From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-80820][LOW] KVM: x86: Fix shadow paging use-after-free due to unexpected GFN Date: Fri, 04 Sep 2026 18:00:53 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-80820 X-AL-KERNEL-Priority: LOW X-AL-KERNEL-Severity: LOW X-AL-KERNEL-Base-Severity: LOW X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 1 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 14d1e55dfd2cf4711bff164a6aaaddb783552134 List-Id: CVE: CVE-2026-80820 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: KVM: x86: Fix shadow paging use-after-free due to unexpected GFN Commit: 14d1e55dfd2cf4711bff164a6aaaddb783552134 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=14d1e55dfd2cf4711bff164a6aaaddb783552134 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80820 Analysis date: Fri, 04 Sep 2026 18:00:53 -0400 ActionableScore: 1 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate / Low-like Manual review required: NO Summary: No one-sentence report phrase was found. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-80820 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80820 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: LOW Manual review required: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-80820 LOW https://www.kernelcve.org/list/?q=CVE-2026-80820 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:H/UI:N/S:U/C:N/I:N/A:H';*CWE-835;CWE-834;**CWE-400;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.1';DESCR 'XFS online scrub can livelock when an AGI unlinked inode bucket contains a circular list. The scrub and repair code walks i_next_unlinked until NULLAGINO, so corrupted metadata can make the same inode numbers repeat indefinitely and keep the scrub path busy. For the CVSS the PR:H is used because reliable triggering normally requires administrator level control over XFS scrub/repair or over a mounted corrupted filesystem image. The issue is not network reachable and requires local filesystem access plus the relevant XFS online scrub functionality. Impact is denial of service through CPU livelock or a stuck maintenance operation. No memory corruption, information disclosure, or privilege escalation primitive is evident from the code change.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1) YES DISK VIRT INIT UAF HARDWARE LINUS MEMORY DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=1 1. ActionableScore * Conservative score: 1 * Paranoid score: 1 * Final recommended bucket: **Ordinary Moderate / Low-like**:: 2. Signal breakdown * Reliable kernel crash / strong DoS: +1. The bug can livelock XFS online scrub or repair when the AGI unlinked inode list contains a cycle. * Availability impact realistic: +1. The affected scrub or repair operation can get stuck and consume CPU until interrupted or the system is otherwise recovered. * Important filesystem/storage path: +1. The code is in XFS online scrub and repair for AGI unlinked inode metadata. * Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2. Reliable triggering normally requires running XFS online scrub/repair or mounting and operating on a corrupted XFS filesystem image, which is normally administrator controlled. * No memory corruption signal: +0. The patch adds cycle detection with an agino bitmap and does not indicate UAF, OOB access, overwrite, refcount corruption, or stale pointer reuse. * No privilege escalation signal: +0. The issue is a livelock in metadata traversal, not an ownership or permission bypass. 3. Reachability analysis The realistic trigger is local and administrative: a user must cause online scrub or repair to process an XFS AGI unlinked inode bucket that already contains a circular list. Ordinary remote network traffic cannot reach this path. Containers or namespaces do not normally lower the privilege requirement unless the environment delegates filesystem administration or lets an untrusted actor mount attacker-controlled XFS images and invoke scrub. The vulnerable path is not a common default data-plane operation; it is a maintenance/checking path. 4. Severity interpretation This behaves like ordinary Moderate or Low-like DoS, not Important. The theoretical impact is a stuck scrub/repair traversal and CPU consumption. There is no evidence of memory corruption, confidentiality impact, integrity impact, privilege escalation, cross-file write, page-cache corruption, or security-boundary bypass. The paranoid score does not increase because the bug class remains an infinite loop/livelock with admin-gated reachability. 5. One-sentence report phrase XFS online scrub or repair can livelock when a corrupted AGI unlinked inode bucket contains a circular list, causing a local administrator-triggered denial of service without evidence of memory corruption or privilege escalation. 6. Manual review recommendation NO MANUAL CHECK NEEDED. The patch shows a bounded fix for cycle detection in an admin-gated XFS scrub/repair path, with no supported memory corruption or security-boundary bypass primitive. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: KVM: x86: Fix shadow paging use-after-free due to unexpected GFN Commit: 14d1e55dfd2cf4711bff164a6aaaddb783552134 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=14d1e55dfd2cf4711bff164a6aaaddb783552134 Commit description: The shadow MMU computes GFNs for direct shadow pages using sp->gfn plus the SPTE index. This assumption breaks for shadow paging if the guest page tables are modified between VM entries (similar to commit aad885e , "KVM: x86/mmu: Drop/zap existing present SPTE even when creating an MMIO SPTE", 2026-03-27). The flow is as follows: - a PDE is installed for a 2MB mapping, and a page in that area is accessed. KVM creates a kvm_mmu_page consisting of 512 4KB pages; the kvm_mmu_page is marked by FNAME(fetch) as direct-mapped because the guest's mapping is a huge page (and thus contiguous). - the PDE mapping is changed from outside the guest. - the guest accesses another page in the same 2MB area. KVM installs a new leaf SPTE and rmap entry; the SPTE uses the "correct" GFN (i.e. based on the new mapping, as changed in the previous step) but that GFN is outside of the [sp->gfn, sp->gfn + 511] range; therefore the rmap entry cannot be found and removed when the kvm_mmu_page is zapped. - the memslot that covers the first 2MB mapping is deleted, and the kvm_mmu_page for the now-invalid GPA is zapped. However, rmap_remove() only looks at the [sp->gfn, sp->gfn + 511] range established in step 1, and fails to find the rmap entry that was recorded by step 3. - any operation that causes an rmap walk for the same page accessed by step 3 then walks a stale rmap and dereferences a freed kvm_mmu_page. This includes dirty logging or MMU notifier invalidations (e.g., from MADV_DONTNEED). The underlying issue is that KVM's walking of shadow PTEs assumes that if a SPTE is present when KVM wants to install a non-leaf SPTE, then the existing kvm_mmu_page must be for the correct gfn. Because the only way for the gfn to be wrong is if KVM messed up and failed to zap a SPTE... which shouldn't happen, but *actually* only happens in response to a guest write. That bug dates back literally forever, as even the first version of KVM assumes that the GFN matches and walks into the "wrong" shadow page. However, that was only an imprecision until 2032a93 ("KVM: MMU: Don't allocate gfns page for direct mmu pages") came along. Fix it by checking for a target gfn mismatch and zapping the existing SPTE. That way the old SP and rmap entries are gone, KVM installs the rmap in the right location, and everyone is happy. Fixes: 2032a93 ("KVM: MMU: Don't allocate gfns page for direct mmu pages") Fixes: 6aa8b73 ("kvm: userspace interface") Reported-by: Alexander Bulekov Reported-by: Fred Griffoul Cc: stable@vger.kernel.org Signed-off-by: Sean Christopherson Link: https://patch.msgid.link/20260503201029.106481-1-pbonzini@redhat.com/ Signed-off-by: Paolo Bonzini Signed-off-by: Sasha Levin 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=14d1e55dfd2cf4711bff164a6aaaddb783552134 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-80820 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80820 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:H/UI:N/S:U/C:N/I:N/A:L The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 4.1 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: 1 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: LOW KPANIC detected for this report: NO Published priority for this report (same as in Subject): LOW 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).