[CVE-2026-46289][MODERATE 7.0] lib/scatterlist: fix length calculations in extract_kvec_to_sg

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-46289
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: lib/scatterlist: fix length calculations in extract_kvec_to_sg
Commit: 3f17500e86d730c76db638bb3ae52f9b5e496c76
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3f17500e86d730c76db638bb3ae52f9b5e496c76
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46289
Analysis date: Mon, 08 Jun 2026 15:33:54 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate at minimum. Not enough evidence for a Strong Important classification without caller analysis, but this should not be auto-closed.
Manual review required: YES

Summary:
`extract_kvec_to_sg() can create malformed scatterlist entries that cross page boundaries because it uses the remaining kvec length instead of the page bounded segment length, creating a plausible local DoS and a weaker but review worthy memory corruption concern in downstream storage or I/O consumers.`

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

The original Linux kernel CVE announcement for CVE-2026-46289 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46289

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
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CVE-2026-46289	MODERATE	CHECK	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-682;CWE-119;*CWE-787;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'extract_kvec_to_sg can build malformed scatterlist entries because it uses the remaining kvec length instead of the per page segment length when calling sg_set_page. This can make one sg entry extend past the current page boundary and may also over report the number of bytes extracted when the sglist runs out of entries. For the CVSS the PR:L is used because a local user or local process may be able to reach affected kernel I/O paths without full administrative privileges, although reliable triggering depends on a specific caller using ITER_KVEC and buffers that cross page boundaries. The issue is not directly network reachable from the patch alone. Impact is at least local denial of service via kernel crash or I/O failure, and in the worst case may include confidentiality or integrity impact if a downstream sg consumer performs memory access beyond the intended buffer.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 4) 	MAYBE	LEAK TEST MEMORY  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate at minimum. Not enough evidence for a Strong Important classification without caller analysis, but this should not be auto-closed.

## 2. Signal breakdown

Paranoid scoring used for `ActionableScore=5`:

* Local unprivileged trigger: +1
  Potentially reachable through local filesystem or storage I/O paths that use `extract_iter_to_sg()`, but exact callers need confirmation.

* Constrained kernel buffer overwrite / weak indirect corruption candidate: +1
  The bug can create malformed scatterlist entries whose length crosses a page boundary. This may cause downstream I/O or DMA consumers to access memory outside the intended page segment, but the patch does not show attacker-controlled target selection or arbitrary write.

* Privileged kernel/device-management memory corruption path: +1
  Scatterlists are trusted kernel I/O descriptors and may be consumed by storage, netfs, crypto, or DMA-adjacent paths.

* Reliable kernel crash / strong DoS: +1
  A malformed sg entry or incorrect extracted length can plausibly cause I/O failure, invalid memory access, or kernel crash depending on the consumer.

* Important filesystem/storage path: +1
  The affected helper is in generic scatterlist extraction logic and originated from netfs iterator handling.

* Confidentiality impact plausible: +1
  If a downstream consumer reads beyond the intended buffer, unintended kernel or adjacent page data exposure is theoretically possible.

* Integrity impact plausible: +1
  If a downstream consumer writes beyond the intended buffer, unintended memory or I/O buffer modification is theoretically possible.

Subtractions:

* Hard or special triggering conditions: -1
  Requires a specific `ITER_KVEC` caller, a kvec crossing a page boundary, and a downstream sg consumer affected by the malformed length.

* Corruption primitive highly constrained: -1
  No demonstrated arbitrary write, reclaim primitive, target control, or direct user-controlled overwrite payload is shown in this patch.

Conservative score is lower because it counts this primarily as a malformed sg descriptor with DoS potential and uncertain reachability, without assuming C/I impact.

## 3. Reachability analysis

The immediate affected path is not directly network-reachable from the patch alone. Triggering likely requires a local process to reach a kernel path that builds scatterlists from `ITER_KVEC` buffers via `extract_iter_to_sg()`. The commit references netfs history and generic scatterlist code, so filesystem or storage paths are the most relevant areas to inspect.

Namespaces and containers may matter if an unprivileged or reduced-privilege workload can trigger affected filesystem, netfs, splice-like, or storage I/O paths. There is no evidence that full host root is required, but there is also no confirmed simple unprivileged reproducer in the provided patch.

Realistic exploitation conditions are nontrivial because the bug needs a suitable caller and a downstream consumer where an oversized sg entry causes harmful memory access rather than just an error or crash.

## 4. Severity interpretation

This behaves like an Actionable Moderate issue with Important-candidate characteristics. The theoretical concern is memory corruption through malformed scatterlist length accounting, especially if an I/O or DMA consumer trusts the sg entry and accesses beyond the intended page. The realistic evidence in the patch is weaker: it demonstrates incorrect length calculation and return accounting, but not a concrete arbitrary write, UAF, page-cache corruption, or privilege escalation primitive.

Therefore the conservative result is borderline manual review, while the paranoid result is Actionable Moderate and should not be auto-closed.

## 5. One-sentence report phrase

`extract_kvec_to_sg() can create malformed scatterlist entries that cross page boundaries because it uses the remaining kvec length instead of the page bounded segment length, creating a plausible local DoS and a weaker but review worthy memory corruption concern in downstream storage or I/O consumers.`

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a generic scatterlist length accounting bug in an important I/O helper, with plausible downstream memory access beyond the intended buffer. Caller reachability and whether any consumer turns this into C/I impact or only DoS must be checked manually.

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

Patch: lib/scatterlist: fix length calculations in extract_kvec_to_sg
Commit: 3f17500e86d730c76db638bb3ae52f9b5e496c76
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3f17500e86d730c76db638bb3ae52f9b5e496c76

Commit description:

Patch series "Fix bugs in extract_iter_to_sg()", v3.

Fix bugs in the kvec and user variants of extract_iter_to_sg.  This series
is growing due to useful remarks made by sashiko.dev.

The main bugs are:
- The length for an sglist entry when extracting from
  a kvec can exceed the number of bytes in the page. This
  is obviously not intended.
- When extracting a user buffer the sglist is temporarily
  used as a scratch buffer for extracted page pointers.
  If the sglist already contains some elements this scratch
  buffer could overlap with existing entries in the sglist.

The series adds test cases to the kunit_iov_iter test that demonstrate all
of these bugs.  Additionally, there is a memory leak fix for the test
itself.

The bugs were orignally introduced into kernel v6.3 where the function
lived in fs/netfs/iterator.c.  It was later moved to lib/scatterlist.c in
v6.5.  Thus the actual fix is only marked for backports to v6.5+.

This patch (of 5):

When extracting from a kvec to a scatterlist, do not cross page
boundaries.  The required length was already calculated but not used as
intended.

Adjust the copied length if the loop runs out of sglist entries without
extracting everything.

While there, return immediately from extract_iter_to_sg if there are no
sglist entries at all.

A subsequent commit will add kunit test cases that demonstrate that the
patch is necessary.

Link: https://lkml.kernel.org/r/20260326214905.818170-1-lk@c--e.de
Link: https://lkml.kernel.org/r/20260326214905.818170-2-lk@c--e.de
Fixes: 0185846 ("netfs: Add a function to extract an iterator into a scatterlist")
Signed-off-by: Christian A. Ehrhardt <lk@c--e.de>
Cc: David Gow <davidgow@google.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: <stable@vger.kernel.org>	[v6.5+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/netfs/iterator.c
  lib/scatterlist.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=3f17500e86d730c76db638bb3ae52f9b5e496c76

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

The original Linux kernel CVE announcement for CVE-2026-46289 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46289

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: 3
  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.
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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
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