* [CVE-2026-93800][MODERATE 7.0] btrfs: fix use-after-free on reloc root after error in insert_dirty_subvol() [ Upstream
@ 2026-09-24 22:13 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-24 22:13 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-93800
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: btrfs: fix use-after-free on reloc root after error in insert_dirty_subvol() [ Upstream
Commit: 97a540d72ebcb21853d11f2a56782fe377f358e7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=97a540d72ebcb21853d11f2a56782fe377f358e7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93800
Analysis date: Thu, 24 Sep 2026 18:13:23 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-93800 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93800
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-2026-93800 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-93800 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:H/I:H/A:H';*CWE-416;CWE-415;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A Btrfs relocation error path can leave root reloc_root pointing to a reloc root that has already been freed. A later unmount calls btrfs_put_root on the stale pointer and triggers a confirmed slab use after free with a write operation. This can cause a kernel crash and may provide confidentiality or integrity impact if the freed object can be reused in a controllable way. For the CVSS the PR:L is used for the paranoid score because a reduced capability root, container root, or service account may be able to manage a Btrfs filesystem or initiate balance operations without full host administrator privileges. Exploitation is local and is not directly network reachable. Reliable triggering requires a Btrfs relocation operation and an I O error during relocation. Impact is at least a local denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to kernel memory corruption. Manual review is required.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6) YES OOB DISK INIT UAF SKIP KERNEL_PANIC_PLUS_UAF HARDWARE SYZBOT KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
The conservative and paranoid scores differ because the paranoid score includes one additional point for a concrete post-free refcount mutation through memory whose original object lifetime has already ended. No floor or cap changed either score.
2. Signal breakdown
Positive signals:
* +2 Real UAF and active lifetime violation. `free_reloc_roots()` can free the relocation root while `root->reloc_root` still retains its address. The later unmount path reaches `btrfs_put_root()` using that dangling pointer. KASAN confirms slab use after free.
* +1 Concrete repeated release or refcount operation. The first lifetime transition frees the relocation root through `free_reloc_roots()`. The exact stale reference remains in `root->reloc_root`. A later `btrfs_put_root()` performs a write of size 4 through the stale object, consistent with a lifetime or refcount mutation rather than only a stale read.
* +1 Reliable kernel crash or strong DoS. The supplied KASAN trace demonstrates a reachable post-free write during unmount. This can cause an Oops, panic, or filesystem unavailability.
* +1 Important filesystem path. The defect is in Btrfs relocation and root lifetime management. It affects filesystem transaction recovery, relocation cleanup, and unmount processing rather than passive diagnostic metadata.
* +1 Paranoid only. The second `btrfs_put_root()` actively mutates lifetime state through already-freed slab storage. This is a technically defensible unresolved post-free corruption mechanism, although controlled reclaim or refcount takeover is not demonstrated.
Important non-awarded signals:
* Controlled reclaim or attacker-controlled replacement of the freed `btrfs_root` object is not demonstrated.
* Arbitrary or directed kernel write is not demonstrated. The observed write is a refcount or lifetime-related write through a stale pointer.
* Confidentiality and integrity consequences are not awarded as established facts. The KASAN write confirms corruption potential, but no read-back, protected-data modification, credential modification, or attacker-controlled reuse is shown.
* Remote reachability is not established. The vulnerable path is local filesystem relocation and unmount handling.
* Local unprivileged trigger is not awarded. The supplied material does not establish that an ordinary user can complete Btrfs balance, relocation, induced I O failure, and unmount without elevated filesystem-management rights.
* A hard race penalty is not applied. This is an ordered relocation error and cleanup sequence, not a demonstrated narrow timing race.
* A host-admin penalty is not applied because the complete capability and namespace boundary for Btrfs balance and the required error injection are not fully established.
* Structured stale-object leverage is not separately awarded. The meaningful stale operation currently evidenced is the repeated lifetime mutation itself, and awarding both would double-count the same fact.
* Weak LPE concern is not separately awarded. It would duplicate the repeated post-free refcount mutation signal.
* Additional availability impact is not awarded because the demonstrated crash and filesystem failure are already counted as the strong DoS signal.
Repeated lifetime operation assessment:
* First release can end lifetime: yes. `free_reloc_roots()` reaches `__del_reloc_root()` and frees the relocation root.
* Exact same reference survives: yes. `root->reloc_root` remains non-NULL and points to the freed relocation root.
