[CVE-2026-53175][IMPORTANT] inet: frags: fix use-after-free caused by the fqdir_pre_exit() flush

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

CVE: CVE-2026-53175
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: inet: frags: fix use-after-free caused by the fqdir_pre_exit() flush
Commit: 0e823ca0e7391630784ae7dd0981b7ad170a93d9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0e823ca0e7391630784ae7dd0981b7ad170a93d9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53175
Analysis date: Mon, 29 Jun 2026 10:50:12 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race during inet fragment queue teardown can leave stale skb pointers after `fqdir_pre_exit()` flushes an incomplete queue, allowing a later fragment insertion path to dereference and possibly write through freed skb state, causing kernel memory corruption and likely DoS with potential LPE requiring manual review.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-53175 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53175

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-2026-53175	IMPORTANT	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:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in inet fragment queue teardown can leave fragments_tail and last_run_head pointing to freed skbs after fqdir_pre_exit() flushes an incomplete queue. A fragment that already found the queue before teardown can later resume, pass the incomplete queue check, and dereference the stale skb pointer in inet_frag_queue_insert(). The bug includes a slab UAF with reads from freed skb metadata and a possible write to next_frag on the append path, so this is a memory corruption class issue rather than a simple crash only bug. For the CVSS the PR:L is used because reliable triggering normally requires a local user or process that can control network namespace lifetime or otherwise force the affected namespace teardown. The vulnerable path is packet driven and can involve IPv4, IPv6, nf_conntrack_reasm6, and 6lowpan reassembly, but it is not a pure unauthenticated remote AV:N issue because the teardown race must also be controlled or hit. Impact is at least local denial of service via kernel crash and in the paranoid case may allow confidentiality and integrity impact or privilege escalation due to UAF memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)  SKIP CVE-2026-53175  SKIP The Fixes patch not applied yet, so unlikely that actual: 006a5035b495dec008805df249f92c22c89c3d2e YES  NO NO unknown 	MAYBE	REMOTE UAF NETWORK LINUS IPV6 KPANIC 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: **6**
* Paranoid score: **8**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* **Local unprivileged trigger: +1**. A local user or container workload may be able to create traffic and trigger namespace teardown conditions, especially when unprivileged user namespaces or delegated containers are enabled.
* **Weak LPE concern: +1**. The patch shows a stale freed skb pointer can be dereferenced and the append path can write to `next_frag`. This is stronger than a read-only stale dereference, but no controlled reclaim or reliable object replacement is shown.
* **Generic memory corruption, strong corruption primitive: +2**. This is a slab use-after-free involving freed skb objects, including a possible write-after-free.
* **Real lifetime corruption: +1**. The bug is caused by stale lifetime state after `inet_frag_queue_flush()` frees queued skbs while `fragments_tail` and `last_run_head` still point to them.
* **Reliable kernel crash / strong DoS: +1**. A stale skb dereference in fragment insertion can plausibly crash the kernel.
* **Common networking core / packet-processing path: +1**. The affected inet fragment infrastructure is shared by IPv4, IPv6, nf_conntrack reassembly, and 6lowpan paths.
* **Hard or unreliable race / special timing required: -1**. Triggering requires a race between netns teardown flush and a fragment that already obtained the queue before taking the queue lock.

Paranoid additional signals:

* **Confidentiality impact plausible: +1**. Because this is UAF memory corruption in skb handling, limited information exposure cannot be ruled out during manual triage.
* **Integrity impact plausible: +1**. The write to a freed skb linkage field creates a plausible corruption sink, although no reliable privilege escalation primitive is demonstrated.

Not counted:

* **Remote reachable / network-triggerable: +0**. Packets are involved, but the vulnerable race also requires namespace teardown or an equivalent local lifecycle condition. This is not a clean unauthenticated remote network bug.
* **Strong LPE plausibility: +0**. No attacker-controlled reclaim, callback overwrite, type confusion, or reliable object replacement is demonstrated.

