* [CVE-2026-81000][MODERATE 7.0] net: tun: bound receive headroom
@ 2026-09-11 23:19 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-11 23:19 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-81000
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: tun: bound receive headroom
Commit: 379d85c7f25f3e05a428225e6b8a65613c6e9b9d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=379d85c7f25f3e05a428225e6b8a65613c6e9b9d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-81000
Analysis date: Fri, 11 Sep 2026 19:19:37 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-81000 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-81000
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-81000 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-81000 CHECK WITH IMPACT FROM ORIG NN LOW Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-191;CWE-681;CWE-122;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'An oversized TUN or TAP receive headroom can make SKB_MAX_HEAD(align) underflow in tun_get_user, causing the computed linear length to wrap when converted to size_t. This can let tun_alloc_skb place skb data outside the allocated skb head, which is a memory corruption condition rather than a pure crash only bug. The realistic trigger depends on a special TUN TAP and OVS headroom propagation setup, so the base score uses local access with high attack complexity. For the CVSS the PR:N is used for the paranoid case only when an attacker can inject packets through an already attached TAP or adjacent virtual network endpoint without host credentials. Impact is at least denial of service and in the worst still defensible interpretation may include confidentiality and integrity impact due to out of bounds skb memory corruption. This should be manually reviewed because the primitive is memory corruption in a network data path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 6) YES OOB SIMPLEFIX NETWORK SKB KPANIC PACKET KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=7
ActionableScoreLower=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 5
* Conservative score: 5. No floor or cap applied.
* Raw paranoid score: 7
* Paranoid score: 7. No floor or cap applied.
* Final recommended bucket: **Strong Important candidate**::
2. Signal breakdown
Conservative signals:
* +2 Strong corruption primitive. The patch establishes that an oversized TUN/TAP headroom can make `SKB_MAX_HEAD(align)` underflow, wrap into a large `size_t`, and allow `tun_alloc_skb()` to place `skb->data` outside the allocated skb head.
* +1 Reliable kernel crash / strong DoS. An skb data pointer outside its allocated head is a realistic kernel memory corruption and crash condition.
* +1 Memory layout invariant restoration. The fix restores consistency between headroom, one-page skb head budget, linear data requirements, and 16-bit skb header offset limits.
* +1 Common networking packet-processing path. The bug is in the TUN/TAP skb receive path and can be reached through virtual networking data paths such as OVS/TAP.
Paranoid additional signal:
* +2 Adjacent or virtual-network packet reachability. If an oversized headroom condition already exists in an OVS/TAP setup, an attacker able to inject packets through the attached virtual or adjacent endpoint may reach the vulnerable data path without host credentials.
Non-awarded signals:
* RealUAF not awarded. This is not a lifetime or stale-object bug.
* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated. The primitive is stronger than crash-only, but the patch does not establish arbitrary target selection.
* Confidentiality and integrity bonuses not separately awarded. They remain plausible consequences of the OOB corruption, but no disclosure path or protected-state overwrite is demonstrated.
* No race penalty. This is an arithmetic and bounds bug, not a timing race.
* No host-admin penalty. Host-admin-equivalent privilege is not positively established for every practical trigger model, especially where TAP/OVS is already provisioned.
* No namespace-scoped privilege conclusion. CAP_NET_ADMIN and OVS/TUN setup may be namespace-related in some deployments, but the complete trigger chain is not proven namespace-scoped.
3. Reachability analysis
The vulnerable path is `tun_get_user()` in `drivers/net/tun.c`. The required unsafe state is an oversized `tun->align`, which can be propagated to TUN or TAP through `ndo_set_rx_headroom`, with OVS mentioned as a propagation source.
Creating and configuring TUN/TAP or OVS normally involves privileged network control, commonly CAP_NET_ADMIN or a privileged management process. However, the packet injection stage may be reachable by a lower-privileged actor if a TAP/OVS/VM networking path is already configured. For example, a guest or adjacent virtual-network endpoint may be able to send packets that are ultimately written into the TAP path by the host networking stack or VMM.
