* [CVE-2026-89983][MODERATE 7.0] i2c: core: fix debugfs UAF on adapter removal
@ 2026-09-16 12:13 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-16 12:13 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89983
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: i2c: core: fix debugfs UAF on adapter removal
Commit: dc33a9762538b3250ed6b5644a4d44c748be33a6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dc33a9762538b3250ed6b5644a4d44c748be33a6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89983
Analysis date: Wed, 16 Sep 2026 08:13:21 -0400
ActionableScore: 5
ActionableScore lower bound: 4
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-2026-89983 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89983
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-2026-89983 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89983 CHECK WITH IMPACT FROM ORIG NN MODERATE 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-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in the I2C core can allow a write to the new_device sysfs attribute while i2c_del_adapter() is removing the adapter. The adapter debugfs directory may already be freed, but i2c_device_probe() can still pass adap debugfs as the parent to debugfs_create_dir(), producing a debugfs use-after-free reported by KASAN. The same race can also allow a client to be created after client deregistration, which may leave it unregistered and make adapter removal hang. For the CVSS the PR:L is used for the paranoid score because a local process with delegated access to I2C sysfs or device management paths may be able to trigger the race without full host root. The issue is not network reachable and requires local control plane interaction with the I2C adapter. Impact is at least local denial of service via kernel crash or hang and in worst case may allow confidentiality or integrity impact due to kernel use-after-free, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) SKIP CVE-2026-89983 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE READ DANGER USB INIT UAF HARDWARE DEBUGFS LINUS SYZBOT KPANIC - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 2
* Conservative score: 4
* Raw paranoid score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
Conservative floor applied: 2 -> 4 because a real kernel UAF is established, stale state is actually used, and host-admin-equivalent privilege is not fully proven from the supplied evidence.
Paranoid floor applied: 3 -> 5 because the stale object is an active debugfs/VFS object consumed by directory creation and lookup logic, not merely a fixed stale buffer or passive metadata pointer.
2. Signal breakdown
Positive conservative signals:
* +2 Real UAF. The commit directly shows freed adap->debugfs being passed to debugfs_create_dir(), with KASAN reporting slab-use-after-free in lookup_noperm_common().
* +1 Reliable kernel crash or strong DoS. The demonstrated UAF can crash the kernel, and the same race can also leave a client unregistered and make wait_for_completion hang.
Negative conservative signals:
* -1 Hard timing race. The write to new_device must race adapter removal, which is a teardown window rather than a normal steady-state operation.
Positive paranoid-only signals:
* +1 Weak structured-object concern. The freed debugfs parent is an active structured filesystem/debugfs object consumed by lookup and directory creation paths, so the stale use is more meaningful than a simple fixed-buffer stale read.
Important non-awarded signals:
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Information disclosure not demonstrated.
* Remote reachability not established.
* Concrete repeated release/refcount operation not established.
* Structured stale-object leverage beyond the weak concern is not awarded because attacker-controlled replacement or post-free contents are not shown.
* Extra Availability is not awarded because the crash and hang are counted as the same DoS category.
* C/I impact is not awarded as a scored Actionable signal, although paranoid CVSS may reasonably use C:H/I:H for manual-review sensitivity.
Repeated release/refcount check:
* First release can end lifetime: not applicable to the exact vulnerable operation.
* Exact same reference survives into a second lifetime mutation: not established.
* Second operation is a real lifetime/refcount mutation: not established.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: yes, but due to active structured UAF surface, not due to repeated release/refcount.
3. Reachability analysis
The trigger requires a write to the i2c adapter new_device sysfs attribute while i2c_del_adapter() is removing the adapter. The common deployment expectation is that this is privileged device-management territory, but the supplied evidence does not fully prove that every complete trigger path requires host-admin-equivalent privilege. Therefore HostAdminRequired is UNKNOWN rather than YES.
No evidence shows that the complete operation is namespace-scoped. I2C adapter sysfs and adapter removal are not normal network-namespace scoped operations, so NamespaceScopedPrivilege is NO.
