[CVE-2026-80810][IMPORTANT] io_uring/rsrc: fix folio size overflow in io_vec_fill_bvec()

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

CVE: CVE-2026-80810
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: io_uring/rsrc: fix folio size overflow in io_vec_fill_bvec()
Commit: 45c945107e007b1fb73e21cd220277652a45521a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=45c945107e007b1fb73e21cd220277652a45521a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80810
Analysis date: Fri, 04 Sep 2026 11:59:17 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An integer shift overflow in io_uring registered-buffer handling can make io_vec_fill_bvec compute a tiny folio_size for gigantic hugetlb folios, causing kernel bvec out-of-bounds writes and reads, with local DoS impact and plausible privilege-escalation concern on affected architectures.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80810 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80810

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-2026-80810	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-80810 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-787;CWE-125;CWE-190;CWE-758;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'An integer shift overflow in io_uring io_vec_fill_bvec can compute folio_size from int 1 instead of unsigned long 1UL when imu folio_shift is 32 or higher. On affected 64 bit systems with gigantic hugetlb folios, the segment estimator can allocate a small bvec array while the fill loop walks the same iovec in tiny chunks and writes past the end of res_bvec while also reading past imu bvec. For the CVSS the PR:L is used because a local user or process with access to io_uring and suitable hugepage backed registered buffers can reach this path without full administrator privileges, although hugepage availability is configuration dependent. The issue is not network reachable and requires local code execution on an affected architecture and configuration. Impact is at least local denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact due to kernel out-of-bounds write and read memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-80810 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	IMPROVEONLY IO_URING KPANIC 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered positive signals:

* Local unprivileged trigger: +1. The affected path is reachable by a local process using io_uring registered buffers, assuming access to suitable hugepage backed memory.
* Memory corruption, strong corruption primitive: +2. The commit explicitly describes writes past the end of the allocated res_bvec array and reads past imu->bvec.
* Weak LPE concern conservative: +1. Kernel OOB write exists, but target and payload control are constrained and no reclaim or arbitrary write primitive is demonstrated.
* Strong LPE plausibility paranoid: +2. In the paranoid interpretation, repeated kernel OOB writes in io_uring buffer construction are enough to justify LPE concern for manual review.
* Local unprivileged high-control kernel data-plane API refinement: +1. io_uring gives local users relatively fine grained control over registered buffers and I/O vector layout.
* Memory layout invariant restoration: +1. The patch restores consistency between folio size computation, bvec segment estimation, and the later fill loop.
* Reliable kernel crash / strong DoS: +1. OOB write and OOB read in kernel memory can realistically crash the kernel.
* Confidentiality impact plausible: +1. OOB read past imu->bvec is present, although no direct disclosure channel is demonstrated.
* Integrity impact plausible: +1. OOB write past res_bvec is present.

Negative signals:

* Hard or configuration-dependent trigger: -1. Reaching the bug requires folio_shift >= 32.
* Rare architecture / hugepage condition: -1. The described practical cases require gigantic hugetlb pages such as 16G pages on arm64 with 64K pages or similar powerpc configurations, while x86_64 is not affected by this specific overflow.
* Corruption primitive constrained or not fully attacker-controlled: -1. The overwrite is through bio_vec construction and the payload/target are not shown to be arbitrary.

Call-site confidence: high. The commit message describes both the estimator and fill-loop disagreement, and the changed helper is directly in the io_uring registered-buffer path.

## 3. Reachability analysis

A local process can trigger the vulnerable path through io_uring registered buffers if it can arrange a registered buffer backed by a folio with shift 32 or larger. This is not network reachable. Namespace and container impact depends on whether the environment exposes io_uring and suitable hugetlb resources to untrusted workloads. In typical deployments, the gigantic hugepage requirement makes exploitation configuration-dependent, but the affected API class is still security-relevant because io_uring is a high-control local kernel interface.

## 4. Severity interpretation

This is not an ordinary Moderate cleanup bug. The patch fixes an integer shift overflow that causes a real mismatch between allocation sizing and later array population, producing kernel out-of-bounds write and out-of-bounds read behavior. Realistic exploitation is limited by architecture and hugepage requirements, so the conservative score stays in Actionable Moderate range. The paranoid score reaches Strong Important candidate because the primitive is kernel memory corruption in a local high-control API and may plausibly support more than DoS after manual exploitability review.

## 5. One-sentence report phrase

An integer shift overflow in io_uring registered-buffer handling can make io_vec_fill_bvec compute a tiny folio_size for gigantic hugetlb folios, causing kernel bvec out-of-bounds writes and reads, with local DoS impact and plausible privilege-escalation concern on affected architectures.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch explicitly describes kernel OOB write and OOB read in io_uring, which is a local high-control API. Even though triggering requires uncommon gigantic hugetlb folios and affected architectures, the memory corruption primitive is strong enough to require manual security review rather than autoclosure.

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UPSTREAM PATCH SUMMARY
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Patch: io_uring/rsrc: fix folio size overflow in io_vec_fill_bvec()
Commit: 45c945107e007b1fb73e21cd220277652a45521a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=45c945107e007b1fb73e21cd220277652a45521a

Commit description:

io_vec_fill_bvec() computes the folio size with a plain int 1:

	unsigned long folio_size = 1 << imu->folio_shift;

imu->folio_shift is unsigned int and comes from folio_shift() of the
folio backing the registered buffer, so it can be 32 or more on a 64 bit
kernel. Shifting int 1 that far is undefined, and on x86 and arm64 the
count is taken modulo 32, so a shift of 34 yields 4 rather than 16G.
Every other folio_shift shift in this file already uses 1UL.

The result is that the segment estimate and the fill loop disagree.
io_estimate_bvec_size() sizes the bvec array with the real shift:

	max_segs += (iov[i].iov_len >> shift) + 2;

so a 1M iovec on a 16G folio is charged 2 segments, while
io_vec_fill_bvec() then walks the same iovec in folio_size chunks of 4
bytes and writes res_bvec[bvec_idx] a quarter of a million times, past
the end of the array it was given. src_bvec is advanced once per
iteration as well, so imu->bvec is read past its end at the same time.
validate_fixed_range() only checks that the range is inside the
registered buffer and does not bound the segment count.

Reaching it needs a folio with a shift of at least 32, which means a
gigantic hugetlb page: 16G on arm64 with 64K pages, where
CONT_PMD_SHIFT is 34 and hugetlb_add_hstate(CONT_PMD_SHIFT - PAGE_SHIFT)
registers that size, and likewise on powerpc. x86_64 tops out at 1G, so
a shift of 30, which still fits in int and is unaffected.

Use 1UL, as the rest of the file does.

Fixes: 9ef4cbb ("io_uring: add infra for importing vectored reg buffers")
Cc: stable@vger.kernel.org
Signed-off-by: Ali Ahmet Memis <ali@iusegentoo.com>
Link: https://patch.msgid.link/20260802163030.51005-1-ali@iusegentoo.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  io_uring/rsrc.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=45c945107e007b1fb73e21cd220277652a45521a

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

The original Linux kernel CVE announcement for CVE-2026-80810 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80810

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:L/I:L/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: 6
  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).