* [CVE-2026-99055][MODERATE REGULAR] tcp: reject net_iov in zerocopy receive mapping hints
@ 2026-10-27 1:11 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-27 1:11 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99055
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: tcp: reject net_iov in zerocopy receive mapping hints
Commit: 563d3564513496b9ce3b3ba7a5154427554d0f06
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=563d3564513496b9ce3b3ba7a5154427554d0f06
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99055
Analysis date: Mon, 26 Oct 2026 21:11:50 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
TCP zerocopy receive can dereference a NULL page for a `net_iov` fragment, causing a kernel crash on a specialized and not yet hardware-validated receive path.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99055 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99055
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-2026-99055 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-99055 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 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:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';**CWE-476;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.9';DESCR 'A NULL pointer dereference in can_map_frag can occur when TCP zerocopy receive processes an unreadable skb containing a net_iov fragment after copying a readable prefix. skb_frag_page returns NULL for net_iov and the old path passes that value to PageCompound. The demonstrated primitive is a kernel NULL dereference and denial of service without evidence of UAF, OOB access, information disclosure, or an arbitrary write. For the CVSS the PR:N value applies to the worst case where a network service automatically uses this receive path for data controlled by an unauthenticated remote peer. Exploitation still has high complexity because it requires CONFIG_NET_DEVMEM, a supported DMA buf bound RX queue, a suitable fragment layout, and hardware reachability that has not yet been tested end to end. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.9 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore 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_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 1 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 KASAN NULLPTR NETWORK SKB HARDWARE LINUS TEST MEMORY DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 1.
* Conservative score: 1; no floor or cap applied.
* Raw paranoid score: 1.
* Paranoid score: 1; remote reachability remains unproven and receives no bonus.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
* +1 Reliable kernel crash/strong DoS: `skb_frag_page()` returns NULL for a `net_iov`, and `PageCompound()` dereferences it. KASAN demonstrated the fault and panic under `panic_on_warn`.
* +0 weak corrupt concern: this is a NULL dereference, not a write, UAF, OOB access, or useful stale-object operation.
* No remote-reachability bonus: actual hardware end-to-end reachability was not tested and requires `CONFIG_NET_DEVMEM`, a suitable NIC, and a DMA-buf-bound RX queue.
* No rarity penalty: the prerequisites constrain exposure, but the supplied evidence does not prove sufficiently rare hardware or an attacker-inaccessible deployment.
* No extra Availability point: it would duplicate the same crash.
* No confidentiality, integrity, authorization, B3-PENDING, B4-PENDING, or B5-R primitive is shown.
3. Reachability analysis
The diagnostic directly injected the `net_iov`; it does not prove a normal attacker can create the fragment. A real path requires prior host-side device and RX-queue configuration plus supported hardware. A remote peer may influence TCP data and fragment sequencing once such a service exists, but the complete remote trigger is unverified. Required host privilege, namespace capability scope, and UID requirements therefore remain unknown. No timing race is involved.
4. Primitive and severity reading
The exact primitive is a deterministic NULL pointer dereference while evaluating an unreadable non-page-backed skb fragment. It is CLASS_C crash/DoS behavior, with no active-state corruption or later corrupted-state consumption. The patch-supported effect is a kernel fault; arbitrary write, disclosure, UAF, and privilege escalation are unsupported. ImportantCandidate is NO and auto-downgrade is allowed because no material non-crash primitive remains.
5. One-sentence report phrase
TCP zerocopy receive can dereference a NULL page for a `net_iov` fragment, causing a kernel crash on a specialized and not yet hardware-validated receive path.
6. Manual review advice
MANUAL CHECK RECOMMENDED
Confirm real NIC reachability, queue provisioning requirements, and whether remotely supplied traffic can reliably produce the required fragment sequence.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: tcp: reject net_iov in zerocopy receive mapping hints
Commit: 563d3564513496b9ce3b3ba7a5154427554d0f06
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=563d3564513496b9ce3b3ba7a5154427554d0f06
Commit description:
After copying a readable prefix, receive_fallback_to_copy() asks
tcp_zerocopy_set_hint_for_skb() where page mapping can resume. If the
next skb is unreadable, find_next_mappable_frag() passes its net_iov
fragment to can_map_frag().
skb_frag_page() returns NULL for a net_iov, but can_map_frag()
dereferences it in PageCompound(). A v7.2 KASAN run on a connected TCP
socket with 64 readable bytes followed by a 4096-byte NET_IOV_DMABUF
fragment reported:
BUG: KASAN: null-ptr-deref in can_map_frag
tcp_zerocopy_receive -> can_map_frag
Kernel panic - not syncing: KASAN: panic_on_warn set
The diagnostic inserted the net_iov directly because the test host has
no devmem-capable NIC. Hardware end-to-end reachability remains untested
and requires CONFIG_NET_DEVMEM plus a supported DMA-buf-bound RX queue.
Reject all net_iov fragments before skb_frag_page(). This covers both
DMABUF and IOURING net_iov types while leaving page-backed checks
unchanged. With the guard, the same queue copied the readable prefix,
returned a 4096-byte skip hint, and completed without a fault.
Fixes: 9f6b619 ("net: support non paged skb frags")
Cc: [email protected]
Signed-off-by: Daehyeon Ko <[email protected]>
Reviewed-by: Mina Almasry <[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/ipv4/tcp.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=563d3564513496b9ce3b3ba7a5154427554d0f06
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99055 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99055
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:N/A:H
The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS score: 5.9
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: unknown
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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