There is no network trigger. The issue is local control-plane interaction with I2C adapter lifetime and sysfs client creation.
Prior host setup may be needed in practical deployments because an I2C adapter and writable sysfs control path must exist. This is not scored as a rare subsystem penalty because I2C is common, but the vulnerable operation is still not ordinary unprivileged user activity.
The race is a teardown race. It is likely harder than a simple deterministic syscall bug, but syzbot reached it and device_remove_file() is used specifically to drain in-flight writers, confirming the window is real.
4. Primitive and severity interpretation
The exact primitive is a real use-after-free of the adapter debugfs directory pointer. i2c_del_adapter() can free or invalidate the adapter debugfs directory before the adapter device is fully unregistered, while new_device remains writable. A racing writer can still reach i2c_device_probe(), which uses the freed adap->debugfs pointer as a parent for debugfs_create_dir().
This is an active structured-object UAF, not fixed stale storage. The stale object participates in debugfs/VFS lookup and directory creation. The demonstrated consequence is kernel crash or adapter-removal hang.
No controlled replacement, arbitrary read/write, callback hijack, or demonstrated privilege escalation is established. The stronger consequence remains unresolved rather than proven.
PrimitiveClass is CLASS_B because a real kernel lifetime violation exists, but there is no demonstrated Class A primitive such as controlled reclaim, directed write, or practical LPE.
ImportantCandidate is YES because the UAF is concrete and involves active structured kernel filesystem state, making automatic Moderate downgrade unsafe without manual review.
AutoDowngradeAllowed is NO because unresolved exploitation leverage of a real active-object UAF would be lost by automatic closure, even though Strong Important severity is not independently proven.
Likely operational severity is Actionable Moderate, with Strong Important review warranted if product-specific reachability shows below-host-admin access or if further analysis finds controllable debugfs object reuse.
5. One-sentence report phrase
A race between I2C adapter removal and writes to the new_device sysfs attribute can pass a freed adapter debugfs directory to debugfs_create_dir(), causing a real kernel use-after-free and local DoS, with unresolved active-object exploitation risk requiring manual review.
6. Manual review recommendation
MANUAL CHECK REQUIRED
A real UAF is demonstrated in active debugfs/VFS object handling, and the complete privilege boundary is not fully resolved from the supplied evidence.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: i2c: core: fix debugfs UAF on adapter removal
Commit: dc33a9762538b3250ed6b5644a4d44c748be33a6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dc33a9762538b3250ed6b5644a4d44c748be33a6
Commit description:
i2c_del_adapter() frees the adapter's debugfs directory before it
unregisters the adapter device, but the new_device sysfs attribute
stays writable until device_del(). A write racing with removal still
reaches i2c_device_probe(), which passes the freed adap->debugfs to
debugfs_create_dir() as the new client's parent:
BUG: KASAN: slab-use-after-free in lookup_noperm_common+0x407/0x430
Read of size 4 at addr ffff88803ef87810 by task syz.0.61/6090
lookup_noperm_common+0x407/0x430
simple_start_creating+0x9c/0x110
debugfs_start_creating+0xdb/0x1a0
debugfs_create_dir+0x24/0x350
i2c_device_probe+0x814/0xbf0
It's technically possible to create a client after i2c_deregister_clients
has run. That client will never be unregistered and make
wait_for_completion hang.
Close the window by removing the new_device attribute at the start of
i2c_del_adapter(). device_remove_file() will drain any clients left.
Fixes: 73febd7 ("i2c: create debugfs entry per adapter")
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=23ad911c819b923238b7
Signed-off-by: Vasileios Almpanis <[email protected]>
Cc: <[email protected]> # v6.8+
Tested-by: [email protected]
Signed-off-by: Andi Shyti <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
drivers/i2c/i2c-core-base.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=dc33a9762538b3250ed6b5644a4d44c748be33a6
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89983 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89983
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: 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
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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