[CVE-2026-53191][MODERATE REGULAR] io_uring/net: inherit IORING_CQE_F_BUF_MORE across bundle recv retries

[CVE-2026-53191][MODERATE REGULAR] io_uring/net: inherit IORING_CQE_F_BUF_MORE across bundle recv retries

[AL-KERNEL]

CVE: CVE-2026-53191
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: io_uring/net: inherit IORING_CQE_F_BUF_MORE across bundle recv retries
Commit: f40570fda3f3a1f96aeaa4aef665ba274b2810b5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f40570fda3f3a1f96aeaa4aef665ba274b2810b5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53191
Analysis date: Thu, 25 Jun 2026 06:37:15 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review recommended / Actionable Moderate not proven
Manual review required: YES

Summary:
An io_uring bundle recv flag propagation bug can cause userspace to advance a provided buffer ring past a buffer entry still used by the kernel, leading to buffer ownership desynchronization and possible application-level data corruption or DoS.

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

The original Linux kernel CVE announcement for CVE-2026-53191 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53191

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: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-53191	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:L/I:L/A:H';CWE-670;*CWE-664;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.8';DESCR 'io_uring bundle recv can lose IORING_CQE_F_BUF_MORE across retry completion flag merging when provided buffer rings are used in incremental mode. Userspace may then believe that a partially consumed buffer is fully released and may advance the buffer ring head past an entry that the kernel still uses. This can desynchronize kernel and userspace buffer ownership and may cause data corruption or application level denial of service in software that relies on this io_uring mode. For the CVSS the PR:L is used because a local process must create the io_uring receive request with provided buffer rings, incremental consumption, and bundle receive enabled. The issue is not a direct network reachable kernel bug, although remote traffic may help trigger symptoms in a service that already uses this specific receive configuration. Impact is mainly integrity and availability at the userspace buffer management level, with no clear evidence of kernel memory corruption or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Actionable Moderate not proven (with actual score 4)	YES	IO_URING  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended / Actionable Moderate not proven**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local process can create the relevant io_uring recv operation with provided buffer rings, incremental consumption, and bundle recv.

* Local unprivileged high-control kernel data-plane API: +1
  io_uring with provided buffer rings gives userspace fine-grained control over buffer ownership and completion semantics.

* Integrity impact plausible: +1
  Userspace may advance the provided buffer ring head past an entry that the kernel still considers partially consumed, causing application-level buffer ownership desynchronization and possible data corruption.

Paranoid-only signal:

* Weak/indirect corruption candidate: +1
  The bug creates a stale or incorrect ownership signal for a buffer still used by the kernel. This is not a proven kernel memory corruption primitive, but it is subtle enough to justify manual review.

Not counted:

* Remote reachable: +0
  Network traffic may help trigger symptoms in an affected service, but the vulnerable setup must already exist locally. This is not directly remote-triggerable as a kernel interface.

* Generic memory corruption: +0
  The patch does not show UAF, OOB write, double free, arbitrary write, type confusion, or reclaimable stale kernel object reuse.

* Reliable kernel crash / strong DoS: +0
  The described impact is buffer state desynchronization, not a demonstrated kernel panic.

* Privilege escalation plausible: +0
  No concrete LPE primitive is shown.

## 3. Reachability analysis

The bug is reachable by a local user or local process using io_uring recv with provided buffer rings in incremental mode and bundle recv retries. Namespaces may matter because io_uring availability and restrictions vary by distribution and container policy, but the interface is generally a local user-accessible kernel API when not administratively restricted.

The issue is not directly network reachable. A remote peer could influence packet sizes, timing, or retry behavior only if a local service already uses this specific io_uring receive configuration. The path is not a generic TCP receive bug.

## 4. Severity interpretation

This behaves more like a borderline actionable Moderate than an Important-class kernel vulnerability. The realistic impact is application-level data corruption or denial of service due to incorrect buffer ownership reporting. Theoretical concern exists because io_uring buffer lifetime and ownership bugs can be subtle, but the patch does not demonstrate kernel memory corruption, privilege escalation, or a direct kernel crash.

## 5. One-sentence report phrase

An io_uring bundle recv flag propagation bug can cause userspace to advance a provided buffer ring past a buffer entry still used by the kernel, leading to buffer ownership desynchronization and possible application-level data corruption or DoS.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is a subtle io_uring provided-buffer ownership bug. It should not be treated as Important without a stronger kernel memory corruption primitive, but it also should not be blindly auto-closed without checking real affected service patterns.

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

Patch: io_uring/net: inherit IORING_CQE_F_BUF_MORE across bundle recv retries
Commit: f40570fda3f3a1f96aeaa4aef665ba274b2810b5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f40570fda3f3a1f96aeaa4aef665ba274b2810b5

Commit description:

When a bundle recv retries inside io_recv_finish(), the merge logic OR
the saved cflags from the previous iteration with the cflags returned by
the new iteration:
  cflags = req->cqe.flags | (cflags & CQE_F_MASK);

Bits listed in CQE_F_MASK are inherited from the new iteration, and all
other bits (notably IORING_CQE_F_BUFFER and the buffer ID) come from the
saved cflags. Before this change CQE_F_MASK covered only
IORING_CQE_F_SOCK_NONEMPTY and IORING_CQE_F_MORE.

When using provided buffer rings (IOU_PBUF_RING_INC) with incremental
mode, and bundle recv, io_kbuf_inc_commit() can leave the head ring
entry partially consumed, __io_put_kbufs() then sets
IORING_CQE_F_BUF_MORE on the returned cflags so userspace knows the
buffer ID will be reused for subsequent completions.

Because IORING_CQE_F_BUF_MORE was not in CQE_F_MASK, the merge above
silently dropped it whenever the final retry iteration partially
consumed the buffer, and the subsequent req->cqe.flags = cflags &
~CQE_F_MASK save would have left a stale IORING_CQE_F_BUF_MORE in the
carried-over cflags had one been present. Userspace would then
wrongfully advance it ring head past an entry the kernel still uses.

Add IORING_CQE_F_BUF_MORE to CQE_F_MASK so it is both inherited from the
new iteration into the user-visible CQE and stripped from the saved
cflags between iterations.

Cc: stable@vger.kernel.org
Signed-off-by: Clément Léger <cleger@meta.com>
Assisted-by: Claude:claude-opus-4.6
Fixes: ae98dbf ("io_uring/kbuf: add support for incremental buffer consumption")
Link: https://patch.msgid.link/20260604160715.2482972-1-cleger@meta.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  io_uring/net.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=f40570fda3f3a1f96aeaa4aef665ba274b2810b5

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

The original Linux kernel CVE announcement for CVE-2026-53191 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53191

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: 4

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).