From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-80726][IMPORTANT] KVM: x86/mmu: WARN and clear role.invalid when creating a child shadow page Date: Thu, 03 Sep 2026 05:36:47 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-80726 X-AL-KERNEL-Priority: IMPORTANT X-AL-KERNEL-Severity: IMPORTANT X-AL-KERNEL-Base-Severity: IMPORTANT X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 8 X-AL-KERNEL-ActionableScore-Lower: 6 X-AL-KERNEL-Commit: 9b7984692c18b22d6d61af3f53887fca7fddb0f1 List-Id: CVE: CVE-2026-80726 Priority: IMPORTANT AL-KERNEL base severity: IMPORTANT KPANIC flag: YES Patch: KVM: x86/mmu: WARN and clear role.invalid when creating a child shadow page Commit: 9b7984692c18b22d6d61af3f53887fca7fddb0f1 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9b7984692c18b22d6d61af3f53887fca7fddb0f1 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80726 Analysis date: Thu, 03 Sep 2026 05:36:47 -0400 ActionableScore: 8 ActionableScore lower bound: 6 Actionable bucket: Strong Important candidate / Actionable Moderate at minimum Manual review required: YES Summary: A KVM x86 MMU shadow page can inherit an invalid role from its parent, violating active MMU page lifetime invariants and causing a slab use-after-free write in host kernel KVM code reachable by a local user with /dev/kvm access. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-80726 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80726 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-2026-80726 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-80726 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-664;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A slab use after free in KVM x86 MMU shadow page handling can occur when a child shadow page inherits an invalid role from its parent and violates the invariant that invalid pages are not kept on the active MMU page list. The observed KASAN report shows a write after free in __kvm_mmu_get_shadow_page, which makes this more severe than a warning or NULL pointer crash. For the CVSS the PR:L is selected because a local user or process with access to /dev/kvm can run a VM and drive the affected KVM MMU paths through ioctls and guest execution. The issue is not directly network reachable. Impact is at least local denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because the primitive is memory corruption in host kernel KVM MMU state.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8) YES VIRT UAF HARDWARE KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=8 ActionableScoreLower=6 ## 1. ActionableScore * Conservative score: 6 * Paranoid score: 8 * Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: ## 2. Signal breakdown Conservative signals: * Local unprivileged trigger: +1. The trace shows UID 1000 reaching the affected KVM ioctl and KVM_RUN path, assuming access to /dev/kvm. * Weak LPE concern: +1. The bug is a host kernel UAF write in KVM MMU state, but the patch does not show controlled reclaim, object replacement, callback control, or arbitrary write. * Memory corruption, strong corruption primitive: +2. KASAN reports slab-use-after-free with Write of size 8 in __kvm_mmu_get_shadow_page. * Real lifetime corruption: +1. The commit explicitly describes invalid shadow pages violating MMU page lifetime/list invariants and leading to UAF. * Reliable kernel crash / strong DoS: +1. The KASAN trace shows a concrete crash path from normal KVM execution. * Broad/default/common subsystem: +1. KVM is widely deployed and /dev/kvm is commonly accessible to VM-running users. * Legacy / non-default execution mode: -1. The affected path appears tied to KVM shadow MMU behavior, which may be less common than EPT/NPT in typical production guests. Paranoid additions: * Strong LPE plausibility instead of weak LPE concern: +1 net increase. Host kernel write-after-free in KVM MMU page management is a high-value memory corruption class. * Confidentiality impact plausible: +1. In the paranoid interpretation, host kernel MMU memory corruption may expose or influence sensitive kernel or guest/host state. * Integrity impact plausible: +1. In the paranoid interpretation, stale MMU page/list corruption may affect host kernel memory-management integrity. ## 3. Reachability analysis The realistic trigger is local access to KVM through /dev/kvm plus ability to run a VM that drives the affected MMU paths. This is PR:L in CVSS-style reasoning, not PR:H, because many deployments grant /dev/kvm access to non-root users in a kvm group or to VM management service accounts. The issue is not directly network reachable. A remote attacker would need an additional path to control a local VM process, guest workload, CI runner, cloud tenant VM, or virtualization management layer. Namespace and container behavior depends on whether /dev/kvm is exposed. If /dev/kvm is