[CVE-2026-72217][IMPORTANT] SUNRPC: Bound-check xdr_buf_to_bvec() stores before writing

[CVE-2026-72217][IMPORTANT] SUNRPC: Bound-check xdr_buf_to_bvec() stores before writing

[AL-KERNEL]

CVE: CVE-2026-72217
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: SUNRPC: Bound-check xdr_buf_to_bvec() stores before writing
Commit: 4a1148f2739d5089c3ca8ae2e9d1053e219ab5df
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4a1148f2739d5089c3ca8ae2e9d1053e219ab5df
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72217
Analysis date: Sun, 16 Aug 2026 05:29:50 -0400
ActionableScore: 9
ActionableScore lower bound: 8
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
SUNRPC `xdr_buf_to_bvec()` can write one `bio_vec` past the caller-provided array before bounds checking, allowing a network-reachable NFS client to corrupt adjacent kernel slab memory and potentially crash the server or cause broader kernel memory corruption.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72217 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72217

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-2026-72217	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:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-119;*CWE-20;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.8';DESCR 'SUNRPC xdr_buf_to_bvec can write one bio_vec past the caller supplied array because the old code stored into bvec before checking count against bvec_size, and the head branch had no bounds check. In the nfsd_vfs_write path the rq_bvec array is allocated with exactly rq_maxpages entries, so the surplus store can corrupt adjacent slab memory with bv_len and bv_offset values influenced by RPC payload layout and sizes. For the CVSS the PR:L is used because reliable triggering normally requires an NFS client identity or export access capable of sending write RPC traffic to the server. The issue is network reachable when an NFS server is exposed to the attacker, but NFS services are commonly limited to trusted storage or data center networks rather than the public Internet. Impact is at least denial of service via kernel memory corruption and in the paranoid case may include confidentiality or integrity impact, so this should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 9)  SKIP CVE-2026-72217 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 2eb2b93581813b74c7174961126f6ec38eadb5a7 YES  NO NO unknown 	MAYBE	REMOTE WRITE OOB SIMPLEFIX NETWORK TEST KPANIC MEMORY  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=9
ActionableScoreLower=8

## 1. ActionableScore

* Conservative score: 8
* Paranoid score: 9
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Remote reachable / network-triggerable: +2
  The affected path is SUNRPC/NFS server-side handling. A malicious or authorized NFS client can influence RPC payload layout and sizes.

* Memory corruption, strong corruption primitive: +2
  The bug is an out-of-bounds write of one `bio_vec` past the caller-provided array. The commit explicitly says the store can land in adjacent slab memory.

* Weak LPE concern: +1
  The overwrite target is adjacent slab memory and not fully attacker-selected, but this is still kernel heap corruption in a network service with partially attacker-influenced fields.

* Reliable kernel crash / strong DoS: +1
  Adjacent slab corruption in the NFS server path can realistically produce kernel crash or service disruption.

* Confidentiality impact plausible: +1
  Not directly demonstrated, but kernel heap corruption in a remote server path can plausibly affect object state in ways that may expose data in worst-case triage.

* Integrity impact plausible: +1
  The OOB write modifies adjacent kernel memory. Even if constrained, integrity impact is plausible.

* Broad/default/common subsystem or broadly deployed exposure: +1
  NFS/SUNRPC is a common server subsystem, especially in storage and data-center environments.

No subtraction is applied for rare configuration. NFS is often access-controlled and not Internet-facing, but it is not a rare or exotic kernel path.

## 3. Reachability analysis

The bug is triggerable from the network when an NFS server is reachable and the attacker can send relevant RPC write traffic. In realistic deployments this often means access to an exported NFS share or to a trusted storage network, so it is not necessarily public-Internet reachable. Namespaces do not materially reduce the server-side risk once the NFS service is exposed. The path is not a local-only ioctl or admin-only configuration path. Exploitation conditions depend on shaping the xdr buffer so that the caller bvec budget is exactly exhausted and the next head/page/tail store occurs one slot past the allocated `rq_bvec` array.

## 4. Severity interpretation

This behaves closer to an Important-class kernel vulnerability than an ordinary Moderate issue. The realistic demonstrated impact is kernel memory corruption and likely DoS. The theoretical worst case includes confidentiality or integrity impact because the bug writes beyond a slab allocation in a network-reachable server path, although the overwrite is constrained to a `bio_vec`-shaped store and does not directly show arbitrary write or controlled object replacement. Conservative handling should still require manual review because remote kernel OOB writes are historically high-risk even when the first visible symptom is crash prevention.

## 5. One-sentence report phrase

SUNRPC `xdr_buf_to_bvec()` can write one `bio_vec` past the caller-provided array before bounds checking, allowing a network-reachable NFS client to corrupt adjacent kernel slab memory and potentially crash the server or cause broader kernel memory corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a network-reachable kernel out-of-bounds write in the NFS/SUNRPC server path, with adjacent slab corruption explicitly described and client-influenced length or offset fields.

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

Patch: SUNRPC: Bound-check xdr_buf_to_bvec() stores before writing
Commit: 4a1148f2739d5089c3ca8ae2e9d1053e219ab5df
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4a1148f2739d5089c3ca8ae2e9d1053e219ab5df

Commit description:

xdr_buf_to_bvec() writes a bio_vec into the caller's array before
testing whether that slot is in range, and the head branch performs
the store with no check at all. When the caller's budget is exactly
used up, the next store lands one element past the end of the array.
The overflow label returns count - 1, which masks the surplus store
but cannot undo it.

rq_bvec, the array passed by nfsd_vfs_write(), is allocated to
exactly rq_maxpages entries with no slack. The OOB store can land in
adjacent slab memory; the bv_len and bv_offset fields written there
are derived from client-supplied RPC payload sizes.

Move the in-range check ahead of the store in the head, page-loop,
and tail branches. With the check at the top of each sequence, count
is incremented only after a successful store, so the overflow label
can return count directly.

Reported-by: Chris Mason <clm@meta.com>
Fixes: 2eb2b93 ("SUNRPC: Convert svc_tcp_sendmsg to use bio_vecs directly")
Cc: stable@vger.kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sunrpc/xdr.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=4a1148f2739d5089c3ca8ae2e9d1053e219ab5df

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

The original Linux kernel CVE announcement for CVE-2026-72217 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72217

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:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.8

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: 8
  Paranoid ActionableScore: 9

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).