From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-74400][MODERATE REGULAR] bpf: fix crash in bpf_[set|remove]_dentry_xattr for negative dentries [ Upstream Date: Sat, 15 Aug 2026 20:36:56 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-74400 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: 3 X-AL-KERNEL-ActionableScore-Lower: 0 X-AL-KERNEL-Commit: e003f3a4be738f650e283e92d07017c3fef8da52 List-Id: CVE: CVE-2026-74400 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: bpf: fix crash in bpf_[set|remove]_dentry_xattr for negative dentries [ Upstream Commit: e003f3a4be738f650e283e92d07017c3fef8da52 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e003f3a4be738f650e283e92d07017c3fef8da52 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74400 Analysis date: Sat, 15 Aug 2026 20:36:56 -0400 ActionableScore: 3 ActionableScore lower bound: 0 Actionable bucket: Borderline manual review recommended / ordinary Moderate if BPF is privileged-only Manual review required: YES Summary: A negative dentry passed to BPF xattr kfuncs can produce a NULL inode dereference or panic-on-warn condition, causing a local denial of service when privileged or preloaded BPF LSM code reaches this path. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-74400 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74400 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-2026-74400 MODERATE CHECK WITH IMPACT FROM ORIG NN LOW 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-476;CWE-617;CWE-703;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'bpf_set_dentry_xattr and bpf_remove_dentry_xattr can dereference a NULL inode when a negative dentry is passed to the BPF xattr kfuncs. A BPF LSM program can encounter such a dentry in hooks such as security_inode_create before the inode exists, and inode_lock on the NULL inode can crash the kernel. Related xattr permission helpers also used WARN_ON for a NULL inode, which can become a denial of service on systems with panic_on_warn enabled. For the CVSS the PR:L is used in the paranoid view because a local unprivileged user may be able to trigger an already loaded BPF LSM program that calls these kfuncs on a negative dentry, or some environments may delegate BPF capabilities to a service account. The issue is not network reachable. Impact is denial of service only via kernel crash or panic_on_warn, with no evidence of UAF, OOB access, information disclosure, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / ordinary Moderate if BPF is privileged-only (with actual score 2) SKIP CVE-2026-74400 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 56467292794b800164df20c076c409ac548e56ec YES NO NO unknown MAYBE NULLPTR DISK BPF SIMPLEFIX IMPROVEONLY LINUS INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 ActionableScoreLower=0 ## 1. ActionableScore * Conservative score: 0 * Paranoid score: 3 * Final recommended bucket: **Borderline manual review recommended / ordinary Moderate if BPF is privileged-only**:: ## 2. Signal breakdown Conservative signals: * Reliable kernel crash / strong DoS: +1 `bpf_set_dentry_xattr()` and `bpf_remove_dentry_xattr()` can call `inode_lock(NULL)` for a negative dentry, causing a NULL pointer dereference. * Availability impact realistic: +1 The crash path is concrete, and `WARN_ON(!inode)` can also become DoS on systems with `panic_on_warn`. * Requires admin/root/CAP_* in typical deployments: -2 Loading or attaching the relevant BPF program normally requires privileged BPF access such as `CAP_BPF`, `CAP_SYS_ADMIN`, or root-controlled configuration. Conservative total: `+1 +1 -2 = 0` Paranoid signals: * Local unprivileged trigger: +1 If a privileged BPF LSM program is already loaded and calls these kfuncs from hooks such as `security_inode_create`, an unprivileged local user may be able to trigger the negative dentry path indirectly. * Reliable kernel crash / strong DoS: +1 The NULL dereference before `inode_lock()` is a direct and credible crash primitive. * Availability impact realistic: +1 The affected path can produce a host kernel crash or panic-on-warn DoS depending on configuration. Paranoid total: `+1 +1 +1 = 3` Not awarded: * No generic memory corruption score. This is NULL dereference / WARN-based DoS, not UAF, OOB write, double free, type confusion, or refcount corruption. * No LPE score. There is no attacker-controlled reclaim, object replacement, callback dispatch, write primitive, or privilege-escalation chain. * No confidentiality or integrity impact. The patch does not support info leak or unauthorized modification. * No BPF high-control unprivileged API bonus. In the evaluated configuration, unprivileged BPF is assumed disabled or privileged-gated. * Call-site confidence: medium. The direct crash call sites are visible, but real-world triggerability depends on what BPF LSM programs are installed. ## 3. Reachability analysis The direct caller must be a BPF program using the xattr kfuncs with a negative dentry. In typical deployments this is privileged because loading or attaching such BPF programs is gated by BPF capabilities or root-controlled policy. However, once such a BPF LSM program is installed, a local unprivileged user might trigger the relevant hook path by performing filesystem operations that create negative dentries, such as file creation attempts. Containers and namespaces matter only if BPF loading is delegated or if a host-level BPF LSM program observes container filesystem operations. The path is not network reachable. ## 4. Severity interpretation This behaves like an ordinary Moderate issue under conservative product assumptions because it is a privileged BPF-triggered NULL dereference with DoS impact only. The paranoid interpretation is borderline actionable because the actual trigger may be indirect from an unprivileged local user if a vulnerable BPF LSM policy is already installed. There is no realistic evidence of privilege escalation or memory corruption beyond NULL dereference, so it should not be treated as Important based on the supplied patch alone. ## 5. One-sentence report phrase A negative dentry passed to BPF xattr kfuncs can produce a NULL inode dereference or panic-on-warn condition, causing a local denial of service when privileged or preloaded BPF LSM code reaches this path. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED Manual review is recommended to determine whether the affected product ships or enables BPF LSM programs that call these kfuncs and whether unprivileged users can indirectly trigger those hooks. If BPF use is strictly privileged and no such preloaded program exists, this can be treated as ordinary Moderate or lower priority. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: bpf: fix crash in bpf_[set|remove]_dentry_xattr for negative dentries [ Upstream Commit: e003f3a4be738f650e283e92d07017c3fef8da52 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e003f3a4be738f650e283e92d07017c3fef8da52 Changed files: fs/bpf_fs_kfuncs.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=e003f3a4be738f650e283e92d07017c3fef8da52 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-74400 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74400 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:L/PR:H/UI:N/S:U/C:N/I:N/A:H 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: 0 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: 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).