passed into a container or sandbox, a reduced-privilege container user may reach the bug without full host root. If /dev/kvm is not exposed, local reachability is significantly reduced. Call-site confidence: high. The commit includes the relevant KASAN allocation, free, and use-after-free stack traces through kvm_arch_vcpu_ioctl_run, kvm_mmu_free_roots, mmu_alloc_root, and __kvm_mmu_get_shadow_page. ## 4. Severity interpretation This is not an ordinary Moderate cleanup. The patch is only three lines, but the commit message directly states that violating the KVM MMU invariant can lead to use-after-free, and the trace shows an actual slab UAF write in host kernel KVM code. The conservative interpretation is Actionable Moderate because the demonstrated impact is a local KVM-triggered host kernel crash with real memory corruption. The paranoid interpretation is Strong Important candidate because KVM MMU UAF writes affect high-value host kernel memory-management state, and similar bug classes can sometimes become guest-to-host or local privilege escalation issues even when the initial report only shows KASAN. ## 5. One-sentence report phrase A KVM x86 MMU shadow page can inherit an invalid role from its parent, violating active MMU page lifetime invariants and causing a slab use-after-free write in host kernel KVM code reachable by a local user with /dev/kvm access. ## 6. Manual review recommendation MANUAL CHECK REQUIRED Reason: explicit slab-use-after-free with a write primitive in KVM MMU state, local non-root reachability through /dev/kvm in common deployments, and plausible escalation concern beyond simple DoS. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: KVM: x86/mmu: WARN and clear role.invalid when creating a child shadow page Commit: 9b7984692c18b22d6d61af3f53887fca7fddb0f1 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9b7984692c18b22d6d61af3f53887fca7fddb0f1 Commit description: Explicitly clear role.invalid when deriving a child shadow page's role from its parent to harden against bugs elsewhere in KVM, as violating KVM's invariant that invalid pages are NOT on the list of active MMU pages leads to use-after-free due to __kvm_mmu_prepare_zap_page() using list_add() instead of list_move() when processing an invalid shadow page, i.e. makes a bad situation far worse. Yell loudly if the parent is invalid, as it means KVM has missed a validity check, i.e. KVM is attempting to map memory using an invalid/obsolete root, but continue on as the child is otherwise still a valid shadow page. ================================================================== BUG: KASAN: slab-use-after-free in __kvm_mmu_get_shadow_page+0x1817/0x1860 [kvm] Write of size 8 at addr ff11000153dd1368 by task repro/853 CPU: 1 UID: 1000 PID: 853 Comm: repro Not tainted 7.2.0-rc2-3aec122bdcaf-next-vm #5 PREEMPT Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 Call Trace: dump_stack_lvl+0x4b/0x70 print_report+0x153/0x49c kasan_report+0xbc/0xf0 __kvm_mmu_get_shadow_page+0x1817/0x1860 [kvm] mmu_alloc_root+0x141/0x320 [kvm] kvm_mmu_load+0x612/0x20f0 [kvm] kvm_arch_vcpu_ioctl_run+0x3dd5/0x6150 [kvm] kvm_vcpu_ioctl+0x5e4/0x10d0 [kvm] __x64_sys_ioctl+0x131/0x1b0 do_syscall_64+0x67/0x5f0 entry_SYSCALL_64_after_hwframe+0x4b/0x53 Allocated by task 853: kasan_save_stack+0x20/0x40 kasan_save_track+0x14/0x30 __kasan_slab_alloc+0x5f/0x70 kmem_cache_alloc_noprof+0xfe/0x2e0 __kvm_mmu_topup_memory_cache+0x135/0x530 [kvm] paging64_page_fault+0x318/0x1e30 [kvm] kvm_mmu_do_page_fault+0x21d/0x630 [kvm] kvm_mmu_page_fault+0x18c/0x17b0 [kvm] kvm_arch_vcpu_ioctl_run+0x1f35/0x6150 [kvm] kvm_vcpu_ioctl+0x5e4/0x10d0 [kvm] __x64_sys_ioctl+0x131/0x1b0 do_syscall_64+0x67/0x5f0 entry_SYSCALL_64_after_hwframe+0x4b/0x53 Freed by task 853: kasan_save_stack+0x20/0x40 kasan_save_track+0x14/0x30 kasan_save_free_info+0x3b/0x60 __kasan_slab_free+0x43/0x70 kmem_cache_free+0xe2/0x400 kvm_mmu_commit_zap_page.part.0+0x1e2/0x310 [kvm] kvm_mmu_free_roots+0x283/0x560 [kvm] kvm_arch_vcpu_ioctl_run+0x33c8/0x6150 [kvm] kvm_vcpu_ioctl+0x5e4/0x10d0 [kvm] __x64_sys_ioctl+0x131/0x1b0 do_syscall_64+0x67/0x5f0 entry_SYSCALL_64_after_hwframe+0x4b/0x53 Reported-by: Hyunwoo Kim Fixes: a770f6f ("KVM: MMU: Inherit a shadow page's guest level count from vcpu setup") Cc: stable@vger.kernel.org Signed-off-by: Sean Christopherson Signed-off-by: Paolo Bonzini Signed-off-by: Greg Kroah-Hartman Changed files: arch/x86/kvm/mmu/mmu.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=9b7984692c18b22d6d61af3f53887fca7fddb0f1 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-80726 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80726 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: 6 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: YES 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 alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).