From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-45915][LOW] fat: avoid parent link count underflow in rmdir [ Upstream Date: Wed, 27 May 2026 21:37:13 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-45915 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: 3 X-AL-KERNEL-ActionableScore-Lower: 1 X-AL-KERNEL-Commit: 7fe0de287e931e07cb96ecf1f449b2ebdb0e1115 List-Id: CVE: CVE-2026-45915 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: fat: avoid parent link count underflow in rmdir [ Upstream Commit: 7fe0de287e931e07cb96ecf1f449b2ebdb0e1115 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7fe0de287e931e07cb96ecf1f449b2ebdb0e1115 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45915 Analysis date: Wed, 27 May 2026 21:37:13 -0400 ActionableScore: 3 ActionableScore lower bound: 1 Actionable bucket: Borderline, manual review recommended Manual review required: YES Summary: A corrupted FAT/VFAT image with an invalid parent directory link count can let local `rmdir()` drive `i_nlink` to zero and trigger a WARN, causing possible DoS when `panic_on_warn` is enabled. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-45915 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45915 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: YES A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-45915 LOW CHECK Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-191;CWE-682;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A corrupted FAT or VFAT filesystem image can contain a parent directory inode with an invalid low link count. When a local user removes a subdirectory, msdos_rmdir() or vfat_rmdir() used to call drop_nlink() unconditionally and could drive the parent i_nlink to zero, triggering a WARN in the VFS link count path. For the CVSS the PR:L is used because the attacker needs local ability to access a mounted writable FAT filesystem and issue rmdir, but full administrator privileges are not required once the crafted image is mounted. The issue is not network reachable. Impact is denial of service through WARN or possible panic_on_warn behavior. No supported evidence indicates information disclosure, integrity impact beyond the already corrupted filesystem image, kernel memory corruption, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2) MAYBE OOB DISK SIMPLEFIX IMPROVEONLY LINUS LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 ActionableScoreLower=1 ## 1. ActionableScore * Conservative score: 1 * Paranoid score: 3 * Final recommended bucket: **Borderline, manual review recommended**:: ## 2. Signal breakdown * Local unprivileged trigger after mount: +1 Once a corrupted FAT or VFAT image is mounted writable and accessible, a local user can issue `rmdir()` on the affected directory. * Availability impact realistic: +1 The demonstrated impact is a WARN in `drop_nlink()`. This can become stronger DoS if `panic_on_warn` is enabled. * Important filesystem/storage path: +1 in paranoid scoring FAT/VFAT is widely used for removable media, but the bug requires an already corrupted image and affects link-count accounting rather than kernel memory safety. * Requires special crafted/corrupted filesystem state: -1 conservative The issue depends on a corrupted FAT image with inconsistent parent link count. * No memory corruption: +0 The patch shows an `i_nlink` underflow/WARN prevention, not UAF, OOB write, arbitrary write, or heap corruption. * No strong integrity impact: +0 The filesystem is already corrupt. The kernel is not shown to create broad persistent metadata corruption beyond the invalid input state. ## 3. Reachability analysis The attacker needs a corrupted FAT/VFAT filesystem image to be mounted writable, then needs local access to remove a subdirectory. If users can mount arbitrary images or insert removable media that is automounted writable, the practical trigger is easier. If mounting removable or loopback filesystems is restricted to trusted administrators, reachability is much lower. Namespaces may matter if unprivileged mounts or container-managed removable media are allowed, but by default this is not a network-reachable issue. Realistic exploitation is WARN or `panic_on_warn` DoS, not privilege escalation. ## 4. Severity interpretation This behaves like ordinary Moderate or Low-like DoS, with a borderline case for environments that automatically mount untrusted FAT media writable. The theoretical impact is availability via WARN/panic. The realistic evidence does not support memory corruption, data disclosure, LPE, or kernel-created filesystem corruption. ## 5. One-sentence report phrase A corrupted FAT/VFAT image with an invalid parent directory link count can let local `rmdir()` drive `i_nlink` to zero and trigger a WARN, causing possible DoS when `panic_on_warn` is enabled. ## 6. Manual review recommendation NO MANUAL CHECK NEEDED No manual check is needed for normal server deployments. Consider manual review only for products that auto-mount untrusted removable FAT media writable or enable `panic_on_warn`. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: fat: avoid parent link count underflow in rmdir [ Upstream Commit: 7fe0de287e931e07cb96ecf1f449b2ebdb0e1115 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7fe0de287e931e07cb96ecf1f449b2ebdb0e1115 Changed files: fs/fat/namei_msdos.c fs/fat/namei_vfat.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=7fe0de287e931e07cb96ecf1f449b2ebdb0e1115 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-45915 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45915 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:N/A:L The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 5.5 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: 3 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).