* [CVE-2026-89532][MODERATE 7.0] svcrdma: Fix pcl_for_each_segment for empty chunks
@ 2026-09-12 2:09 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-12 2:09 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89532
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: svcrdma: Fix pcl_for_each_segment for empty chunks
Commit: 9c5a03c3dc505c0295339b0a1b9e4fe36447e482
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9c5a03c3dc505c0295339b0a1b9e4fe36447e482
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89532
Analysis date: Fri, 11 Sep 2026 22:09:45 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
A remote peer on an RPC/RDMA fabric can advertise a zero-segment Write or Reply chunk, causing `pcl_for_each_segment` to underflow its segment bound and perform an out-of-bounds kernel read until a GPF or kernel crash occurs.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89532 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89532
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-2026-89532 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89532 CHECK WITH IMPACT FROM ORIG NN IMPORTANT Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H';CWE-191;CWE-125;*CWE-20;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.1';DESCR 'An out-of-bounds read in the SUNRPC svc RDMA parsed chunk list can occur when a remote peer supplies a Write or Reply chunk with zero segments. The pcl_for_each_segment macro computes an inclusive upper bound using ch_segcount minus 1, so a zero value underflows and makes the loop walk beyond the ch_segments flexible array. When Send-With-Invalidate is negotiated, svc_rdma_get_inv_rkey can iterate this malformed chunk and read segment metadata from unrelated memory until the kernel faults. For the CVSS the PR:N is used because no local account or target side privilege is required beyond network reachability to the RPC/RDMA service. The issue is network adjacent rather than broadly Internet reachable in typical deployments because RPC/RDMA is usually used inside trusted storage or data center networks. Impact is at least denial of service via kernel crash. For the paranoid score, limited confidentiality impact is retained because the primitive is an out-of-bounds kernel read, but no arbitrary write or clear privilege escalation path is shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4) SKIP CVE-2026-89532 UNKNOWN SKIP No affected files built, so skip this CVE NO - - unknown MAYBE WRITE OOB KPANIC - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=4
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
## 1. ActionableScore
* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::
No floor or cap changed the score.
## 2. Signal breakdown
Positive signals:
* +2 Remote or network-adjacent wire reachability. The zero-segment Write or Reply chunk is stated to be reachable from the wire through RPC/RDMA input.
* +1 Concrete integer underflow and OOB read primitive. `ch_segcount == 0` makes `ch_segcount - 1` wrap to `0xffffffff`, causing iteration beyond the `ch_segments` flexible array.
* +1 Reliable kernel crash or strong DoS. The described path reads `segment->rs_handle` from unrelated memory until a general protection fault occurs.
Non-awarded signals:
* RealUAF not awarded. No object lifetime end, stale reference after free, or repeated release/refcount mutation is established.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality not awarded. The OOB read is internal to the kernel and no disclosure path to userspace, network, or device-visible output is shown.
* Integrity not awarded. No unauthorized write or state modification is shown.
* Strong LPE or strong exploitation leverage not awarded. The patch supports OOB read to crash, not controlled memory corruption.
* Extra Availability not awarded because the same GPF/kernel crash is already counted once.
* Rare subsystem penalty not applied. RPC/RDMA is specialized, but the input establishes wire reachability for configured services and does not prove a rare or physical-only requirement.
## 3. Reachability analysis
The trigger is a malformed RPC/RDMA chunk received over the transport. The attacker can advertise a Write or Reply chunk with zero segments, which passes the described validation because only segment counts greater than `rc_maxpages` are rejected.
Host administrator privileges are not required for the attacker once the RPC/RDMA service is reachable. Therefore `HostAdminRequired=NO`. This does assume the service is already configured and exposed by the victim, but that is a deployment exposure condition rather than an attacker privilege requirement.
The issue is best treated as network-adjacent rather than broad Internet exposure. RPC/RDMA is normally used in storage, NFS/RDMA, or data-center fabrics. It is not typically exposed to the public Internet.
No namespace-scoped privilege issue is involved. The trigger is remote protocol input, not local namespace capability use.
No hard race is involved. The exploit condition is a malformed chunk state and a negotiated Send-With-Invalidate path, not a narrow timing window.
## 4. Primitive and severity interpretation
The exact primitive is a remote, attacker-influenced integer underflow in segment iteration, leading to an out-of-bounds kernel read and then a general protection fault.
