From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-98464][MODERATE 7.0] RISC-V: KVM: Synchronize hrtimer callback during teardown Date: Mon, 19 Oct 2026 00:16:42 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-98464 X-AL-KERNEL-Priority: MODERATE 7.0 X-AL-KERNEL-Severity: MODERATE 7.0 X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 5 X-AL-KERNEL-ActionableScore-Lower: 4 X-AL-KERNEL-Commit: 9bdbae64ae0dab93041ebc57ec4e93def793fc2d List-Id: CVE: CVE-2026-98464 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: RISC-V: KVM: Synchronize hrtimer callback during teardown Commit: 9bdbae64ae0dab93041ebc57ec4e93def793fc2d Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9bdbae64ae0dab93041ebc57ec4e93def793fc2d Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98464 Analysis date: Mon, 19 Oct 2026 00:16:42 -0400 ActionableScore: 5 ActionableScore lower bound: 4 Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review Manual review required: YES Summary: A malicious RISC-V guest can induce a timing-sensitive host KVM vCPU use-after-free by racing an armed non-Sstc hrtimer callback with SBI-requested VM teardown, causing a confirmed post-free host-kernel write and potential host denial of service. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98464 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98464 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-2026-98464 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98464 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME (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-667;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 in RISC-V KVM timer teardown can free a vCPU while its non-Sstc hrtimer callback is still running. The callback can then enter kvm_riscv_vcpu_set_interrupt and write through a freed vCPU object, with KASAN confirming a slab use-after-free and a four byte write in do_raw_spin_lock. A guest can supply the trigger by arming the timer through SBI TIME and requesting shutdown through SBI legacy shutdown or SRST when the VMM destroys the VM after KVM_EXIT_SYSTEM_EVENT. For the CVSS the PR:L reflects that control of a running guest is required, while host administrator privileges and a post-launch host ioctl are not required. AC:H applies because successful exploitation depends on winning a teardown timing window and natural reproduction was unreliable. The issue is not network reachable by itself. Impact is at least host kernel denial of service and may include confidentiality or integrity impact because a callback writes into a freed slab object. Reliable privilege escalation or guest to host escape is not demonstrated, but the confirmed UAF write warrants manual review. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 5 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 MODERATE to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. KP and FD specialist checks stage IMPORTANT to MODERATE_KPANIC because a specific evidence based protection or manual review veto was applied and FD false positive review allowed a downgrade. 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_HIGH_BASED_ON_GUESSCVSS and V33_ACTIVE_STATE_BOUNDARY_CANDIDATE and IMPORTANT_CANDIDATE_MANUAL_REVIEW and KP_ONLY_DOWNGRADE_VETO and DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 4 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. 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 5) MAYBE DANGER VIRT INIT RACE UAF SKIP HARDWARE LINUS TEST KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS V33_ACTIVE_STATE_BOUNDARY_CANDIDATE IMPORTANT_CANDIDATE_MANUAL_REVIEW KP_ONLY_DOWNGRADE_VETO DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=5 ActionableScoreLower=4 PrimitiveClass=CLASS_B RealUAF=YES HostAdminRequired=NO NamespaceScopedPrivilege=NO ImportantCandidate=YES ManualReview=REQUIRED AutoDowngradeAllowed=NO RequiresHostUID0=NO AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw conservative score: 3. * Conservative score: 4. REAL UAF FLOOR 4 changes 3 to 4. * Raw paranoid score: 3. * Paranoid score: 5. The active structured vCPU use, confirmed post-free write, guest-to-host boundary, and unresolved non-crash consequences satisfy the manual-review safety floor and V33 candidate rule. * Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**:: 2. Signal breakdown Positive signals: * +2 Real UAF. `kvm_destroy_vcpus()` frees the enclosing vCPU while the hrtimer callback can still call `kvm_riscv_vcpu_set_interrupt()`. * +1 Strong DoS concern. KASAN confirms a host-kernel slab use-after-free with a four-byte write in `do_raw_spin_lock`. A host fault or crash is technically credible. * +1 Attacker-influenced active-object lifetime. Guest SBI TIME arms the callback, and guest SBI shutdown or SRST induces the VMM teardown that frees the vCPU. The supplied call path states that no post-launch host ioctl is required. Negative signals: * -1 Hard timing condition. Natural execution reached the relevant ordering in 12 of 200 traced runs, while 1500 stock-kernel stress iterations produced no KASAN report. Important non-awarded signals: * No additional availability point is awarded for the same potential host crash. * Controlled slab reclaim or attacker-shaped replacement contents are not demonstrated. * Arbitrary or directed host-kernel write is not demonstrated. The observed write is associated with locking the stale vCPU state. * Strong guest-to-host exploitation leverage +2 is not awarded merely because this is a host KVM UAF reached from guest activity. * Confidentiality impact, host privilege escalation, and VM escape are not demonstrated. * Remote network reachability is not established. * No concrete repeated release or refcount mutation exists. The bug is callback use after vCPU destruction, not a double put or second release. * B5-R, B3-PENDING, V32, V34, and B4-PENDING do not apply. V33 assessment: * Write: a four-byte host-kernel write occurs through a freed vCPU object in `do_raw_spin_lock`. * Privileged consumer: the host hrtimer callback enters `kvm_riscv_vcpu_set_interrupt()` and operates on active host KVM interrupt state. * Boundary: guest SBI operations induce timer activity and VM teardown across the guest-to-host virtualization boundary. * Consequence: host UAF and corruption are demonstrated. Controlled host compromise is unresolved rather than proven. 