).