* Second operation is a real lifetime or refcount mutation: yes. `btrfs_put_root()` accesses the freed object and KASAN reports a write of size 4.
* Concrete repeated release or refcount signal awarded: yes, +1.
* Paranoid repeated-refcount leverage awarded: yes, +1. The reason is the concrete write-like lifetime mutation through freed storage, not generic theoretical heap reuse.
* FLOOR 5 applied: not needed. The conservative raw arithmetic already reaches 5.
3. Reachability analysis
The trigger is local. The demonstrated sequence requires a Btrfs filesystem relocation or balance operation, an error from `btrfs_update_reloc_root()` such as an I O failure, subsequent transaction abort handling, and a later filesystem unmount.
The supplied reproduction uses a loop-backed Btrfs filesystem and syzkaller. It does not establish that an ordinary unprivileged user can create the required storage topology, mount the filesystem, initiate balance, induce the required I O failure, and unmount it.
Btrfs management operations commonly involve `CAP_SYS_ADMIN`-like authority, but the supplied patch does not fully show the capability checks, their user-namespace scope, or whether a reduced-capability service or delegated environment can complete the entire sequence. Therefore:
* HostAdminRequired is `UNKNOWN`, not `YES`.
* NamespaceScopedPrivilege is `UNKNOWN`.
* No local-unprivileged trigger bonus is awarded.
* No host-admin penalty is applied merely from an assumption.
No special physical hardware is required. The reported test uses a loop device, but reliable reproduction requires an induced relocation-time failure and therefore is operationally more complex than a normal filesystem operation. This complexity is not scored as a hard race penalty because the defect is a deterministic ordered error path once the I O failure occurs.
4. Primitive and severity interpretation
The exact primitive is a real slab use after free involving an active `struct btrfs_root` relocation object. It is not fixed asynchronous storage and not merely a poisoned-pointer crash. The relocation root is freed while another root retains a stale ownership and lifetime reference in `root->reloc_root`.
This is stronger than an ordinary stale cleanup dereference:
A. Ordinary stale cleanup dereference:
`stale pointer -> field read or immediate fault -> crash`
B. This issue:
`reloc root freed -> root->reloc_root remains dangling -> later btrfs_put_root on same dangling reference -> write-like lifetime or refcount mutation through freed slab object`
The supplied KASAN trace directly supports category B. The stale operation is not only a read. `btrfs_put_root()` writes into freed memory, and the commit explicitly identifies a use after free or double-free attempt.
The issue remains `CLASS_B`, not `CLASS_A`, because controlled reclaim, attacker-shaped replacement objects, callback control, arbitrary read/write, refcount takeover, data disclosure, or demonstrated privilege escalation are not established.
The demonstrated consequence is local kernel crash or filesystem-related DoS during unmount after a failed relocation. The speculative consequence is more serious kernel memory corruption if an attacker can reliably arrange post-free slab reuse before the second `btrfs_put_root()` mutation. That mechanism is plausible enough to require manual analysis, but it is not established enough to claim arbitrary write or privilege escalation.
`ImportantCandidate=YES` because this is a concrete repeated lifetime mutation through a dangling active filesystem object. `AutoDowngradeAllowed=NO` because automatically treating it as ordinary crash-only cleanup would discard unresolved post-free refcount corruption leverage.
Likely operational severity is Actionable Moderate at minimum. It is a strong Important candidate if further analysis confirms below-host-admin reachability, controllable error injection, slab reuse control, or corruption of a security-relevant replacement object.
5. One-sentence report phrase
A Btrfs relocation error can leave a freed relocation root referenced by root->reloc_root, causing a later unmount to execute btrfs_put_root on stale memory and perform a confirmed post-free refcount write that can crash the kernel and requires manual exploitation review.
6. Manual review recommendation
MANUAL CHECK REQUIRED
A confirmed UAF is followed by a concrete second lifetime or refcount mutation on the same dangling reference. Controlled reclaim and privilege boundaries are unresolved, so this must not be automatically downgraded to ordinary crash-only behavior.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: btrfs: fix use-after-free on reloc root after error in insert_dirty_subvol() [ Upstream
Commit: 97a540d72ebcb21853d11f2a56782fe377f358e7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=97a540d72ebcb21853d11f2a56782fe377f358e7
Changed files:
fs/btrfs/relocation.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=97a540d72ebcb21853d11f2a56782fe377f358e7
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-93800 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93800
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:N/I:N/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: 5
Paranoid ActionableScore: 6
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
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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