* [CVE-2026-89543][MODERATE 7.0] sunrpc: fix use-after-free in __rpc_clnt_handle_event and __rpc_clnt_remove_pipedir
@ 2026-09-12 10:05 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-12 10:05 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89543
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: sunrpc: fix use-after-free in __rpc_clnt_handle_event and __rpc_clnt_remove_pipedir
Commit: e769fcde3cc73e847b1eb3acd40c04a291cb0c0c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e769fcde3cc73e847b1eb3acd40c04a291cb0c0c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89543
Analysis date: Sat, 12 Sep 2026 06:05:44 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
A SUNRPC pipefs lifecycle bug can leave pipefs dentries referencing a freed rpc_clnt after mount or unmount state becomes stale, allowing later rpc_info_open or rpc_show_info access to trigger a local kernel use after free and likely DoS.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89543 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89543
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-2026-89543 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89543 CHECK WITH IMPACT FROM ORIG NN IMPORTANT Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-672;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A use after free can occur in the SUNRPC pipefs lifecycle because clnt pipefs_sb may become stale across mount, unmount, or remount events. If cleanup is skipped, pipefs dentries can remain with inode private data that still references a freed rpc_clnt object, and later rpc_info_open or rpc_show_info access can dereference that freed object. For the CVSS the PR:L is used for the paranoid score because some deployments may expose the relevant mount namespace or service lifecycle control to a local user, container root, or service account rather than only to full host root. The issue is not directly network reachable and is triggered through local rpc_pipefs and SUNRPC client lifecycle paths. Impact is at least a local denial of service via kernel crash and in worst case may allow confidentiality, integrity, and availability impact due to a kernel use after free.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) YES REMOTE UAF NETWORK LINUS KPANIC KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6 YES NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
## 1. ActionableScore
* Raw conservative score: 3
* +2 Real UAF / lifetime violation.
* +1 realistic kernel crash / local DoS from dereferencing freed `rpc_clnt`.
* Conservative score: 4
* Applied REAL UAF FLOOR 4: 3 -> 4.
* Host-admin-equivalent privilege is not positively established from the supplied evidence, but below-host-admin reachability is also not proven.
* Raw paranoid score: 4
* +2 Real UAF / lifetime violation.
* +1 realistic kernel crash / local DoS.
* +1 escaped active structured stale object: pipefs dentries can survive cleanup and later drive `rpc_info_open()` / `rpc_show_info()` on freed `rpc_clnt`.
* Paranoid score: 5
* Applied REAL UAF FLOOR 5: 4 -> 5.
* Reason: the stale reference is not only an immediate cleanup fault. It escapes into persistent pipefs dentries and is later consumed by active info/open/show paths.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
## 2. Signal breakdown
Positive signals:
* +2 Real UAF: the commit directly states that pipefs dentries may retain inode private data pointing to a freed `rpc_clnt`.
* +1 Reliable kernel crash / DoS: later `rpc_info_open()` or `rpc_show_info()` can dereference the freed object.
* +1 Paranoid structured stale-object concern: the stale object is an active `rpc_clnt`, and the stale reference survives normal cleanup in a pipefs dentry and can be reached later through user-visible file operations.
Non-awarded or withheld signals:
* Controlled reclaim not demonstrated.
* Arbitrary read/write not demonstrated.
* Concrete privilege escalation not demonstrated.
* Remote reachability not established.
* Concrete repeated release/refcount mutation not established.
* First release / final free of `rpc_clnt`: implied by freed `rpc_clnt`, but the patch does not show a second `put` / `release` on the same exact dangling reference.
* Exact same reference survives: yes, via inode private data in stale pipefs dentries.
* Second operation is a lifetime/refcount mutation: no, the visible later operations are `rpc_info_open()` / `rpc_show_info()` dereferences.
* +1 Concrete repeated release/refcount was not awarded.
* Paranoid repeated-refcount leverage +1 was not awarded.
* Weak LPE was not separately awarded because no specific reuse, replacement, callback, credential, or write primitive is shown.
* Extra Availability was not awarded because the crash/DoS is already counted once.
* Confidentiality/Integrity bonuses were not awarded because a concrete disclosure or unauthorized modification path is not established, even though `rpc_show_info()` makes this worth manual review.
Negative signals:
* No -2 host-admin penalty: host-admin-equivalent privilege is plausible for rpc_pipefs mount/remount handling, but it is not positively established as required for the complete trigger from supplied evidence.
* No race penalty: the issue is a lifecycle/state mismatch across mount, unmount, remount, and cleanup paths, not necessarily a narrow timing race.
* No rare hardware/config penalty: SUNRPC/rpc_pipefs is not a physical rare-hardware-only path.
## 3. Reachability analysis
The vulnerable condition is local and tied to SUNRPC/rpc_pipefs client lifecycle handling. The trigger appears to involve late pipefs mount, unmount, or remount behavior plus RPC client cleanup, followed by access to stale pipefs entries through `rpc_info_open()` or `rpc_show_info()`.
