[CVE-2026-64102][IMPORTANT] RDMA/siw: Reject MPA FPDU length underflow before signed receive math

[CVE-2026-64102][IMPORTANT] RDMA/siw: Reject MPA FPDU length underflow before signed receive math

[AL-KERNEL]

CVE: CVE-2026-64102
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: RDMA/siw: Reject MPA FPDU length underflow before signed receive math
Commit: 683f7cfbf514193d63c0efa079f3352bde84c2e0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=683f7cfbf514193d63c0efa079f3352bde84c2e0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64102
Analysis date: Sun, 19 Jul 2026 23:34:52 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious connected Soft iWARP peer can send a malformed MPA FPDU with an underflowed length that reaches `skb_copy_bits()` as a huge copy size, causing KASAN-confirmed UAF/OOB read behavior and at least a remote DoS with plausible confidentiality or integrity concerns.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64102 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64102

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-2026-64102	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';CWE-191;CWE-195;*CWE-416;CWE-125;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H';Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '9.8';DESCR 'A peer controlled MPA FPDU length in RDMA siw can be smaller than the fixed DDP/RDMAP header length for the selected opcode. This causes an integer underflow in fpdu_part_rem and the resulting negative signed length can flow through memory checks and into skb_copy_bits as a huge copy length. KASAN reports a use after free read in skb_copy_bits with a multi gigabyte read size, so the issue should be treated as a memory corruption candidate rather than a simple protocol warning. For the CVSS the PR:N value is used because a malicious connected siw peer can send the malformed FPDU without needing a local account or privileges on the victim system. The issue is network reachable within environments where Soft iWARP RDMA endpoints are exposed, typically storage or data center networks rather than the public Internet. Impact is at least remote denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to the UAF/OOB copy primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	READ OOB DANGER INIT UAF NETWORK SKB KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: 7
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. A malicious connected Soft iWARP peer can send the malformed FPDU over the SIW TCP receive path.
* Generic memory corruption, strong primitive: +2. The patch context shows peer-controlled integer underflow reaching `skb_copy_bits()` as a huge copy length, with KASAN reporting use-after-free read.
* Real lifetime corruption: +1. KASAN reports UAF in `skb_copy_bits()`, not merely a warning or validation failure.
* Reliable kernel crash / strong DoS: +1. The described path can cause a kernel crash through a malformed FPDU.
* Confidentiality impact plausible: +1. A huge copy from stale or out-of-bounds skb memory may expose kernel or stale memory contents depending on destination visibility.
* Rare AND difficult-to-reach subsystem/configuration: -1. SIW/RDMA endpoints are usually deployed in storage or data-center fabrics and are not normally Internet-exposed.

Paranoid delta:

* Integrity impact plausible: +1. The negative length also bypasses memory range validation before a huge copy into the receive target, so a limited integrity concern is defensible even if not proven as arbitrary kernel write.

## 3. Reachability analysis

The bug is triggerable by a connected SIW/iWARP peer that can send a malformed FPDU with an MPA length smaller than the opcode header requirement. No local account or victim-side privileges are required once the SIW endpoint is reachable. This is network-triggerable in RDMA-over-TCP deployments, but practical exposure is usually limited to storage, HPC, or data-center networks rather than the public Internet. Namespaces and containers are not the main factor here. The key precondition is that Soft iWARP is enabled and accepting connections from an untrusted or compromised peer.

Call-site confidence: high. The commit message provides the downstream flow from `siw_get_hdr()` through `siw_tcp_rx_data()`, `siw_check_mem()`, `siw_rx_data()`, and `skb_copy_bits()`, plus a KASAN trace from the real TCP receive path.

## 4. Severity interpretation

This behaves more like an Important-class kernel memory-corruption candidate than an ordinary Moderate DoS. The realistic demonstrated impact is remote or network-adjacent kernel crash. The theoretical higher impact is supported by a concrete primitive: peer-controlled length underflow, signed-to-unsigned copy length expansion, failed memory bounds validation, and KASAN UAF read. There is no demonstrated arbitrary write or exploit chain, so the conservative score stays below Critical-class handling, but the issue should not be auto-closed as a routine RDMA protocol validation bug.

