From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-64130][MODERATE REGULAR] mm/page_alloc: fix initialization of tags of the huge zero folio with init_on_free Date: Sun, 19 Jul 2026 13:58:39 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-64130 X-AL-KERNEL-Priority: MODERATE REGULAR X-AL-KERNEL-Severity: MODERATE REGULAR X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 2 X-AL-KERNEL-ActionableScore-Lower: 1 X-AL-KERNEL-Commit: 738d18f1da3513d17b6f7bf30146cc4ac2480ffd List-Id: CVE: CVE-2026-64130 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: mm/page_alloc: fix initialization of tags of the huge zero folio with init_on_free Commit: 738d18f1da3513d17b6f7bf30146cc4ac2480ffd Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=738d18f1da3513d17b6f7bf30146cc4ac2480ffd Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64130 Analysis date: Sun, 19 Jul 2026 13:58:39 -0400 ActionableScore: 2 ActionableScore lower bound: 1 Actionable bucket: Ordinary Moderate / Low-like, with manual review recommended for MTE metadata exposure Manual review required: YES Summary: An arm64 MTE initialization bug can leave stale allocation tags on the huge zero folio when init_on_free is enabled, allowing a local user to observe non zero tag metadata instead of the documented zero tag state. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-64130 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64130 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 REGULAR 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-2026-64130 MODERATE 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:L/I:L/A:L';CWE-665;CWE-908;*CWE-200;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '4.5';DESCR 'An arm64 MTE initialization bug can leave stale allocation tags on the huge zero folio when init_on_free is enabled. The huge zero folio is mapped through a special PMD, so the normal first user mapping path does not initialize tags with set_pte_at and userspace may observe non zero tags instead of the documented zero tag state. For the CVSS the PR:L is used because a local unprivileged process is needed to create and access the relevant user mappings on an affected arm64 MTE system. The issue is not network reachable and does not provide a demonstrated arbitrary write or UAF primitive. Impact is mainly limited metadata exposure and possible local process disruption from MTE tag faults, with a conservative integrity impact because MTE tag state is security relevant metadata.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like, with manual review recommended for MTE metadata exposure (with actual score 2) SKIP CVE-2026-64130 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: adfb6609c6809e107ded9a1cd46f519c882e64ea YES NO NO unknown MAYBE READ KASAN INIT HARDWARE INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=2 ActionableScoreLower=1 ## 1. ActionableScore * Conservative score: 1 * Paranoid score: 2 * Final recommended bucket: **Ordinary Moderate / Low-like, with manual review recommended for MTE metadata exposure**:: ## 2. Signal breakdown Conservative signals: * Local unprivileged trigger: +1 A local process can create and access the relevant user mappings on an affected arm64 MTE system. * Confidentiality impact plausible: +1 Userspace may observe stale non zero MTE allocation tags from the huge zero folio. This is metadata exposure, not direct page-content disclosure. * Rare AND difficult-to-reach subsystem/configuration: -1 The issue requires arm64 MTE, init_on_free, huge zero folio behavior, and a mapping path through a special PMD. Paranoid additional signal: * Integrity impact plausible: +1 MTE tags are security-relevant metadata. Incorrect initial tag state may weaken expected memory-tagging semantics, but no arbitrary write, UAF, or privilege escalation primitive is shown. Not awarded: * No generic memory corruption. The patch shows stale tag metadata, not UAF, OOB write, double free, or type confusion. * No reliable kernel crash or strong DoS. * No network reachability. * No privilege escalation signal. * No page-cache or Dirty-Pipe-like ownership/COW bypass. ## 3. Reachability analysis The bug is locally reachable by a user process that can create the relevant MTE-enabled mappings on arm64. It is not network reachable. Containers do not fundamentally change the primitive unless the container can access arm64 MTE userspace features and create the required mappings, but the impact remains metadata exposure rather than host memory corruption. The affected path is configuration dependent. It requires arm64 MTE support and init_on_free. The huge zero folio special PMD path is important because ordinary PTE mappings would initialize tags through set_pte_at and related paths. Call-site confidence: high. The relevant allocation hook, tag clearing helper, and huge zero