This is not a UAF and not a repeated lifetime/refcount mutation. It is not an active structured stale-object case. It is a malformed parser state that causes the kernel to walk outside the intended flexible array.
Primitive classification is `CLASS_C` because the demonstrated behavior is primarily invalid read and crash/DoS without an independently useful corruption primitive.
Demonstrated consequence: host kernel crash or fatal fault.
Speculative consequence: limited confidentiality could be considered in CVSS-style paranoid scoring because an OOB read exists, but ActionableScore does not award confidentiality without a concrete disclosure path.
ImportantCandidate is `NO` because no Class A primitive, real UAF, controlled corruption, authorization bypass, COW ownership violation, or demonstrated LPE is established.
AutoDowngradeAllowed is `NO` because this is a wire-reachable kernel parser OOB read with host crash impact and should be manually reviewed before closure, even though it is not an Important-class candidate on the supplied evidence.
Likely operational severity: actionable remote or network-adjacent DoS in environments exposing RPC/RDMA, especially storage or NFS/RDMA fabrics.
## 5. One-sentence report phrase
A remote peer on an RPC/RDMA fabric can advertise a zero-segment Write or Reply chunk, causing `pcl_for_each_segment` to underflow its segment bound and perform an out-of-bounds kernel read until a GPF or kernel crash occurs.
## 6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: this is a wire-reachable kernel transport parser issue with a concrete OOB read and reliable crash impact, although no UAF, arbitrary write, disclosure path, or privilege-escalation primitive is established.
## 7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: svcrdma: Fix pcl_for_each_segment for empty chunks
Commit: 9c5a03c3dc505c0295339b0a1b9e4fe36447e482
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9c5a03c3dc505c0295339b0a1b9e4fe36447e482
Commit description:
When a parsed chunk list contains a chunk whose ch_segcount is zero,
pcl_for_each_segment computes its inclusive upper bound as
&chunk->ch_segments[ch_segcount - 1]. ch_segcount is u32, so the
subtraction wraps to 0xFFFFFFFF and the bound lands far past the
ch_segments flex array. The loop body then walks unrelated memory at
sizeof(struct svc_rdma_segment) stride until it faults.
A zero-segcount chunk is reachable from the wire:
xdr_check_write_chunk() only rejects segcount values greater than
rc_maxpages, and pcl_alloc_write() links a freshly allocated chunk
onto rc_write_pcl/rc_reply_pcl before its segment-fill loop runs,
so a Write or Reply chunk advertising zero segments leaves
ch_segcount == 0 on the list. When the transport has negotiated
Send-With-Invalidate, svc_rdma_get_inv_rkey() iterates all four
PCLs with pcl_for_each_segment and dereferences segment->rs_handle
on each iteration, turning the underflow into an out-of-bounds read
and a general protection fault.
xdr_check_write_list / xdr_check_reply_chunk
pcl_alloc_write()
chunk = pcl_alloc_chunk(...) /* ch_segcount = 0 */
list_add_tail(&chunk->ch_list, &pcl->cl_chunks)
/* fill loop iterates zero times for wire segcount 0 */
svc_rdma_get_inv_rkey()
pcl_for_each_chunk(rc_write_pcl)
pcl_for_each_segment(segment, chunk)
pos <= &ch_segments[0u - 1u] /* 0xFFFFFFFF */
segment->rs_handle /* OOB read -> GPF */
Fix by switching the macro to a half-open upper bound that uses
ch_segcount directly. For ch_segcount == 0 the loop start equals the
loop end and the body is skipped; for ch_segcount > 0 the iteration
range is unchanged. All six existing call sites in
net/sunrpc/xprtrdma/svc_rdma_recvfrom.c and
net/sunrpc/xprtrdma/svc_rdma_rw.c remain correct under the new bound,
so no caller changes are needed.
Fixes: 78147ca ("svcrdma: Add a "parsed chunk list" data structure")
Cc: [email protected]
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <[email protected]>
Acked-by: Jeff Layton <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Chuck Lever <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
net/sunrpc/xprtrdma/svc_rdma_rw.c
include/linux/sunrpc/svc_rdma_pcl.h
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=9c5a03c3dc505c0295339b0a1b9e4fe36447e482
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89532 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89532
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:N/I:N/A:H
The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H
The Best / paranoid CVSS score: 7.1
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: 4
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
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please use reply email interface to add additional manual analyses
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