* [CVE-2026-99156][MODERATE 7.0] scsi: st: Fix use-after-free race between st_remove() and st_open()
@ 2026-10-29 3:22 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-29 3:22 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99156
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: scsi: st: Fix use-after-free race between st_remove() and st_open()
Commit: a1647b653549ff29f0422a63eabf329b842f3822
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a1647b653549ff29f0422a63eabf329b842f3822
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99156
Analysis date: Wed, 28 Oct 2026 23:22:09 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
A concurrent permitted tape open can increment the kref of a freed but still-IDR-published `scsi_tape`, causing confirmed slab UAF corruption and possible subsequent consumption of stale structured state.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99156 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99156
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
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CVE-2026-99156 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-99156 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC (final reconciliation agrees; DECISION: both MODERATE_KPANIC) 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-825;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race between st_remove and st_open can leave a scsi_tape pointer published in st_index_idr after its final reference is dropped and the object is freed. A concurrent st_open can then find the stale pointer and execute kref_get on freed slab memory, producing a confirmed use after free write of size 4. For the CVSS the PR:L value reflects that the attacker needs local permission to open the SCSI tape device. Reliable triggering also depends on concurrent device removal and winning a timing window, so AC:H is appropriate. Kernel memory corruption and a crash are supported by the KASAN report. The constrained reference count increment does not prove privilege escalation, but slab reuse creates a plausible path to broader confidentiality and integrity impact in the paranoid assessment. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 4 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the ActionableScore threshold rule was applied. KP and FD specialist checks stage MODERATE_KPANIC to MODERATE because KP specialist did not establish a qualifying panic. final reconciliation stage MODERATE to MODERATE_KPANIC because the cross model evidence reconciliation rule was applied. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 and DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 4.2 and ActionableScore 4 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 4) MAYBE LOCK RACE LEAK UAF HARDWARE LINUS INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 2.
* Conservative score: 4. REAL UAF FLOOR 4 raises 2 -> 4.
* Raw paranoid score: 3.
* Paranoid score: 4. REAL UAF FLOOR 4 raises 3 -> 4.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +2: Confirmed UAF. `st_remove()` can perform the final `kref_put()` while `tpnt` remains published in `st_index_idr`.
* +1: Plausible kernel crash/DoS, supported by the KASAN slab-UAF report.
* -1 base: A one-time device-removal/open race is required; the attacker must reach the narrow ordering around `st_ref_mutex`. The paranoid reading withholds this penalty because a blocked open establishes much of the required ordering.
* Floor 4: The stale reference is actively used and this is not fixed stale storage with guaranteed immediate fault behavior.
* No separate overwrite point: the four-byte `kref_get()` mutation is the UAF use itself, so counting generic corruption again would double-count it.
* Controlled reclaim, attacker-selected payload/target, arbitrary write, disclosure, LPE, and a second release/refcount decrement are not shown.
* No extra availability point is awarded for the same crash.
3. Reachability analysis
A user with permission to open the SCSI tape device can invoke `st_open()`. Device removal concurrently supplies the lifetime transition, but the evidence does not establish whether the same attacker can initiate removal or whether host administration, hot-unplug, or another actor is required. Thus the complete trigger is compound and HostAdminRequired remains UNKNOWN. No namespace-scoped capability path, special sysctl, or attacker-controlled reclaim is shown.
4. Primitive and severity reading
This is an active structured-object UAF: `kref_get()` writes the reference count in an already freed `scsi_tape`, after which normal `st_open()` processing may consume that object. It is stronger than a read-only stale dereference, but no controlled replacement or CLASS_A primitive is established.
The attacker-invoked open, concrete post-free state mutation, privileged kernel consumer, and credible delegated device-interface/host-kernel boundary satisfy an unknown active-state corruption chain requiring review. Therefore ImportantCandidate is YES and automatic downgrade is unsafe, although Important severity or LPE is not proven.
5. One-sentence report phrase
A concurrent permitted tape open can increment the kref of a freed but still-IDR-published `scsi_tape`, causing confirmed slab UAF corruption and possible subsequent consumption of stale structured state.
6. Manual review advice
MANUAL CHECK REQUIRED
Review device-node delegation, who can induce removal, post-`kref_get()` field consumption, and feasible slab reuse.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: scsi: st: Fix use-after-free race between st_remove() and st_open()
Commit: a1647b653549ff29f0422a63eabf329b842f3822
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a1647b653549ff29f0422a63eabf329b842f3822
Commit description:
st_remove() drops the probe reference on the scsi_tape with kref_put()
and only afterwards removes its index from st_index_idr. If that was the
last reference, scsi_tape_release() frees the structure while it is
still published in the idr.
scsi_tape_get(), called from st_open(), looks up the tape in
st_index_idr and takes a reference under st_ref_mutex and
st_index_lock. remove_cdevs() does not wait for opens that are already
past chrdev_open(), so an open that is blocked on st_ref_mutex while
st_remove() holds it can take the mutex as soon as st_remove() drops it,
find the freed tpnt in the idr and call kref_get() on it:
BUG: KASAN: slab-use-after-free in st_open
Write of size 4 ...
st_open (drivers/scsi/st.c:254 drivers/scsi/st.c:1291)
chrdev_open
...
Freed by task ...:
kfree
st_remove (drivers/scsi/st.c:4512)
Remove the index from the idr before dropping the reference, all under
st_ref_mutex. A concurrent scsi_tape_get() now either runs first and
holds a valid reference that keeps the tape alive, or runs afterwards
and finds nothing, so st_open() fails with -ENXIO.
This restores the ordering st_remove() had before the conversion to
st_index_idr, where the tape was unpublished from scsi_tapes[] before
the final kref_put().
Fixes: 6c648d9 ("[SCSI] st: get rid of scsi_tapes array")
Cc: [email protected]
Reported-by: Dingisoul <[email protected]>
Closes: https://lore.kernel.org/linux-scsi/[email protected]/
Tested-by: Dingisoul <[email protected]>
Signed-off-by: Laurence Oberman <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Martin K. Petersen (Oracle) <[email protected]>
[ loberman: st_remove() still returns int in 6.18; context adjusted ]
Signed-off-by: Laurence Oberman <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
drivers/scsi/st.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=a1647b653549ff29f0422a63eabf329b842f3822
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99156 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99156
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: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: unknown
Paranoid ActionableScore: 4
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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