[CVE-2026-89722][MODERATE REGULAR] PCI/sysfs: Fix out-of-bounds read in pci_write_legacy_io()

[CVE-2026-89722][MODERATE REGULAR] PCI/sysfs: Fix out-of-bounds read in pci_write_legacy_io()

[AL-KERNEL]

CVE: CVE-2026-89722
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: PCI/sysfs: Fix out-of-bounds read in pci_write_legacy_io()
Commit: e892f05f1f790d5721cd8e3c561ed76da7e08bae
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e892f05f1f790d5721cd8e3c561ed76da7e08bae
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89722
Analysis date: Fri, 11 Sep 2026 23:57:58 -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-2026-89722 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89722

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 REGULAR
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-2026-89722	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89722 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:L';CWE-125;CWE-126;*CWE-20;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '4.2';DESCR 'pci_write_legacy_io read 4 bytes from the kernfs write buffer even when userspace wrote only 1 or 2 bytes to the PCI legacy_io sysfs file. This creates a small slab out of bounds read of up to 2 bytes for a 1 byte write and 1 byte for a 2 byte write. The value is used for a legacy PCI I/O port write, so the realistic impact is limited to incorrect I/O data selection and possible KASAN reported kernel warning or local denial of service in debug kernels. For the CVSS the PR:H is used because legacy_io is root only and exists only on Alpha and PowerPC systems with HAVE_PCI_LEGACY. The issue is not network reachable and does not provide a clear privilege escalation path for an unprivileged local user. Impact is mainly low integrity or availability impact, with only limited confidentiality considered in the paranoid score due to the out of bounds read primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	READ KASAN OOB HARDWARE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=1
ActionableScoreLower=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=YES
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 was applied. No cap was required. The paranoid score is emitted as the final ActionableScore because it is higher than the conservative score.

2. Signal breakdown

Conservative signals:

* +1: Limited slab out-of-bounds read. `pci_write_legacy_io()` read a `u32` from a kernfs write buffer even for 1 or 2 byte writes.
* +1: Memory layout invariant violation. The read size did not match the userspace write size and the kernfs allocation size.
* -2: Host-admin-equivalent privilege is positively established. The commit explicitly states that `legacy_io` is root-only.
* 0: No reliable kernel crash signal. KASAN reports the invalid read, but a production host-wide crash is not established.
* 0: No confidentiality signal. The overread bytes are not shown to be copied back to userspace, the network, or another attacker-visible channel.
* 0: No security-boundary integrity signal. The affected interface is root-only, and incorrect PCI I/O byte selection does not establish unauthorized modification by a lower-privileged actor.

Paranoid additional signal:

* +1: Low constrained integrity concern. On PowerPC, the endian and shift behavior could cause the wrong byte or word to be written to legacy PCI I/O port space, but this remains root-only and device/local-interface constrained.

Important non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Use-after-free not demonstrated.
* Remote reachability not established.
* Privilege escalation not established.
* Confidentiality not awarded because the OOB read has no demonstrated disclosure path.
* Extra availability not awarded because KASAN warning/debug detection is not the same as a reliable host-wide DoS.
* Strong corruption not awarded because this is a bounded read of at most 1 or 2 bytes past a small kernfs buffer, not a controlled overwrite or lifetime bug.

3. Reachability analysis

The vulnerable operation is a write to the PCI `legacy_io` sysfs file. The commit explicitly states that this file is root-only and exists only on Alpha and PowerPC, the architectures defining `HAVE_PCI_LEGACY`.

The complete trigger therefore requires host root or equivalent host-admin access. No namespace-scoped reachability is established. No ordinary local unprivileged path is established. No network path is involved.

There is no race requirement. The bug is deterministic once a privileged writer performs a 1 or 2 byte write to the affected sysfs file. The architecture restriction is real, but it is not separately scored beyond the root-only restriction in the conservative arithmetic to avoid over-penalizing the same practical exposure.