HostAdminRequired is therefore UNKNOWN. The evidence does not prove that the attacker must be host admin for all realistic deployments, but it also does not prove that an ordinary local user can independently create the complete vulnerable setup.
NamespaceScopedPrivilege is UNKNOWN. TUN/TAP and network-device operations may be namespace-aware in some paths, but the supplied evidence does not establish that the complete OVS plus headroom propagation plus packet trigger chain can be performed entirely with attacker-obtainable namespace privileges.
No special physical hardware is required. No hard race is involved. Prior privileged setup may exist in practical virtualization or container networking deployments, but attacker-controlled setup is not fully established.
4. Primitive and severity interpretation
The exact primitive is an integer underflow and signed-to-unsigned size conversion issue in skb allocation layout computation. An oversized headroom causes an invalid linear length calculation, and `tun_alloc_skb()` may produce an skb whose `skb->data` points outside the allocated skb head.
This is an active packet-buffer memory corruption primitive, not a fixed stale-storage bug and not a cleanup-only fault. It is also not a UAF and does not involve repeated release/refcount mutation.
PrimitiveClass is CLASS_A because the patch establishes attacker-influenced kernel memory corruption in a packet data path. The demonstrated consequence is at least kernel crash or host DoS. The speculative but technically defensible consequence is stronger memory corruption impact, because packet contents and skb layout are attacker-influenced, although arbitrary read/write or privilege escalation is not demonstrated.
ImportantCandidate is YES because the primitive is stronger than crash-only and occurs in a network data path. AutoDowngradeAllowed is NO because automatically closing this as ordinary Moderate would lose a concrete OOB skb memory-corruption primitive with unresolved exploitation leverage.
Operationally, this should be treated as at least Actionable Moderate and, under the already-configured TAP/OVS attacker model, a Strong Important candidate requiring manual review.
5. One-sentence report phrase
An oversized TUN/TAP receive headroom can underflow skb headroom calculations in `tun_get_user()`, causing `skb->data` to be placed outside the allocated skb head and creating a kernel memory corruption condition reachable through affected virtual networking paths.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: the patch establishes a concrete OOB skb memory-corruption primitive in a TUN/TAP networking path, with plausible below-host-admin packet reachability in already configured OVS/TAP deployments.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: net: tun: bound receive headroom
Commit: 379d85c7f25f3e05a428225e6b8a65613c6e9b9d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=379d85c7f25f3e05a428225e6b8a65613c6e9b9d
Commit description:
tun_get_user() uses tun->align both as skb headroom and when choosing how
much packet data to keep linear. OVS can propagate an oversized headroom
request from another port to TUN or TAP.
When align is larger than the usable space in a one-page skb head,
SKB_MAX_HEAD(align) underflows and the result becomes negative when stored
in good_linear. That value later wraps when assigned to the size_t linear
variable, and tun_alloc_skb() can place skb->data outside the allocated
head.
Bound the headroom stored by TUN to the one-page skb-head budget and the
largest non-sentinel 16-bit skb header offset. Leave one linear byte for
raw TUN and a complete Ethernet header for TAP, including NET_IP_ALIGN.
Also pull the raw-TUN protocol byte and the TAP Ethernet header before
accessing them, so these checks remain safe for nonlinear skbs supplied by
other allocation paths.
Fixes: eaea34b ("net/tun: implement ndo_set_rx_headroom")
Cc: [email protected]
Signed-off-by: Asim Viladi Oglu Manizada <[email protected]>
Reviewed-by: Willem de Bruijn <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
drivers/net/tun.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=379d85c7f25f3e05a428225e6b8a65613c6e9b9d
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-81000 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-81000
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:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 7.5
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: 5
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: MODERATE
KPANIC detected for this report: YES
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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