* [CVE-2026-99018][IMPORTANT] riscv: entry: gate shadow call stack reload on sstatus.SPP, not tp
@ 2026-10-27 4:01 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-27 4:01 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99018
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: riscv: entry: gate shadow call stack reload on sstatus.SPP, not tp
Commit: 6639735367a7d491977e9e5e7afc50b489d81d83
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6639735367a7d491977e9e5e7afc50b489d81d83
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99018
Analysis date: Tue, 27 Oct 2026 00:01:25 -0400
ActionableScore: 8
ActionableScore lower bound: unknown
Actionable bucket: Strong Important candidate
Manual review required: YES
Summary:
An unprivileged RISC-V task can bypass kernel SCS-pointer reload using forged `tp`, yielding attacker-directed kernel writes and return-address control through a poisoned `gp`.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99018 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99018
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: IMPORTANT
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-2026-99018 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-99018 CHECK WITH IMPACT FROM ORIG NN NONE | AEGIS IMPACT IMPORTANT AEGIS_AS=6 LEGACY=IMPORTANT RESULT=SAME (final reconciliation agrees; DECISION: both IMPORTANT) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-807;CWE-123;CWE-269;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '7.8';DESCR 'On RISC V systems with CONFIG_SHADOW_CALL_STACK, handle_exception trusted a comparison involving the user writable tp register to decide whether the kernel shadow call stack pointer must be reloaded. An unprivileged local process can set tp to the linear map address of its own task_struct and enter the kernel with an attacker controlled gp, causing compiler generated shadow stack stores and loads to provide kernel arbitrary write and return address control primitives. For the CVSS the PR:L value is selected because local execution by an unprivileged user is required, while kernel privileges or administrative capabilities are not required. The base AC:H reflects the need to determine or brute force the current task_struct address when KASLR is enabled, while the worst case AC:L applies when that address is predictable or otherwise known. Successful exploitation can plausibly provide full kernel privilege escalation and complete confidentiality, integrity, and availability impact. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7.8 and the production ActionableScore returned 8 with recommendation Strong Important candidate. 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. ActionableScore thresholds stage MODERATE_KPANIC to IMPORTANT because the ActionableScore threshold rule was applied. final reconciliation stage IMPORTANT to IMPORTANT because KP specialist did not establish a qualifying panic. 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 produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 and INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 7.8 and ActionableScore 6 with primitive class CLASS_A and manual review REQUIRED. Its independently reconciled impact was IMPORTANT. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 8) MAYBE KASAN OOB INIT SKIP HARDWARE LINUS TEST INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 KPANIC_UNSUPPORTED_REMOVED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=8
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 8.
* Conservative score: 8; no floor or cap applied.
* Raw paranoid score: 8.
* Paranoid score: 8; no additional speculative effects scored.
* Final recommended bucket: **Strong Important candidate**::
2. Signal breakdown
* +2 strong corruption primitive: user-controlled `gp` causes compiler-generated `sd ra, -8(gp)` to write a return address to an attacker-selected kernel address.
* +2 strong exploit leverage: the matching `ld ra, -8(gp)` supplies direct return-address/control-flow influence, separate from the write primitive.
* +1 local unprivileged trigger: a normal U-mode task controls `tp` and `gp` and can enter the kernel without administrative capabilities.
* +1 practical exploit chain: forged `tp == ¤t` skips SCS reload, preserves poisoned `gp`, and compiler-emitted SCS operations then write and load through it.
* +1 integrity impact: the primitive directly modifies attacker-selected protected kernel memory.
* +1 reliable kernel crash/strong DoS: attacker-controlled shadow-stack address and return state can plausibly produce a fatal kernel control-flow failure.
* No confidentiality or completed privilege-escalation point: arbitrary disclosure, root acquisition, and a working end-to-end LPE are not demonstrated.
* No AC penalty: KASLR may require brute force, but the evidence does not establish a narrow or unrepeatable race.
* No rarity penalty: RISC-V plus `CONFIG_SHADOW_CALL_STACK` limits exposure, but is not proven both rare and difficult to reach.
3. Reachability analysis
The complete trigger is available to an ordinary local RISC-V U-mode process: set user-writable registers, guess or learn its `task_struct` linear-map address, and trap into the kernel. No host UID 0, capability, namespace privilege, prior privileged setup, special device, or race is shown as required. `CONFIG_SHADOW_CALL_STACK` and affected RISC-V entry code are necessary. KASLR can increase attack complexity but repeated attempts are indicated.
4. Primitive and severity reading
This is not a UAF or authorization bypass. It is a direct active kernel control-state corruption primitive: attacker-controlled `gp` becomes the kernel shadow-call-stack pointer, enabling an attacker-directed kernel write and subsequent return-address load. Persistent poisoning through `task_struct.scs_sp` strengthens repeatability. Arbitrary kernel read, completed LPE, and full CIA compromise remain unproven, but the shown write and control-flow primitives are independently sufficient for CLASS_A and Important retention. Auto-downgrade is unsafe.
5. One-sentence report phrase
An unprivileged RISC-V task can bypass kernel SCS-pointer reload using forged `tp`, yielding attacker-directed kernel writes and return-address control through a poisoned `gp`.
6. Manual review advice
MANUAL CHECK REQUIRED
Confirm affected configurations and practical address discovery, while retaining Important because the patch establishes arbitrary-write and control-flow primitives.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: riscv: entry: gate shadow call stack reload on sstatus.SPP, not tp
Commit: 6639735367a7d491977e9e5e7afc50b489d81d83
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6639735367a7d491977e9e5e7afc50b489d81d83
Commit description:
Under CONFIG_SHADOW_CALL_STACK, handle_exception() reloads the kernel
shadow call stack pointer (gp) only when the trap came from userspace.
It made that decision with scs_load_current_if_task_changed, which
compares the trap-time tp (read back from sscratch into s5) against the
current tp:
beq \prev, tp, _skip_scs
scs_load_current
_skip_scs:
tp is a user-writable register. A user task that sets tp to the
linear-map address of its own task_struct (¤t) before trapping
makes the comparison succeed (can achieve by brute force with KASLR
enable under our test), so the kernel skips scs_load_current and
keeps running with the gp that userspace left in place. Under
CONFIG_SHADOW_CALL_STACK gp is the shadow call stack pointer and
load_global_pointer is a no-op, so the compiler-emitted
"sd ra, -8(gp)" / "ld ra, -8(gp)" in every non-leaf function then write
to, and return through, an attacker-controlled address -- an
unprivileged kernel arbitrary-write and control-flow-hijack primitive.
The poisoned gp is saved to task_struct.scs_sp on return, so the reload
is skipped on every subsequent entry as well.
Decide "came from userspace" from the hardware sstatus.SPP bit (already
in s1 at this point) instead of from tp, mirroring the check that
ret_from_exception already uses a few instructions later. SPP is
written by hardware on trap entry and cannot be forged from U-mode.
Legitimate behaviour is unchanged: user entries (SPP=0) reload the task
shadow call stack, nested kernel entries (SPP=1) skip it.
Fixes: d1584d7 ("riscv: Implement Shadow Call Stack")
Signed-off-by: Ziyi Guo <[email protected]>
Reviewed-by: Sami Tolvanen <[email protected]>
Link: https://patch.msgid.link/[email protected]
Cc: [email protected]
Signed-off-by: Paul Walmsley <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
arch/riscv/include/asm/scs.h
arch/riscv/kernel/entry.S
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=6639735367a7d491977e9e5e7afc50b489d81d83
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99018 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99018
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:H/I:H/A:H
The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 7.8
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: 8
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: IMPORTANT
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): IMPORTANT
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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