4. Primitive and severity interpretation

The exact primitive is a small bounded slab out-of-bounds read from the kernfs write buffer. For a 1 byte write, the old code could read up to 2 bytes beyond the allocation. For a 2 byte write, it could read 1 byte beyond the intended allocation.

This is not an active structured object lifetime issue. It is not fixed stale storage. It is not UAF. It is not a repeated release or refcount mutation. It is not a cleanup/destructor lifetime bug.

Primitive class is CLASS_C because the demonstrated behavior is a limited invalid read and incorrect local device I/O value handling without an independently useful exploitation primitive.

The demonstrated consequence is KASAN-reported slab out-of-bounds read and potentially wrong data written to legacy PCI I/O port space on affected PowerPC cases. The speculative consequence would be limited local device misbehavior or debug-kernel warning. There is no supported path to LPE, arbitrary kernel read/write, or sensitive information disclosure.

ImportantCandidate=NO because the primitive is constrained, root-only, architecture-limited, and lacks meaningful exploitation leverage beyond the invalid read and wrong-value write behavior.

AutoDowngradeAllowed=YES because no Class A primitive, meaningful Class B lifetime issue, authorization bypass, shared-page ownership violation, or unresolved strong corruption primitive is established.

Likely operational severity is Low to ordinary Moderate.

5. One-sentence report phrase

A root-only PCI sysfs legacy_io write path could read up to 2 bytes past the kernfs write buffer for short writes, causing KASAN-reported OOB reads and potentially incorrect legacy PCI I/O data on Alpha or PowerPC systems, without evidence of unprivileged reachability, disclosure, or privilege escalation.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the issue is root-only, not network reachable, not a UAF, not a write primitive, and has no demonstrated disclosure or privilege-escalation path.

7. Call-site confidence

Call-site confidence: high

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

Patch: PCI/sysfs: Fix out-of-bounds read in pci_write_legacy_io()
Commit: e892f05f1f790d5721cd8e3c561ed76da7e08bae
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e892f05f1f790d5721cd8e3c561ed76da7e08bae

Commit description:

pci_write_legacy_io() loads 4 bytes from the kernfs write buffer
regardless of how many bytes userspace wrote:

  if (count != 1 && count != 2 && count != 4)
          return -EINVAL;

  return pci_legacy_write(bus, off, *(u32 *)buf, count);

kernfs_fop_write_iter() allocates the buffer with kmalloc(len + 1),
so a 1-byte write to the legacy_io sysfs file allocates 2 bytes and
the unconditional u32 load reads up to 2 bytes past the end of the
allocation, which KASAN reports as a slab-out-of-bounds read.
Similarly, a 2-byte write overreads by 1 byte.

Thus, read only the number of bytes requested using get_unaligned_le16()
and get_unaligned_le32() for the 2 and 4 byte cases, interpreting the
buffer as little-endian to match the byte ordering of PCI I/O port
space.

The PowerPC implementation previously compensated for the generic
code's native-endian 32-bit load by shifting the value into place
for the 1 and 2 byte cases.  The shifts were only correct on
big-endian kernels.

On little-endian PowerPC (POWER8 and later), they extracted the wrong
bytes, so a 1-byte write wrote an out-of-bounds byte instead of the
requested value.  On big-endian, the native load also caused out_le16()
and out_le32() to reverse the user's bytes on the wire for 2 and 4 byte
writes.  The little-endian helpers resolve both issues, so the shifts
are removed.

No changes are needed for the Alpha platform.

The legacy_io file is root-only and exists only on Alpha and PowerPC,
the two architectures that define HAVE_PCI_LEGACY.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260616163131.2763281-1-kwilczynski@kernel.org
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/powerpc/kernel/pci-common.c
  drivers/pci/pci-sysfs.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=e892f05f1f790d5721cd8e3c561ed76da7e08bae

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

The original Linux kernel CVE announcement for CVE-2026-89722 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89722

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:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 4.2

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

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).