From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-45912][MODERATE 7.0] ext4: don't cache extent during splitting extent Date: Wed, 27 May 2026 09:40:31 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-45912 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: 8302b5b4aacdbb378f7b1216bb2ee782b5142415 List-Id: CVE: CVE-2026-45912 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: ext4: don't cache extent during splitting extent Commit: 8302b5b4aacdbb378f7b1216bb2ee782b5142415 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8302b5b4aacdbb378f7b1216bb2ee782b5142415 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45912 Analysis date: Wed, 27 May 2026 09:40:31 -0400 ActionableScore: 5 ActionableScore lower bound: 4 Actionable bucket: Actionable Moderate at minimum Manual review required: NO Summary: No one-sentence report phrase was found. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-45912 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45912 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: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-45912 MODERATE CHECK Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H';*CWE-664;CWE-682;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '6.3';DESCR 'ext4 can leave a stale hole in the extent status tree while splitting an unwritten extent, because extents being modified may be cached during the split. A local user with write access to an ext4 file may be able to create the pattern with fallocate, Direct IO to part of the unwritten extent, and later buffered delayed writes, causing inconsistent space accounting or filesystem metadata state. For the CVSS the PR:L is used because ordinary file write access can influence the affected extent conversion path without administrator privileges. The issue is not network reachable and does not show kernel memory corruption or privilege escalation. Impact is primarily filesystem integrity and availability, not confidentiality.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) YES DISK WARNONLY IMPROVEONLY KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=5 ActionableScoreLower=4 1. ActionableScore * Conservative score: 4 * Paranoid score: 5 * Final recommended bucket: **Actionable Moderate at minimum**:: 2. Signal breakdown * Local unprivileged trigger: +1. A local user with write access to an ext4 file can plausibly influence unwritten extent splitting through fallocate, DIO, and buffered writes. * Filesystem/storage metadata corruption reachable by unprivileged local user: +1. The bug can leave stale extent-status entries and cause incorrect space accounting. * Important filesystem/storage path: +1. ext4 extent splitting, unwritten extent conversion, and delayed allocation accounting are core filesystem paths. * Integrity impact plausible: +1. Incorrect extent status and space accounting can affect filesystem metadata/accounting correctness. * Availability impact realistic: +1. Space accounting errors or later consistency checks can cause operation failure, warnings, or filesystem disruption. * Hard/specific sequence required: -1. Triggering requires a particular unwritten extent split pattern, DIO write to part of the extent, and later buffered delayed write. * No kernel memory corruption: +0. This is filesystem state/accounting inconsistency, not UAF, OOB write, arbitrary write, or LPE. * No confidentiality impact: +0. No read disclosure or cross-file data exposure is shown. 3. Reachability analysis The affected path can be reached by local file operations on ext4, especially a sequence involving unwritten extents, partial Direct IO writes, and later buffered delayed writes. Administrative privileges are not inherently required if the user can create and write files on an ext4 filesystem. Namespaces or containers may still reach the bug if they write to an ext4-backed filesystem and can issue the needed I/O pattern. The issue is not network reachable except indirectly through services that expose local file writes. 4. Severity interpretation This is an actionable Moderate filesystem integrity issue. It is not an Important-class memory corruption bug, but it affects ext4 metadata/accounting correctness in a path potentially reachable by unprivileged local file writes. The realistic impact is filesystem accounting inconsistency and possible availability degradation. Theoretical impact should not be upgraded to privilege escalation without stronger evidence. 5. One-sentence report phrase ext4 may cache stale extent status during unwritten extent splitting, allowing local file write patterns to create incorrect space accounting and filesystem metadata state. 6. Manual review recommendation MANUAL CHECK RECOMMENDED The issue affects a core ext4 metadata/accounting path and may be reachable by unprivileged local file operations, but it does not show kernel memory corruption or privilege escalation. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: ext4: don't cache extent during splitting extent Commit: 8302b5b4aacdbb378f7b1216bb2ee782b5142415 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8302b5b4aacdbb378f7b1216bb2ee782b5142415 Commit description: Caching extents during the splitting process is risky, as it may result in stale extents remaining in the status tree. Moreover, in most cases, the corresponding extent block entries are likely already cached before the split happens, making caching here not particularly useful. Assume we have an unwritten extent, and then DIO writes the first half. [UUUUUUUUUUUUUUUU] on-disk extent U: unwritten extent [UUUUUUUUUUUUUUUU] extent status tree |<- ->| ----> dio write this range First, when ext4_split_extent_at() splits this extent, it truncates the existing extent and then inserts a new one. During this process, this extent status entry may be shrunk, and calls to ext4_find_extent() and ext4_cache_extents() may occur, which could potentially insert the truncated range as a hole into the extent status tree. After the split is completed, this hole is not replaced with the correct status. [UUUUUUU|UUUUUUUU] on-disk extent U: unwritten extent [UUUUUUU|HHHHHHHH] extent status tree H: hole Then, the outer calling functions will not correct this remaining hole extent either. Finally, if we perform a delayed buffer write on this latter part, it will re-insert the delayed extent and cause an error in space accounting. In adition, if the unwritten extent cache is not shrunk during the splitting, ext4_cache_extents() also conflicts with existing extents when caching extents. In the future, we will add checks when caching extents, which will trigger a warning. Therefore, Do not cache extents that are being split. Signed-off-by: Zhang Yi Reviewed-by: Ojaswin Mujoo Reviewed-by: Baokun Li Cc: stable@kernel.org Message-ID: <20251129103247.686136-6-yi.zhang@huaweicloud.com> Signed-off-by: Theodore Ts'o Signed-off-by: Greg Kroah-Hartman Changed files: fs/ext4/extents.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=8302b5b4aacdbb378f7b1216bb2ee782b5142415 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-45912 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45912 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:N/I:L/A:L The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H The Best / paranoid CVSS score: 6.3 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).