From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-80859][MODERATE REGULAR] fuse: fix missing barrier when checking io-uring readiness Date: Fri, 04 Sep 2026 14:34:55 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-80859 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: unknown X-AL-KERNEL-Commit: dd9c835709f4bb3e4256eea7573e4e6e18f956de List-Id: CVE: CVE-2026-80859 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: fuse: fix missing barrier when checking io-uring readiness Commit: dd9c835709f4bb3e4256eea7573e4e6e18f956de Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dd9c835709f4bb3e4256eea7573e4e6e18f956de Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80859 Analysis date: Fri, 04 Sep 2026 14:34:55 -0400 ActionableScore: 3 ActionableScore lower bound: unknown Actionable bucket: Borderline / manual review recommended Manual review required: YES Summary: A missing read memory barrier in the FUSE io_uring readiness path can allow request allocation before queues are ready, reintroducing a local lock-order inversion deadlock and causing denial of service. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-80859 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80859 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-2026-80859 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-80859 CHECK WITH IMPACT FROM ORIG NN MODERATE 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:N/A:H';CWE-362;CWE-662;*CWE-833;BEST CVSS score: '4.7';DESCR 'FUSE request allocation can observe fch initialized as ready while missing the updated io_uring pointer because fuse_block_alloc lacked a read memory barrier matching the write barrier in fuse_chan_set_initialized. This can allow requests to be allocated before io_uring queues are ready and can reintroduce a lock order inversion deadlock. For the CVSS the PR:L is used because a local user or service with access to FUSE and the relevant mount setup may be able to trigger the affected request path, while fully remote triggering is not supported by the patch context. The issue is not network reachable. Impact is denial of service through a local deadlock or hang, with no supported evidence of memory corruption, information disclosure, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) SKIP CVE-2026-80859 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE DEADLOCK DISK INIT SIMPLEFIX HARDWARE LINUS IO_URING DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=NO ManualReview=RECOMMENDED AutoDowngradeAllowed=YES ## 1. ActionableScore * Raw conservative score: 3 * Conservative score: 3 * Raw paranoid score: 3 * Paranoid score: 3 * No floor or cap changed the score. * Final recommended bucket: **Borderline / manual review recommended**:: ## 2. Signal breakdown Positive signals: * +1: Concrete kernel synchronization / memory-ordering defect. `fuse_block_alloc()` could observe `fch->initialized=1` while still seeing stale `fch->io_uring=0`. * +1: Reliable local DoS mechanism. The commit states this can reintroduce a lock-order inversion deadlock. * +1: Below-host-admin local reachability is supported by the supplied PR:L hint and FUSE request path context. This is local, not remote. Non-awarded signals: * +0: RealUAF not awarded. No use-after-free, stale freed object, double release, or refcount mutation is established. * +0: Strong or weak LPE not awarded. The patch supports deadlock only, not memory corruption or privilege escalation. * +0: Confidentiality and integrity not awarded. No disclosure, write primitive, ownership violation, or authorization bypass is shown. * +0: Remote reachability not awarded. This is not triggered by network packets. * +0: Extra availability not awarded. The deadlock/DoS is already counted once. * +0: Race penalty not applied. This is a memory-ordering race, but the evidence does not prove a narrow one-shot externally uncontrollable timing window. ## 3. Reachability analysis The evaluated attacker is a local user or local service that can exercise the relevant FUSE mount/request path using the io-uring backed FUSE setup. The supplied context supports PR:L rather than host-admin-only triggering, so `HostAdminRequired=NO`. No complete namespace-scoped chain is established. The issue does not depend on network namespaces and the supplied context does not prove that all setup and trigger stages are possible inside attacker-created namespaces, so `NamespaceScopedPrivilege=UNKNOWN`. No prior public network exposure exists. A prior FUSE/io-uring configuration or mount path must exist or be creatable by the local actor. No special hardware is required. The issue depends on CPU memory ordering and concurrent visibility of initialized/io_uring state, but no concrete evidence supports treating it as a hard unreproducible race for ActionableScore purposes. ## 4. Primitive and severity interpretation The exact primitive is an ordering violation between `fch->initialized` and `fch->io_uring`. The reader side lacked an `smp_rmb()` in the correct place, so request allocation could proceed before io-uring queues were ready. This is not active structured UAF and not fixed stale storage UAF. It is a Class C invalid-state / deadlock condition. There is no concrete repeated release/refcount operation. There is no first lifetime operation that can free an object, no surviving dangling reference, and no second lifetime mutation. Demonstrated consequence is local denial of service via lock-order inversion deadlock or hang. Speculative consequences beyond DoS are not supported. `ImportantCandidate=NO` because there is no established corruption, boundary bypass, UAF, ownership violation, or LPE primitive. `AutoDowngradeAllowed=YES` because this is a crash/hang class issue without unresolved exploitation leverage beyond availability. Likely operational severity is ordinary to borderline Moderate. ## 5. One-sentence report phrase A missing read memory barrier in the FUSE io_uring readiness path can allow request allocation before queues are ready, reintroducing a local lock-order inversion deadlock and causing denial of service. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED Reason: this is a kernel race / memory-ordering issue in FUSE request allocation, but current evidence supports DoS only and does not establish memory corruption or privilege escalation. ## 7. Call-site confidence Call-site confidence: medium ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: fuse: fix missing barrier when checking io-uring readiness Commit: dd9c835709f4bb3e4256eea7573e4e6e18f956de Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dd9c835709f4bb3e4256eea7573e4e6e18f956de Commit description: fuse_block_alloc() reads fch->initialized and then fch->io_uring. fch->io_uring is set before fch->initialized, ordered by the smp_wmb() in fuse_chan_set_intialized(), but fuse_block_alloc() has no matching read barrier between the two loads. This may lead a CPU to observe fch->initialized=1 but fch->io_uring=0, and skip the check that blocks request allocation until the io-uring queues are ready. This can reintroduce the lock-order inversion deadlock that commit 3393ff9 prevents. Add an smp_rmb() barrier to pair with the smp_wmb() in fuse_chan_set_initialized() to prevent this. Fixes: 3393ff9 ("fuse: block request allocation until io-uring init is complete") Cc: stable@vger.kernel.org Reviewed-by: Bernd Schubert Signed-off-by: Joanne Koong Signed-off-by: Miklos Szeredi Signed-off-by: Greg Kroah-Hartman Changed files: fs/fuse/dev.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=dd9c835709f4bb3e4256eea7573e4e6e18f956de ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-80859 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80859 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: Not available The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 4.7 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: unknown 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).