* [CVE-2026-98079][LOW] btrfs: zstd: fix lost wakeup when waiting for a workspace [ Upstream
@ 2026-09-25 17:07 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-25 17:07 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98079
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: btrfs: zstd: fix lost wakeup when waiting for a workspace [ Upstream
Commit: d8f57049521948df5f50797141472fbbed3b0723
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d8f57049521948df5f50797141472fbbed3b0723
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98079
Analysis date: Fri, 25 Sep 2026 13:07:01 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98079 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98079
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: LOW
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-2026-98079 LOW https://www.kernelcve.org/list/?q=CVE-2026-98079 CHECK WITH IMPACT FROM ORIG NN NONE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;CWE-667;*CWE-833;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A lost wakeup race in the Btrfs Zstd workspace allocator can leave a writer sleeping in TASK_UNINTERRUPTIBLE even though a suitable compression workspace is already available. The race occurs when workspace allocation fails and another task returns a workspace before the waiting writer is published on the wait queue. The writer can then remain blocked until another workspace return happens to generate a later wakeup. For the CVSS the PR:L value is used because a local user or process with write access to a Btrfs filesystem using Zstd compression can reach the affected path without administrative control of the kernel. AC:H is used because triggering depends on allocation failure and a specific timing window between the failed allocation and prepare_to_wait. The issue is not directly network reachable although writes received through a separately configured network file service could invoke the same filesystem path. Impact is denial of service through a blocked writer and potentially degraded filesystem availability. No memory corruption primitive or supported path to privilege escalation is present.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) MAYBE DISK SIMPLEFIX SKIP - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
* +1 Local unprivileged trigger. A user with write access to a Btrfs filesystem using Zstd compression can reach the workspace allocation path.
* +1 Availability impact. The lost wakeup can leave a writer indefinitely blocked in TASK_UNINTERRUPTIBLE until another qualifying workspace return generates a wakeup.
* No race penalty. The commit explicitly describes the race window as wide because allocation can enter reclaim.
* No host-admin penalty. The vulnerable write path is reachable below host-admin privilege once a writable Btrfs filesystem using Zstd is available.
* No memory-corruption points. There is no UAF, OOB access, stale object, overwrite, or lifetime mutation.
* No extra strong-DoS point. A host crash, filesystem shutdown, or persistent host-wide failure is not established.
* No confidentiality or integrity points. The bug only loses a wakeup and does not expose or modify protected data.
* No filesystem-integrity point. The affected compression workspace wait path does not corrupt extents, metadata, ownership, COW state, or stored data.
* No remote-reachability point. A separate network filesystem service could indirectly submit writes, but direct network exposure is not inherent to this bug.
3. Reachability analysis
An ordinary local process with permission to write files on a Btrfs filesystem using Zstd compression can enter the affected path. The system must already have a suitable Btrfs filesystem and Zstd compression in use, but the attacker does not need host-administrator privilege to perform ordinary permitted writes.
No capability or namespace-specific privilege is required for the vulnerable operation, so NamespaceScopedPrivilege is NO rather than YES or UNKNOWN. There is no special hardware, boot mode, or unusual device prerequisite.
Triggering requires workspace allocation failure followed by a workspace return before the waiter becomes visible. This is a timing race, but the commit states that the window is wide because kvmalloc may enter reclaim. Repeated filesystem writes and memory pressure can increase exposure, so a hard-race penalty is not justified. Direct remote triggering is not established.
4. Primitive and severity interpretation
The exact defect is a lost wakeup caused by scheduling without rechecking the workspace pool after publishing the waiter. The result is an indefinitely blocked writer despite an available workspace.
This is not stale storage, an active-object UAF, a repeated release, or a refcount mutation. PrimitiveClass is CLASS_C because the demonstrated behavior is availability loss without an independently useful corruption or security-boundary primitive.
The practical consequence is a blocked write operation and possible accumulation of tasks in uninterruptible sleep. Host-wide failure is a theoretical escalation of workload impact, not a patch-supported corruption or persistent resource-exhaustion primitive.
ImportantCandidate is NO because there is no memory corruption, ownership violation, authorization bypass, data-integrity failure, or privilege-escalation mechanism. AutoDowngradeAllowed is YES because no unresolved exploitation primitive would be lost by treating this as an ordinary availability issue.
5. One-sentence report phrase
A lost wakeup race in the Btrfs Zstd workspace allocator can leave a local writer indefinitely blocked in TASK_UNINTERRUPTIBLE, causing limited denial of service without memory corruption or privilege escalation.
6. Manual review recommendation
NO MANUAL CHECK NEEDED
The patch establishes a bounded lost-wakeup availability defect with no UAF, corruption, security-boundary bypass, or Important-grade exploitation primitive.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: btrfs: zstd: fix lost wakeup when waiting for a workspace [ Upstream
Commit: d8f57049521948df5f50797141472fbbed3b0723
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d8f57049521948df5f50797141472fbbed3b0723
Changed files:
fs/btrfs/zstd.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=d8f57049521948df5f50797141472fbbed3b0723
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98079 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98079
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:L
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS score: 4.7
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: unknown
Paranoid ActionableScore: 2
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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW
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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