[CVE-2026-64293][MODERATE 7.0] iommufd: Use sizeof(*hdr) instead of sizeof(hdr) in veventq read

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

CVE: CVE-2026-64293
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: iommufd: Use sizeof(*hdr) instead of sizeof(hdr) in veventq read
Commit: 04a177f91160ee18da98f5689482cf0f589ec869
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=04a177f91160ee18da98f5689482cf0f589ec869
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64293
Analysis date: Sat, 25 Jul 2026 06:34:46 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum / manual review required, not Strong Important on current evidence
Manual review required: YES

Summary:
A sizeof pointer versus sizeof object bug in iommufd vEVENTQ read can undercount the copied event header on 32-bit systems, allowing copy_to_user to write past the caller supplied buffer and potentially corrupt a local VMM or service process memory.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64293 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64293

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-2026-64293	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-467;CWE-131;*CWE-787;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.8';DESCR 'iommufd vEVENTQ read uses sizeof(hdr) instead of sizeof(*hdr) in the bounds check for normal vEVENT records. On 32-bit systems this undercounts the header by 4 bytes, allowing read() to copy more bytes to the user buffer than the caller supplied in count. The immediate primitive is an out of bounds write into user memory, not kernel memory corruption. For the CVSS the PR:L is used because a local process must be able to access iommufd and reach the vEVENTQ read path, but full host administrator privileges are not necessarily required in delegated virtualization setups. The issue is not network reachable. Impact is primarily local userspace memory corruption or process crash, with a paranoid concern for privileged VMM or service memory corruption if such a component reads attacker influenced vEVENT data.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum / manual review required, not Strong Important on current evidence (with actual score 4)	YES	IMPROVEONLY  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	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: **Actionable Moderate at minimum / manual review required, not Strong Important on current evidence**::

## 2. Signal breakdown

Conservative signals:

* Local trigger: +1. A local process must access the iommufd vEVENTQ read path.
* Weak or constrained corruption candidate: +1. The bug can make the kernel write past the user supplied read buffer, but this is an out of bounds write into user memory, not kernel memory corruption.
* Integrity impact plausible in limited scope: +1. The immediate integrity impact is corruption of the caller userspace memory.
* Constrained payload or target: -1. The copied data is the vEVENT header and payload, not an arbitrary attacker selected kernel write primitive.
* Rare/currently limited condition: -1. The concrete mismatch exists on 32-bit today because sizeof(void *) is 4 while the header is 8. On 64-bit it is currently latent unless the header grows.

Paranoid additional signals:

* Privileged virtualization service concern: +1. iommufd is commonly used by VMM or device assignment infrastructure, so corruption of a privileged userspace reader is a defensible manual-review concern.
* Availability impact plausible: +1. A privileged VMM or management process could crash if it reads attacker influenced vEVENT data into a too-small buffer.
* Security boundary concern, weak: +1. If a less privileged VM or device-facing actor can influence events consumed by a more privileged VMM, the userspace overwrite may cross a practical privilege boundary.

Not awarded:

* No remote/network trigger.
* No kernel memory corruption.
* No demonstrated arbitrary write, UAF, type confusion, reclaim primitive, or direct LPE.
* No Dirty-Pipe-like shared page or page-cache corruption primitive.

## 3. Reachability analysis

The bug is reachable through the local iommufd vEVENTQ read path. It requires a process with access to the relevant iommufd object and a vulnerable read buffer size. Full host root is not necessarily required in delegated virtualization setups, but ordinary unprivileged users may not have this path by default depending on device permissions and product configuration.

The concrete current impact is mainly 32-bit specific. On 64-bit, sizeof(hdr) and sizeof(*hdr) currently both evaluate to 8, so the issue is mostly latent unless the header layout changes. The path is not network reachable by itself. Containers and namespaces matter only if iommufd access is delegated into a reduced-privilege environment.

Call-site confidence: medium. The vulnerable copy_to_user behavior is clear from the provided function context, but event producer control and real VMM usage patterns are not fully shown in the patch.

## 4. Severity interpretation

This behaves more like an Actionable Moderate than an Important-class kernel vulnerability. The primitive is a kernel-mediated userspace buffer overwrite, not kernel memory corruption. Realistic direct impact is userspace memory corruption or process crash.

The paranoid score is higher because iommufd is a virtualization/device-management interface and the reader may be a privileged VMM or service. However, without evidence of attacker-controlled payload, attacker-selected privileged target memory, or a demonstrated VMM compromise chain, this should not be treated as a Strong Important candidate.

## 5. One-sentence report phrase

A sizeof pointer versus sizeof object bug in iommufd vEVENTQ read can undercount the copied event header on 32-bit systems, allowing copy_to_user to write past the caller supplied buffer and potentially corrupt a local VMM or service process memory.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug has an out of bounds userspace write primitive at a virtualization related kernel API boundary. It is not kernel memory corruption, but the possibility of corrupting a privileged VMM or delegated service makes auto-closing unsafe without checking actual product exposure, architecture support, and iommufd permissions.

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

Patch: iommufd: Use sizeof(*hdr) instead of sizeof(hdr) in veventq read
Commit: 04a177f91160ee18da98f5689482cf0f589ec869
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=04a177f91160ee18da98f5689482cf0f589ec869

Commit description:

The bound-check in iommufd_veventq_fops_read() for the normal vEVENT
path uses sizeof(hdr) where the surrounding code uses sizeof(*hdr):

	if (!vevent_for_lost_events_header(cur) &&
	    sizeof(hdr) + cur->data_len > count - done) {

hdr is declared as struct iommufd_vevent_header *, so sizeof(hdr)
evaluates to the size of the pointer.  Surrounding code uses
sizeof(*hdr) consistently:

	if (done >= count || sizeof(*hdr) > count - done) {
	...
	if (copy_to_user(buf + done, hdr, sizeof(*hdr))) {
	...
	done += sizeof(*hdr);

struct iommufd_vevent_header is currently 8 bytes (two __u32 fields,
flags and sequence), so on 64-bit (sizeof(void *) == 8) the two
expressions happen to be equal and the check works as intended.

On 32-bit (sizeof(void *) == 4) the check under-counts the header by
4 bytes: a vEVENT whose data_len causes 8 + cur->data_len to exceed
count - done while 4 + cur->data_len does not will pass the check,
then the loop will copy_to_user 8 bytes of header followed by data_len
bytes of payload, writing past the user-supplied buffer.

It is also a latent bug for any future expansion of struct
iommufd_vevent_header beyond sizeof(void *) on 64-bit; the check
should not depend on the type happening to match the host pointer
width.

Use sizeof(*hdr) to match the rest of the function and the actual
amount that will be copied.

Fixes: e36ba5a ("iommufd: Add IOMMUFD_OBJ_VEVENTQ and IOMMUFD_CMD_VEVENTQ_ALLOC")
Link: https://patch.msgid.link/r/20260430175630.67078-1-kai.aizen.dev@gmail.com
Cc: stable@vger.kernel.org
Reported-by: Kai Aizen <kai.aizen.dev@gmail.com>
Signed-off-by: Kai Aizen <kai.aizen.dev@gmail.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/iommu/iommufd/eventq.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=04a177f91160ee18da98f5689482cf0f589ec869

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

The original Linux kernel CVE announcement for CVE-2026-64293 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64293

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:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 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.
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).