[CVE-2026-64426][MODERATE REGULAR] io_uring/nop: fix file reference leak with IOSQE_FIXED_FILE

[CVE-2026-64426][MODERATE REGULAR] io_uring/nop: fix file reference leak with IOSQE_FIXED_FILE

[AL-KERNEL]

CVE: CVE-2026-64426
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: io_uring/nop: fix file reference leak with IOSQE_FIXED_FILE
Commit: 722869fcff598fad20d5ab79c305897a7534708b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=722869fcff598fad20d5ab79c305897a7534708b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64426
Analysis date: Sat, 25 Jul 2026 10:22:32 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A local io_uring NOP request with mismatched fixed-file flags can leak fget file references, allowing repeated local abuse to consume kernel resources and potentially cause denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-64426 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64426

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-2026-64426	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-772;*CWE-401;CWE-404;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A file reference leak in io_uring NOP handling can occur when IOSQE_FIXED_FILE is set but IORING_NOP_FIXED_FILE is not set. The request is marked as fixed file during generic initialization, but io_nop can still acquire a normal fget reference and completion will not drop it because REQ_F_FIXED_FILE remains set. A local user can potentially repeat this pattern to accumulate unreleased file references and consume kernel resources. For the CVSS the PR:L is used because triggering requires a local process able to submit io_uring requests, not remote network access. The issue is not network reachable. Impact is denial of service through resource exhaustion. No confidentiality or integrity impact is supported because the primitive is a reference leak rather than UAF, OOB access, or arbitrary write.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 3)	YES	LEAK TEST IO_URING SYZBOT  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local process able to submit io_uring NOP requests can trigger the mismatched IOSQE_FIXED_FILE and IORING_NOP_FIXED_FILE path.

* Availability impact realistic: +1
  Repeated submissions can leak file references and consume kernel resources. This is resource exhaustion, not immediate memory corruption.

Paranoid-only signal:

* Local unprivileged high-control kernel data-plane API refinement: +1
  io_uring gives local users fine-grained request construction and lifetime control. This raises triage interest slightly, but here the primitive is still only a reference leak.

Not applied:

* Generic memory corruption: +0
  The patch shows a file reference leak, not UAF, double-free, OOB write, type confusion, or arbitrary write.

* Privilege escalation plausible: +0
  No reclaim, stale callback, overwrite, refcount takeover, or object confusion primitive is shown.

* Reliable kernel crash / strong DoS: +0 conservative
  The issue is a gradual leak. It may lead to DoS under sustained abuse, but not a single-shot reliable crash.

* Confidentiality / Integrity impact: +0
  No information disclosure or write/corruption primitive is supported.

## 3. Reachability analysis

A local user with access to io_uring can submit the affected NOP request pattern. The issue is not network reachable. Namespaces or containers may matter only if io_uring is available to the contained workload, but the attacker still needs local code execution in that environment. The affected path is a local kernel API path and does not require CAP_SYS_ADMIN in the generic case, although some products may restrict io_uring for untrusted users.

Call-site confidence: high. The patch and commit message directly show acquisition through io_file_get_normal() and missing release due to REQ_F_FIXED_FILE.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like local resource exhaustion issue. The theoretical worst case is local DoS through repeated file reference leaks. Realistic exploitation evidence does not support memory corruption, privilege escalation, confidentiality impact, or integrity impact. It should not be treated as Important-class based on the provided patch.

## 5. One-sentence report phrase

A local io_uring NOP request with mismatched fixed-file flags can leak fget file references, allowing repeated local abuse to consume kernel resources and potentially cause denial of service.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a local resource leak with no demonstrated memory corruption, no network reachability, and no plausible privilege escalation primitive in the patch context.

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

Patch: io_uring/nop: fix file reference leak with IOSQE_FIXED_FILE
Commit: 722869fcff598fad20d5ab79c305897a7534708b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=722869fcff598fad20d5ab79c305897a7534708b

Commit description:

NOP file-acquisition support choses between a fixed (registered) file and
a normal fget()'d file based on its own IORING_NOP_FIXED_FILE flag in
sqe->nop_flags. However, a request's REQ_F_FIXED_FILE is set
independently from the generic IOSQE_FIXED_FILE sqe flag during request
init, before the issue handler runs.

If a NOP is submitted with IOSQE_FIXED_FILE set (so REQ_F_FIXED_FILE is
set) but without IORING_NOP_FIXED_FILE, io_nop() takes the normal path
and grabs a real reference via io_file_get_normal(). On completion,
io_put_file() only drops the reference when REQ_F_FIXED_FILE is clear,
so the fget()'d file is never released and leaks:

  BUG: memory leak
  unreferenced object 0xffff88800f42c240 (size 176):
    kmem_cache_alloc_noprof+0x358/0x440
    alloc_empty_file+0x57/0x180
    path_openat+0x44/0x1e50
    do_file_open+0x121/0x200
    do_sys_openat2+0xa7/0x150
    __x64_sys_openat+0x82/0xf0

Decide between fixed and normal file acquisition from REQ_F_FIXED_FILE,
the same way io_assign_file() does for every other opcode, and fold
IORING_NOP_FIXED_FILE into REQ_F_FIXED_FILE at prep time.

Cc: stable@vger.kernel.org
Fixes: a85f310 ("io_uring/nop: add support for testing registered files and buffers")
Reported-by: syzbot+2cd473471e77bda12b0e@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?id=879092631b98f73a28ea405adacfa5bb34a14a25
Signed-off-by: Vasileios Almpanis <vasilisalmpanis@gmail.com>
Link: https://patch.msgid.link/20260615144619.482749-1-vasilisalmpanis@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  io_uring/nop.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=722869fcff598fad20d5ab79c305897a7534708b

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

The original Linux kernel CVE announcement for CVE-2026-64426 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64426

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:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 2
  Paranoid ActionableScore: 3

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).