## 3. Reachability analysis

The realistic trigger is local or container-adjacent. An attacker needs to create or influence fragmented packet processing while a network namespace is being torn down. On systems with unprivileged user namespaces enabled, a local unprivileged user may be able to create a user namespace and network namespace, generate fragmented traffic, and force teardown timing. In stricter deployments, this may require container privileges or administrative namespace control.

The affected path is in common networking fragment reassembly infrastructure and is not a rare driver-only path. IPv4, IPv6, nf_conntrack IPv6 reassembly, and 6lowpan share the relevant flush behavior. However, pure remote exploitation is not supported by the patch context because the teardown race is a local lifecycle condition.

## 4. Severity interpretation

This is above ordinary Moderate because the bug is a real race-based UAF in a shared networking path and includes a possible write-after-free to skb linkage. The conservative interpretation is **Actionable Moderate** because exploitation requires a hard race and local namespace lifecycle control. The paranoid interpretation reaches **Strong Important candidate** because UAF plus writable stale skb state can sometimes become privilege escalation after deeper object lifetime and reclaim analysis.

Theoretical impact includes local privilege escalation or broader memory corruption. Realistic evidence from the patch supports at least kernel crash and memory corruption, but does not demonstrate a reliable LPE chain.

## 5. One-sentence report phrase

A race during inet fragment queue teardown can leave stale skb pointers after `fqdir_pre_exit()` flushes an incomplete queue, allowing a later fragment insertion path to dereference and possibly write through freed skb state, causing kernel memory corruption and likely DoS with potential LPE requiring manual review.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

This is a race-based slab UAF in common networking fragment infrastructure with a possible write-after-free sink. Even though reliable exploitation is not proven and the race is local lifecycle dependent, the memory corruption class and shared networking exposure make auto-close unsafe.

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

Patch: inet: frags: fix use-after-free caused by the fqdir_pre_exit() flush
Commit: 0e823ca0e7391630784ae7dd0981b7ad170a93d9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0e823ca0e7391630784ae7dd0981b7ad170a93d9

Commit description:

On netns teardown, fqdir_pre_exit() walks the fqdir rhashtable and
flushes every fragment queue that is not yet complete using
inet_frag_queue_flush(). That helper frees all the skbs queued on the
fragment queue but does not set INET_FRAG_COMPLETE, and leaves
q->fragments_tail and q->last_run_head pointing at the freed skbs.
The queue itself stays in the rhashtable.

fqdir_pre_exit() first lowers high_thresh to 0 to stop new queue lookups,
but it cannot stop a fragment that already obtained the queue through
inet_frag_find() earlier and stalled just before taking the queue lock.
Once that fragment resumes after the flush and takes the queue lock,
it passes the INET_FRAG_COMPLETE check and then dereferences the freed
fragments_tail. inet_frag_queue_insert() reads FRAG_CB() and ->len of
that pointer and, on the append path, writes ->next_frag, causing a
slab use-after-free. IPv6, nf_conntrack_reasm6 and 6lowpan reassembly
share the same flush path and are affected as well.

Reset rb_fragments, fragments_tail and last_run_head in
inet_frag_queue_flush() so a flushed queue no longer points at the
freed skbs. A fragment that resumes after the flush and takes the
queue lock then finds an empty queue and starts a new run instead of
dereferencing the freed fragments_tail. ip_frag_reinit() already
performed this reset after its own flush, so drop the now duplicate
code there.

Cc: stable@vger.kernel.org
Fixes: 006a503 ("inet: frags: flush pending skbs in fqdir_pre_exit()")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Link: https://patch.msgid.link/ah6ukYq5G98LshdA@v4bel
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv4/inet_fragment.c
  net/ipv4/ip_fragment.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=0e823ca0e7391630784ae7dd0981b7ad170a93d9

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

The original Linux kernel CVE announcement for CVE-2026-53175 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53175

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: 8

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).