folio mapping behavior are directly described by the patch and commit message. ## 4. Severity interpretation This behaves more like an ordinary Moderate or Low-like issue than an Important vulnerability. The theoretical concern is that stale MTE allocation tags are exposed to userspace and violate documented MTE initialization semantics. The realistic evidence supports limited metadata exposure and possible local process-level disruption from tag faults, not kernel memory corruption or LPE. Paranoid scoring raises the issue slightly because MTE tags are security metadata, but the patch does not support a strong confidentiality, integrity, or availability impact. ## 5. One-sentence report phrase An arm64 MTE initialization bug can leave stale allocation tags on the huge zero folio when init_on_free is enabled, allowing a local user to observe non zero tag metadata instead of the documented zero tag state. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED Manual review is recommended because the issue exposes MTE security metadata and depends on architecture-specific memory-tagging semantics. It does not currently justify Important handling because there is no demonstrated UAF, arbitrary write, privilege escalation, or system-wide DoS. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: mm/page_alloc: fix initialization of tags of the huge zero folio with init_on_free Commit: 738d18f1da3513d17b6f7bf30146cc4ac2480ffd Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=738d18f1da3513d17b6f7bf30146cc4ac2480ffd Commit description: __GFP_ZEROTAGS semantics are currently a bit weird, but effectively this flag is only ever set alongside __GFP_ZERO and __GFP_SKIP_KASAN. If we run with init_on_free, we will zero out pages during __free_pages_prepare(), to skip zeroing on the allocation path. However, when allocating with __GFP_ZEROTAG set, post_alloc_hook() will consequently not only skip clearing page content, but also skip clearing tag memory. Not clearing tags through __GFP_ZEROTAGS is irrelevant for most pages that will get mapped to user space through set_pte_at() later: set_pte_at() and friends will detect that the tags have not been initialized yet (PG_mte_tagged not set), and initialize them. However, for the huge zero folio, which will be mapped through a PMD marked as special, this initialization will not be performed, ending up exposing whatever tags were still set for the pages. The docs (Documentation/arch/arm64/memory-tagging-extension.rst) state that allocation tags are set to 0 when a page is first mapped to user space. That no longer holds with the huge zero folio when init_on_free is enabled. Fix it by decoupling __GFP_ZEROTAGS from __GFP_ZERO, passing to tag_clear_highpages() whether we want to also clear page content. Invert the meaning of the tag_clear_highpages() return value to have clearer semantics. Reproduced with the huge zero folio by modifying the check_buffer_fill arm64/mte selftest to use a 2 MiB area, after making sure that pages have a non-0 tag set when freeing (note that, during boot, we will not actually initialize tags, but only set KASAN_TAG_KERNEL in the page flags). $ ./check_buffer_fill 1..20 ... not ok 17 Check initial tags with private mapping, sync error mode and mmap memory not ok 18 Check initial tags with private mapping, sync error mode and mmap/mprotect memory ... This code needs more cleanups; we'll tackle that next, like decoupling __GFP_ZEROTAGS from __GFP_SKIP_KASAN. [akpm@linux-foundation.org: s/__GPF_ZERO/__GFP_ZERO/, per David] Link: https://lore.kernel.org/20260421-zerotags-v2-1-05cb1035482e@kernel.org Fixes: adfb660 ("mm/huge_memory: initialise the tags of the huge zero folio") Signed-off-by: David Hildenbrand (Arm) Reviewed-by: Catalin Marinas Tested-by: Lance Yang Cc: Brendan Jackman Cc: Dev Jain Cc: Johannes Weiner Cc: Liam Howlett Cc: Lorenzo Stoakes (Oracle) Cc: Mark Brown Cc: Michal Hocko Cc: Mike Rapoport Cc: Ryan Roberts Cc: Suren Baghdasaryan Cc: Will Deacon Cc: Zi Yan Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman Changed files: arch/arm64/memory-tagging-extension.rst arch/arm64/include/asm/page.h arch/arm64/mm/fault.c include/linux/gfp_types.h include/linux/highmem.h mm/page_alloc.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=738d18f1da3513d17b6f7bf30146cc4ac2480ffd ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-64130 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64130 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:N/A:L The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L The Best / paranoid CVSS score: 4.5 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: 1 Paranoid ActionableScore: 2 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: NO Published priority for this report (same as in Subject): MODERATE REGULAR 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).