The complete privilege boundary is unresolved from the supplied patch. Mounting or remounting rpc_pipefs normally suggests administrative involvement, but the evidence does not positively prove that full host-admin-equivalent privilege is required for every practical trigger path. It also does not prove that an ordinary unprivileged user can complete the full sequence.
Namespace-scoped privilege is unknown. The supplied evidence does not establish that all required mount, SUNRPC lifecycle, and pipefs access operations are available inside attacker-created namespaces.
No direct network trigger is established. NFS/SUNRPC may involve network services, but this bug is in local pipefs lifecycle and stale dentry handling rather than packet processing.
Prior setup may exist in practice if rpc_pipefs is already mounted and SUNRPC/NFS clients are active, but the supplied evidence does not establish a required separate privileged setup as part of the vulnerability model.
Race repeatability is not the main issue. The bug is primarily stale lifecycle state: `clnt->pipefs_sb` is not refreshed on mount events and not cleared on unmount/failure paths.
## 4. Primitive and severity interpretation
Exact primitive: stale pipefs dentry inode private data can point to a freed `rpc_clnt`, causing a real use-after-free when later pipefs info paths access it.
Storage type: active structured object, not fixed stale storage. `rpc_clnt` is a nontrivial kernel object whose fields are consumed by later SUNRPC info/open/show logic.
Cleanup-only vs broader active use: not purely cleanup-only. The stale reference remains in pipefs dentries and can later be reached through `rpc_info_open()` or `rpc_show_info()`.
Concrete repeated release/refcount operation: not established. This is not scored as a double-release/refcount case.
Class: CLASS_B. A real kernel lifetime/UAF primitive exists, but no controlled reclaim, arbitrary R/W, callback control, or demonstrated LPE is supplied.
Demonstrated consequence: local kernel crash / DoS is the concrete minimum consequence.
Speculative consequence: stronger C/I or LPE impact is possible in the generic sense for UAFs, but not established by the patch. The more relevant unresolved concern is that a stale active object escapes into a user-visible pipefs path and may be consumed after free.
ImportantCandidate=YES because this is a real escaped active-object UAF with unresolved exploitation leverage beyond a shallow cleanup-only fault.
AutoDowngradeAllowed=NO because automatically closing this as ordinary Moderate could lose a real stale-object lifetime primitive that survives cleanup and is later reachable through pipefs file operations.
Likely operational severity: Actionable Moderate at minimum. It needs manual review to decide whether the stale `rpc_clnt` consumption can expose data, corrupt state, or remain only a crash/DoS.
## 5. One-sentence report phrase
A SUNRPC pipefs lifecycle bug can leave pipefs dentries referencing a freed rpc_clnt after mount or unmount state becomes stale, allowing later rpc_info_open or rpc_show_info access to trigger a local kernel use after free and likely DoS.
## 6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: real UAF is established, the stale reference escapes cleanup through persistent pipefs dentries, and later active file operations consume the freed `rpc_clnt`; controlled exploitation is not proven, but automatic downgrade would be unsafe.
## 7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: sunrpc: fix use-after-free in __rpc_clnt_handle_event and __rpc_clnt_remove_pipedir
Commit: e769fcde3cc73e847b1eb3acd40c04a291cb0c0c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e769fcde3cc73e847b1eb3acd40c04a291cb0c0c
Commit description:
Normal client creation goes through rpc_setup_pipedir(), which records
clnt->pipefs_sb, but the mount-event path in __rpc_clnt_handle_event()
calls rpc_setup_pipedir_sb() directly and never refreshes that field.
The umount path also removes the directory without clearing
clnt->pipefs_sb.
After a late pipefs mount or any remount, rpc_clnt_remove_pipedir()
compares the current superblock against a stale pipefs_sb pointer and
skips cleanup, leaving pipefs dentries whose inode private data still
points at a freed rpc_clnt, leading to a potential use-after-free during
subsequent rpc_info_open() or rpc_show_info() calls.
Fix this by properly updating clnt->pipefs_sb upon mount events and
clearing it during unmount or failure paths.
Fixes: bfca5fb ("SUNRPC: Fix RPC client cleaned up the freed pipefs dentries")
Cc: [email protected]
Reported-by: Yuan Tan <[email protected]>
Reported-by: Xin Liu <[email protected]>
Reviewed-by: Ren Wei <[email protected]>
Assisted-by: Codex:gpt-5.4
Signed-off-by: Luxiao Xu <[email protected]>
Signed-off-by: Trond Myklebust <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
net/sunrpc/clnt.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=e769fcde3cc73e847b1eb3acd40c04a291cb0c0c
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89543 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89543
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:H/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 7
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: 4
Paranoid ActionableScore: 5
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
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