* [CVE-2026-99033][MODERATE REGULAR] net/packet: guard the ll header push in packet_rcv_spkt()
@ 2026-10-26 20:12 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-26 20:12 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99033
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/packet: guard the ll header push in packet_rcv_spkt()
Commit: d8a24b9e1774a93943caf68d48c5e17be432220b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d8a24b9e1774a93943caf68d48c5e17be432220b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99033
Analysis date: Mon, 26 Oct 2026 16:12:16 -0400
ActionableScore: 1
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
A CAP_NET_RAW packet socket bound to a headerless CAN interface can process an unset MAC-header sentinel as a negative push length and panic the kernel.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99033 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99033
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-2026-99033 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-99033 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC) 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-20;CWE-191;CWE-754;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'packet_rcv_spkt can call skb_push with an invalid negative length when a device lacking a visible link layer header leaves skb mac_header at the 0xFFFF sentinel. The bounds check reaches skb_under_panic and BUG from softirq context which causes a full system panic. The base path requires a local process with CAP_NET_RAW to create and bind a SOCK_PACKET socket to an affected interface such as slcan. For the CVSS the PR:N paranoid value applies only to an adjacent CAN sender when a suitable socket is already open by another local process. That scenario has AC:H because the attacker must depend on the preexisting socket and an affected driver receive path. The demonstrated primitive is a checked skb underflow followed by panic and there is no evidence of UAF OOB write information disclosure or privilege escalation. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 5.3 and ActionableScore 2 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1) MAYBE REMOTE DANGER SKB HARDWARE PACKET DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=1
ActionableScoreLower=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 0.
* Conservative score: 0; no floor or cap applied.
* Raw paranoid score: 1.
* Paranoid score: 1; no floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
* +1 Reliable kernel crash: the invalid negative `skb_push()` length reaches `skb_under_panic()` and `BUG` in softirq context, producing the demonstrated full-system panic.
* -1 Reduced privilege, base model: creating the required `SOCK_PACKET` socket is gated by `CAP_NET_RAW`, a narrower privilege than effective host administration.
* Paranoid difference: with a suitable socket already open and bound to the affected interface, an adjacent CAN sender can supply the triggering frame without holding that capability. No privilege penalty is applied to that actor, but the preexisting socket/interface requirement prevents a remote-reachability bonus.
* No OOB-write or memory-corruption point: `skb_push()` detects the underflow before performing an out-of-bounds data write.
* No UAF, disclosure, integrity impact, authorization bypass, LPE, or second availability point is shown.
* B5-R does not apply: there is no retry-accumulated heap OOB write or later consumption of corrupted structured state.
* B3-PENDING does not apply: no shared/file-backed write occurs.
3. Reachability analysis
The demonstrated path requires an affected headerless interface such as `slcan`, a `CAP_NET_RAW` process opening and binding a legacy `SOCK_PACKET` socket, and receipt of a frame. Numeric UID 0 is unnecessary. Existing `slcan` provisioning is prior setup, but the evidence does not establish that the complete setup and trigger are available within attacker-created namespaces; therefore namespace-scoped privilege remains unknown. No difficult race is involved.
4. Primitive and severity reading
The exact primitive is checked SKB underflow leading directly to a kernel BUG and host panic. It is Class C crash/DoS behavior, not an unchecked OOB write or active corrupted-state primitive. The panic is proven; non-crash exploitation is unsupported. ImportantCandidate is therefore NO, and normal Moderate handling remains permissible.
5. One-sentence report phrase
A CAP_NET_RAW packet socket bound to a headerless CAN interface can process an unset MAC-header sentinel as a negative push length and panic the kernel.
6. Manual review advice
MANUAL CHECK RECOMMENDED
Confirm affected interface provisioning and whether a less-privileged adjacent sender can reach a preexisting packet socket in the product deployment.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: net/packet: guard the ll header push in packet_rcv_spkt()
Commit: d8a24b9e1774a93943caf68d48c5e17be432220b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d8a24b9e1774a93943caf68d48c5e17be432220b
Commit description:
packet_rcv_spkt() restores the link layer header with
skb_push(skb, skb->data - skb_mac_header(skb));
That subtraction is only meaningful when the device actually has a link
layer header. packet_rcv() and tpacket_rcv() both wrap it in
dev_has_header(), which is also the predicate the block comment at the
top of the file states the restore in terms of; packet_rcv_spkt() does
not. commit d549699 ("net/packet: fix packet receive on L3
devices without visible hard header") introduced the helper and changed
the two call sites, and this one stayed behind.
A device without a visible ll header can leave skb->mac_header at the
0xFFFF sentinel that __alloc_skb() initialises it to. A CAN skb does:
init_can_skb() sets pkt_type and ip_summed but does not reset the
headers, and commit 9f10374 ("can: remove private CAN skb
headroom infrastructure") dropped the skb_reset_*_header() calls that
used to be there. skb_mac_header() is then 0xFFFF, the length becomes
a large negative number and skb_push() reports it through
skb_under_panic() -- from softirq context, so it is a full system panic
even with panic_on_oops=0:
skbuff: skb_under_panic: text:ffffffff8bd21bc1 len:-65455 put:-65471 head:... data:... tail:0x50 end:0x180 dev:can0
kernel BUG at net/core/skbuff.c:214!
RIP: 0010:skb_panic+0x50/0x60
Call Trace:
<IRQ>
skb_push+0x38/0x40
packet_rcv_spkt+0xe1/0x170
__netif_receive_skb_core.constprop.0+0x7e8/0xd30
...
Kernel panic - not syncing: Fatal exception in interrupt
The socket type is reachable: packet_create() accepts SOCK_PACKET
alongside SOCK_RAW and SOCK_DGRAM behind the same CAP_NET_RAW check,
and neither the socket length nor a capability check keeps it away
from a CAN interface.
The missing skb_reset_*_header() calls in init_can_skb() are a
regression in their own right and are being fixed separately, but a
packet socket should not turn a link layer that did not initialise its
mac header into a kernel panic. Guard the push the way the other two
receive paths do.
Tested on v7.3-rc5 under QEMU with a slcan device on a pty, which is
the driver RX path: vcan does not reproduce it, because can_send()
resets the headers on the way out. One SOCK_PACKET socket bound to
can0 and one frame written into the line discipline panics an
unpatched kernel with the trace above; the same image with this patch
prints no panic and powers off normally. Both kernels are this tree,
defconfig plus CONFIG_CAN_SLCAN=y, differing only in this hunk.
Fixes: d549699 ("net/packet: fix packet receive on L3 devices without visible hard header")
Cc: [email protected]
Signed-off-by: Quchaosheng <[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:
net/core/skbuff.c
net/packet/af_packet.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=d8a24b9e1774a93943caf68d48c5e17be432220b
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99033 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99033
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:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
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: 0
Paranoid ActionableScore: 1
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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