* [CVE-2026-99016][IMPORTANT] riscv: futex: untag the user pointer before the atomic access
@ 2026-10-26 21:10 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-26 21:10 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99016
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: riscv: futex: untag the user pointer before the atomic access
Commit: 2e5c80bbc062a8dda33e687ebf7be8f074ee4f7d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2e5c80bbc062a8dda33e687ebf7be8f074ee4f7d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99016
Analysis date: Mon, 26 Oct 2026 17:10:13 -0400
ActionableScore: 7
ActionableScore lower bound: unknown
Actionable bucket: Strong Important candidate
Manual review required: YES
Summary:
A local unprivileged RISC-V process can exploit tagged-pointer validation mismatch in futex atomics to perform a directed 32-bit RMW on an arbitrary canonical kernel address and obtain a comparison oracle.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99016 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99016
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-2026-99016 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-99016 CHECK WITH IMPACT FROM ORIG NN NONE | AEGIS IMPACT IMPORTANT AEGIS_AS=7 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-20;CWE-822;CWE-123;BEST CVSS score: '7.8';DESCR 'The RISC V futex helpers validated an untagged user address but passed the original tagged pointer to supervisor mode atomic instructions. With CONFIG_RISCV_ISA_SUPM, tagged address ABI, Sv57, and a suitable PMLEN value, an unprivileged process can make `FUTEX_WAKE_OP` perform a constrained atomic read modify write on an arbitrary canonical kernel address. The comparison result also provides a read oracle, while modification of kernel memory creates a plausible privilege escalation and kernel crash path. For the CVSS the PR:L value is selected because exploitation requires execution by a local unprivileged process, but no administrative capability is required. The issue is not directly network reachable. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7.8 and the production ActionableScore returned 7 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 8.3 and ActionableScore 7 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 7) MAYBE OOB SKIP HARDWARE LINUS INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 KPANIC_UNSUPPORTED_REMOVED - - checked
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ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=7
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=CONFIRMED
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 7.
* Conservative score: 7; no floor or cap applied.
* Raw paranoid score: 7.
* Paranoid score: 7; no additional speculative effects scored.
* Final recommended bucket: **Strong Important candidate**::
2. Signal breakdown
* +2 — Strong corruption primitive: `access_ok()` validates the untagged address, but the raw tagged pointer reaches a 32-bit atomic RMW, permitting selection of a canonical kernel address.
* +2 — Strong exploit leverage: the target is arbitrary kernel virtual memory and the attacker selects a futex atomic operation and operand; this is materially stronger than a fixed overwrite.
* +1 — Local unprivileged trigger: the supplied call-path proof identifies ordinary futex use plus attacker-controlled tagged-address setup, without an administrative capability.
* +1 — Confidentiality plausible: the `FUTEX_OP_CMP_*` result supplies a limited read oracle for the targeted kernel word.
* +1 — Integrity plausible: the atomic operation actually modifies attacker-selected protected kernel memory.
* +1 — Reliable crash/strong DoS plausible from atomic modification of an invalid or sensitive kernel target; no extra availability point is counted.
* -1 — Deployment restriction: exploitation specifically requires RISC-V SUPM support, tagged-address ABI use, Sv57, and a suitable PMLEN; these are concrete boot/configuration prerequisites unavailable for an attacker to create.
Not awarded: completed LPE, arbitrary payload, code execution, remote reachability, or a separate practical exploit-chain point. NEW52/B5-R and NEW53/B3-PENDING do not apply.
3. Reachability analysis
The supported actor is a normal local process invoking `FUTEX_WAKE_OP` or the cmpxchg helper after enabling tagged addresses with `prctl`. No host UID 0, capability, namespace privilege, prior administrator-created object, race, or special physical device is required. The host must already run the specific RISC-V Sv57/SUPM configuration; an attacker cannot enable those platform prerequisites.
4. Primitive and severity reading
The primitive is a check/use address mismatch: validation uses `untagged_addr(uaddr)`, while supervisor-mode inline assembly dereferences the original tagged pointer. This yields a directed 32-bit kernel atomic RMW and comparison-based read oracle. It is CLASS_A, not a UAF or cleanup defect. Kernel-memory modification is proven; LPE and arbitrary code execution remain unproven. ImportantCandidate is YES and auto-downgrade is disallowed because a below-host-admin directed kernel write survives the configuration restriction.
This is not an authorization DENY-to-ALLOW defect, so B4 authorization-state preservation is inapplicable.
5. One-sentence report phrase
A local unprivileged RISC-V process can exploit tagged-pointer validation mismatch in futex atomics to perform a directed 32-bit RMW on an arbitrary canonical kernel address and obtain a comparison oracle.
6. Manual review advice
MANUAL CHECK REQUIRED
The proven directed kernel write is Important-grade even though practical LPE and affected deployment prevalence require confirmation.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: riscv: futex: untag the user pointer before the atomic access
Commit: 2e5c80bbc062a8dda33e687ebf7be8f074ee4f7d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2e5c80bbc062a8dda33e687ebf7be8f074ee4f7d
Commit description:
access_ok() checks untagged_addr(uaddr), but arch_futex_atomic_op_inuser()
and futex_atomic_cmpxchg_inatomic() hand the raw uaddr to the inline asm
amoswap/amoadd/.../lr.w+sc.w through the "+m" (*uaddr) operand. When the
tagged address ABI is enabled (CONFIG_RISCV_ISA_SUPM, prctl
PR_SET_TAGGED_ADDR_CTRL with PMLEN != 0), those two addresses differ: a
user pointer whose top PMLEN bits hold a tag passes access_ok() after
untagging, but the atomic is then performed on the still-tagged raw
address.
Supervisor-mode data accesses are not subject to the U-mode pointer
masking (that is governed by menvcfg.PMM, which the kernel does not set),
so hardware does not strip the tag for the kernel's own access. With
Sv57 and PMLEN=16 the tag bits overlap the canonical-address bits, so a
tagged pointer can name a canonical kernel virtual address (e.g. in the
linear map) whose untagged form is a valid user address. An unprivileged
process can thus make FUTEX_WAKE_OP perform an atomic read-modify-write on
an arbitrary kernel address, with the matching FUTEX_OP_CMP_* result
serving as a read oracle.
get_user()/put_user()/raw_copy_{to,from}_user() already untag the pointer
after the access_ok() check; do the same in the futex helpers so the
atomic operates on the address that was actually validated.
Fixes: 2e17430 ("riscv: Add support for the tagged address ABI")
Reviewed-by: Samuel Holland <[email protected]>
Signed-off-by: Ziyi Guo <[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/futex.h
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=2e5c80bbc062a8dda33e687ebf7be8f074ee4f7d
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99016 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99016
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: Not available
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: 7
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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