From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-63875][IMPORTANT] arm64: tlb: Flush walk cache when unsharing PMD tables [ Upstream Date: Sun, 19 Jul 2026 17:16:31 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-63875 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: 7 X-AL-KERNEL-ActionableScore-Lower: 5 X-AL-KERNEL-Commit: dced308d7d6a0de1c09d2058f38f1aaaf5cbb914 List-Id: CVE: CVE-2026-63875 Priority: IMPORTANT AL-KERNEL base severity: IMPORTANT KPANIC flag: YES Patch: arm64: tlb: Flush walk cache when unsharing PMD tables [ Upstream Commit: dced308d7d6a0de1c09d2058f38f1aaaf5cbb914 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dced308d7d6a0de1c09d2058f38f1aaaf5cbb914 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63875 Analysis date: Sun, 19 Jul 2026 17:16:31 -0400 ActionableScore: 7 ActionableScore lower bound: 5 Actionable bucket: Strong Important candidate / Actionable Moderate at minimum Manual review required: YES Summary: An arm64 TLB maintenance flaw can leave stale PMD table state in the walk cache after huge_pmd_unshare(), allowing later page-table walks to use outdated non-leaf translation state and potentially causing denial of service or memory isolation impact on systems using affected hugepage mappings. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63875 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63875 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-2026-63875 IMPORTANT CHECK WITH IMPACT FROM ORIG NN LOW 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-664;*CWE-416;CWE-693;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'An arm64 TLB maintenance bug can leave a stale PMD table entry in the walk cache after huge_pmd_unshare unshares a PMD table. A later page table walk may use outdated non-leaf translation state, which can produce incorrect address translations and may affect memory isolation or crash the system. For the CVSS the PR:L is used because triggering requires local code execution and control over mappings that can reach PMD unshare paths, but it does not necessarily require full administrator privileges when hugetlb access is available. The issue is not network reachable and depends on arm64 hardware behavior plus hugepage or hugetlb mapping patterns. Impact is at least local denial of service and in the paranoid case may include confidentiality and integrity impact due to stale page table state.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7) SKIP CVE-2026-63875 UNKNOWN SKIP No affected files built, so skip this CVE NO - - unknown MAYBE HARDWARE KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7 - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=7 ActionableScoreLower=5 ## 1. ActionableScore * Conservative score: 5 * Paranoid score: 7 * Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: ## 2. Signal breakdown Conservative score signals: * Local unprivileged trigger: +1. A local process may be able to exercise hugetlb or huge PMD unshare paths if hugepage access is available. * Weak or indirect corruption candidate: +1. The stale walk-cache PMD entry can cause incorrect page-table walks, but the patch does not show an attacker-controlled overwrite or reclaim primitive. * Weak LPE concern: +1. Incorrect translations in MMU/page-table state can plausibly affect memory isolation, but practical privilege escalation is not demonstrated. * Availability impact realistic: +1. Incorrect page-table walks can plausibly crash or destabilize the system. * Broad/default security-sensitive subsystem: +1. This is arm64 MMU/TLB maintenance code, which is a high-value memory-management path. * Rare or configuration-dependent trigger: -1. Practical triggering depends on arm64 plus hugepage/hugetlb PMD unshare behavior. Paranoid additional interpretation: * Confidentiality impact plausible: +1. Stale non-leaf translation state may expose memory that should no longer be reachable through that page-table context. * Integrity impact plausible: +1. Incorrect translations may allow writes through stale mappings in a worst-case interpretation. ## 3. Reachability analysis The bug is local, not network reachable. Triggering requires execution on an affected arm64 system and a mapping pattern that reaches huge_pmd_unshare() and PMD table unsharing. In typical deployments this likely requires hugetlb or hugepage availability, which may be restricted by configuration, limits, filesystem permissions, or administrative setup. However, it does not necessarily require full host root if hugepage use is delegated to local users or workloads. Namespaces and containers may matter if hugepage resources are exposed to containerized workloads. Call-site confidence: medium. The direct patch is small, but the changed flag is in the architecture TLB flush path and the commit clearly ties it to huge_pmd_unshare() and stale walk-cache state. ## 4. Severity interpretation This is not an ordinary validation bug or simple DoS-only issue. It restores a missing walk-cache invalidation condition for unshared PMD tables, and stale page-table walk state is security-sensitive because it can affect address translation correctness. Realistic exploitation evidence is limited, so the conservative score stays in Actionable Moderate territory. The paranoid score reaches Strong Important candidate because stale MMU/page-table state can plausibly affect confidentiality and integrity, even without a demonstrated arbitrary write primitive. ## 5. One-sentence report phrase An arm64 TLB maintenance flaw can leave stale PMD table state in the walk cache after huge_pmd_unshare(), allowing later page-table walks to use outdated non-leaf translation state and potentially causing denial of service or memory isolation impact on systems using affected hugepage mappings. ## 6. Manual review recommendation MANUAL CHECK REQUIRED Reason: this is a memory-management correctness issue involving stale page-table walk-cache state. Even though no concrete exploit primitive is demonstrated, the affected path can influence address translation and memory isolation, so it should not be auto-closed. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: arm64: tlb: Flush walk cache when unsharing PMD tables [ Upstream Commit: dced308d7d6a0de1c09d2058f38f1aaaf5cbb914 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dced308d7d6a0de1c09d2058f38f1aaaf5cbb914 Changed files: arch/arm64/include/asm/tlb.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=dced308d7d6a0de1c09d2058f38f1aaaf5cbb914 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63875 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63875 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: 5 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: 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).