## 5. One-sentence report phrase

A malicious connected Soft iWARP peer can send a malformed MPA FPDU with an underflowed length that reaches `skb_copy_bits()` as a huge copy size, causing KASAN-confirmed UAF/OOB read behavior and at least a remote DoS with plausible confidentiality or integrity concerns.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is network-triggerable in SIW deployments and the patch context shows a concrete memory-corruption path, not just input validation or a warning.

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

Patch: RDMA/siw: Reject MPA FPDU length underflow before signed receive math
Commit: 683f7cfbf514193d63c0efa079f3352bde84c2e0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=683f7cfbf514193d63c0efa079f3352bde84c2e0

Commit description:

A malicious connected siw peer can send an iWARP FPDU whose MPA length
field (c_hdr->mpa_len, 16 bit big-endian, peer-controlled) is smaller
than the fixed DDP/RDMAP header for the announced opcode. Soft-iWARP
parses the full header in siw_get_hdr() based on iwarp_pktinfo[opcode]
.hdr_len, but never compares mpa_len against that header length.

siw_tcp_rx_data() then derives

    srx->fpdu_part_rem = be16_to_cpu(mpa_len) - fpdu_part_rcvd
                         + MPA_HDR_SIZE;

where fpdu_part_rcvd equals iwarp_pktinfo[opcode].hdr_len at this
point. For a tagged WRITE (hdr_len 16, MPA_HDR_SIZE 2) the smallest
on-wire mpa_len of 0 yields fpdu_part_rem = -14, and any mpa_len below
hdr_len - MPA_HDR_SIZE underflows to a negative int.

The signed value then flows into siw_proc_write()/siw_proc_rresp() as

    bytes = min(srx->fpdu_part_rem, srx->skb_new);

is handed to siw_check_mem() as an int len (whose interval check
addr + len > mem->va + mem->len is satisfied for a valid base when
len is negative), and reaches siw_rx_data() -> siw_rx_kva() /
siw_rx_umem() -> skb_copy_bits() as a signed copy length. The header
copy branch in skb_copy_bits() promotes that to size_t, producing a
multi-gigabyte read.

KASAN under a KUnit harness that drives the real kernel TCP receive
path -- a loopback AF_INET socketpair, the malformed FPDU written via
kernel_sendmsg, sk_data_ready firing in softirq, tcp_read_sock
dispatching to siw_tcp_rx_data -- reports:

    BUG: KASAN: use-after-free in skb_copy_bits+0x284/0x480
    Read of size 4294967295 at addr ffff888 ...
    Call Trace:
     skb_copy_bits
     siw_rx_kva
     siw_rx_data
     siw_check_mem
     siw_proc_write
     siw_tcp_rx_data
     __tcp_read_sock
     siw_qp_llp_data_ready
     tcp_data_ready
     tcp_data_queue

Add the missing invariant at the earliest point where the peer header
is fully assembled. iwarp_pktinfo[*].hdr_len - MPA_HDR_SIZE is exactly
the value the siw transmitter uses as the minimum mpa_len for each
opcode (drivers/infiniband/sw/siw/siw_qp.c:33), so this matches the
protocol contract. Out-of-range FPDUs terminate the connection with
TERM_ERROR_LAYER_LLP / LLP_ETYPE_MPA / LLP_ECODE_FPDU_START -- which
is RFC 5044 Section 8 error code 3 ("Marker and ULPDU Length fields
do not agree on the start of an FPDU"), the correct framing-error
class for this inconsistency.

Fixes: 8b6a361 ("rdma/siw: receive path")
Link: https://patch.msgid.link/r/20260513175325.2042630-2-michael.bommarito@gmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Assisted-by: Claude:claude-opus-4-7
Acked-by: Bernard Metzler <bernard.metzler@linux.dev>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/infiniband/sw/siw/siw_qp.c
  drivers/infiniband/sw/siw/siw_qp_rx.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=683f7cfbf514193d63c0efa079f3352bde84c2e0

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

The original Linux kernel CVE announcement for CVE-2026-64102 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64102

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:L/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 9.8

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: 7
  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).