3. Reachability analysis The evaluated attacker is malicious guest code. It can arm the non-Sstc timer with SBI TIME and request shutdown with SBI legacy shutdown or SRST. A VMM configured to honor `KVM_EXIT_SYSTEM_EVENT` then destroys the VM, supplying the teardown side without another attacker-controlled host ioctl. The VMM creates the vCPU and performs its destruction, but the supplied evidence establishes that the guest can induce both relevant runtime events through the supported SBI interface. This is therefore guest-induced after launch rather than a trigger requiring cooperation from a malicious host administrator. Host UID 0 and host-administrator powers are not required by the guest attacker. Namespace capabilities are not involved. Prior VMM creation of the VM is a deployment prerequisite, not attacker host-admin privilege. The race is timing-sensitive and close to one-shot for each VM teardown. Repeated attempts may require VMM VM recreation, which is not shown to be under direct guest control. 4. Primitive and severity interpretation The exact primitive is an asynchronous write-capable use-after-free of an active structured `kvm_vcpu` object. It is not fixed stale storage and is not merely a passive metadata inconsistency. The callback continues normal host interrupt processing after the enclosing vCPU has been freed. This is CLASS_B rather than CLASS_A. The post-free write and guest-triggered host lifetime violation are concrete, but controlled reclaim, attacker-selected destination, attacker-selected payload, callback hijacking, arbitrary host access, and guest escape are not established. The demonstrated consequence is host-kernel memory corruption detected by KASAN, with a credible host crash or DoS outcome. A stronger integrity consequence is technically unresolved because active freed vCPU state is modified, but privilege escalation is not established. ImportantCandidate is YES under V33 because attacker-influenced corruption of active structured state, a privileged host consumer, and a credible guest-to-host boundary are independently identified. AutoDowngradeAllowed is NO because automatically reducing this to an ordinary crash-only issue would discard the unresolved active-state corruption and virtualization-boundary analysis. Likely operational severity is Actionable Moderate. Important classification remains a mandatory review candidate rather than a proven conclusion. 5. One-sentence report phrase A malicious RISC-V guest can induce a timing-sensitive host KVM vCPU use-after-free by racing an armed non-Sstc hrtimer callback with SBI-requested VM teardown, causing a confirmed post-free host-kernel write and potential host denial of service. 6. Manual review recommendation MANUAL CHECK REQUIRED The confirmed write-capable UAF affects active host KVM state across a guest-to-host boundary, while reclaim behavior and consequences beyond host crash remain unresolved. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: RISC-V: KVM: Synchronize hrtimer callback during teardown Commit: 9bdbae64ae0dab93041ebc57ec4e93def793fc2d Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9bdbae64ae0dab93041ebc57ec4e93def793fc2d Commit description: The non-Sstc hrtimer callback clears next_set before its final uses of the enclosing vCPU. If teardown observes next_set as false while the callback is still running, kvm_riscv_vcpu_timer_cancel() skips hrtimer_cancel() and kvm_destroy_vcpus() can free the vCPU before the callback enters kvm_riscv_vcpu_set_interrupt(). A guest can arm the timer with SBI TIME and request shutdown with SBI legacy shutdown or SRST. A VMM that honors KVM_EXIT_SYSTEM_EVENT and destroys the VM supplies the teardown side of the race; no post-launch host ioctl is needed to arm or request teardown. On upstream master 62cc902 , generic KASAN reported: BUG: KASAN: slab-use-after-free in do_raw_spin_lock Write of size 4 at addr ff60000005e58898 kvm_riscv_vcpu_set_interrupt kvm_riscv_vcpu_hrtimer_expired __hrtimer_run_queues hrtimer_interrupt The object was allocated by KVM_CREATE_VCPU and freed concurrently by: kvm_destroy_vcpus kvm_arch_destroy_vm kvm_destroy_vm __fput For deterministic validation, I added mdelay(1000) immediately after the existing next_set = false assignment. This only widens the existing post-clear callback window. A no-delay trace build naturally reached the callback-after-teardown-start/before-deinit ordering in 12 of 200 runs, but 1,500 stock-kernel stress iterations did not produce a KASAN report, so natural reproduction is timing-sensitive. Always invoke hrtimer_cancel() for an initialized timer. Preserve the existing -EINVAL result when the timer is no longer set, but only after synchronizing with a running callback. With this patch, hrtimer_cancel() blocked for the full widened callback window before vCPU destruction. KASAN reported no error in 100 fixed-and-widened runs or 200 fix-only timing-sweep runs. Fixes: 3a9f66c ("RISC-V: KVM: Add timer functionality") Cc: stable@vger.kernel.org Assisted-by: OpenAI:GPT-5.6 Signed-off-by: Myeonghun Pak Reviewed-by: Anup Patel Link: https://lore.kernel.org/r/20260731163550.46991-1-mhun512@gmail.com Signed-off-by: Anup Patel Signed-off-by: Greg Kroah-Hartman Changed files: arch/riscv/kvm/vcpu_timer.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=9bdbae64ae0dab93041ebc57ec4e93def793fc2d ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98464 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98464 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: 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 alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).