From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-98543][MODERATE REGULAR] bpf: fs/xattr: don't assume the inode is locked in path_unlink/path_rmdir Date: Sun, 18 Oct 2026 22:53:37 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-98543 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: unknown X-AL-KERNEL-Commit: 7504b677c4370c9e4b153c553ff8831c093b1ff6 List-Id: CVE: CVE-2026-98543 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: bpf: fs/xattr: don't assume the inode is locked in path_unlink/path_rmdir Commit: 7504b677c4370c9e4b153c553ff8831c093b1ff6 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7504b677c4370c9e4b153c553ff8831c093b1ff6 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98543 Analysis date: Sun, 18 Oct 2026 22:53:37 -0400 ActionableScore: 2 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate Manual review required: NO Summary: A BPF LSM verifier locking mismatch allows sleepable path unlink and path rmdir programs to mutate victim inode xattrs without holding i_rwsem, creating a local filesystem race with possible metadata inconsistency or kernel failure. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98543 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98543 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: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-98543 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98543 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=0 LEGACY=MODERATE RESULT=DIFF V39_AEGIS_ONE_STEP_UP=LOW->MODERATE TEMP_DISTANCE_MERGE_SKIP=AEGIS_NO_POSITIVE_SEVERITY_EVIDENCE (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC) 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:H';CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.8';DESCR 'A locking mismatch in BPF LSM xattr handling allows a sleepable program attached to path_unlink or path_rmdir to use xattr mutation kfunc variants that assume the victim inode i_rwsem is already held. The security hooks run before vfs_unlink or vfs_rmdir acquires that lock, so xattr mutation can race with inode removal or other filesystem operations. The resulting unsynchronized access may cause extended attribute inconsistency, filesystem specific corruption, or a kernel crash. For the CVSS the PR:L reflects the paranoid case where BPF LSM management is delegated to reduced-capability root or a service account. Ordinary systems generally require administrative privileges to load and attach BPF LSM programs, which is represented by PR:H in the base score. The issue is local and is not directly network reachable. Impact is at least limited integrity and availability loss. A reliable privilege escalation primitive is not established, but filesystem dependent race consequences require manual review. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.8 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage LOW to MODERATE because KP specialist did not establish a qualifying panic and the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were KPANIC_UNSUPPORTED_REMOVED and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 0.0 and ActionableScore 0 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) MAYBE LOCK DISK BPF LINUS KPANIC_UNSUPPORTED_REMOVED V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=2 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=UNKNOWN NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=NO ManualReview=RECOMMENDED AutoDowngradeAllowed=YES RequiresHostUID0=UNKNOWN AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO ## 1. ActionableScore * Raw conservative score: 2 * Conservative score: 2 * Raw paranoid score: 2 * Paranoid score: 2 * No floor or cap changed either score. * Final recommended bucket: **Ordinary Moderate**:: ## 2. Signal breakdown Positive signals: * +1 Weak or indirect active-state corruption candidate. The verifier selects xattr kfunc variants that assume `i_rwsem` is held, although the affected path hooks run before that lock is acquired. The resulting xattr mutation is concretely unsynchronized. * +1 Important filesystem path. The defect affects mutation of inode extended attributes and therefore operates on persistent filesystem metadata rather than diagnostic state. Negative signals: * No negative privilege score. Loading and attaching a sleepable BPF LSM program is privilege-gated, but the supplied evidence does not resolve whether the complete operation requires host-admin-equivalent authority or can be delegated. * No race penalty. A concurrency window exists, but there is no evidence that it is narrow, one-shot, or difficult to repeat. * No niche-subsystem penalty. BPF LSM xattr kfunc use is specialized, but specialization alone is insufficient. Material signals not awarded: * No UAF, stale-reference use, OOB access, repeated release, or refcount corruption is established. * No controlled reclaim, arbitrary kernel write, callback influence, or attacker-selected adjacent object is demonstrated. * No reliable kernel crash is documented. Filesystem-specific crashes remain possible but unresolved. * No separate confidentiality or integrity point is awarded. The BPF program is already permitted to request the xattr mutation, and corruption of another protected object is not demonstrated. * No authorization bypass exists. The patch corrects locking selection rather than an ALLOW-versus-DENY decision. * No remote or ordinary unprivileged trigger bonus is awarded. * V33, B3-PENDING, B4-PENDING, and B5-R do not apply because there is no confirmed cross-boundary active-state corruption, shared-backing write, authorization bypass, or retry-accumulated heap OOB write. ## 3. Reachability analysis The vulnerable state requires a sleepable BPF LSM program attached to `path_unlink` or `path_rmdir` and using a dentry xattr mutation kfunc. The verifier then rewrites the call to a locked variant that does not acquire the victim inode lock. BPF program loading and LSM attachment are privilege-gated. The supplied material does not establish whether the complete sequence requires host-admin-equivalent capability in the initial user namespace or is available through a delegated BPF service or token. Therefore `HostAdminRequired=UNKNOWN`. No evidence establishes that all loading, attachment, and trigger stages are available through an attacker-created namespace. Therefore `NamespaceScopedPrivilege=UNKNOWN`. After prior attachment, an unlink or rmdir operation can invoke the program. An ordinary process might initiate that filesystem operation, but this does not prove that the same process controls the BPF program, xattr operation, or concurrent race. No network trigger or special hardware requirement exists. ## 4. Primitive and severity interpretation The exact primitive is an xattr mutation performed without the victim inode `i_rwsem` because the verifier incorrectly assumes that the calling hook already holds it. This is active structured inode state, not fixed stale storage. However, the supplied code establishes improper synchronization rather than a lifetime violation or concrete heap corruption. There is no first operation that frees an object and no second operation through a dangling reference. The demonstrated consequence is violation of inode and xattr serialization requirements. Filesystem metadata inconsistency or a kernel fault is technically possible, but the exact downstream result depends on the filesystem implementation and is not established by this patch. The issue is therefore `CLASS_C`, not Class A or Class B. `ImportantCandidate=NO` because no independently useful corruption, ownership, authorization, or cross-boundary primitive is demonstrated. `AutoDowngradeAllowed=YES` because unresolved consequences do not rise beyond a synchronization defect with possible filesystem-specific DoS or metadata damage. Likely operational severity is ordinary Moderate, with source-level review recommended for affected filesystem xattr handlers. ## 5. One-sentence report phrase A BPF LSM verifier locking mismatch allows sleepable path unlink and path rmdir programs to mutate victim inode xattrs without holding i_rwsem, creating a local filesystem race with possible metadata inconsistency or kernel failure. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED Review representative filesystem xattr handlers to determine whether concurrent unlink, rmdir, setxattr, or removexattr operations can produce a concrete crash, persistent metadata corruption, or lifetime violation. ## 7. Call-site confidence Call-site confidence: medium ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: bpf: fs/xattr: don't assume the inode is locked in path_unlink/path_rmdir Commit: 7504b677c4370c9e4b153c553ff8831c093b1ff6 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7504b677c4370c9e4b153c553ff8831c093b1ff6 Commit description: bpf_lsm_has_d_inode_locked() makes the verifier rewrite bpf_[set|remove]_dentry_xattr() to the _locked variants, which assume that the caller already holds the inode's i_rwsem. The path_unlink and path_rmdir hooks are listed, but security_path_unlink() and security_path_rmdir() run before vfs_unlink()/vfs_rmdir() take the victim inode's i_rwsem, so a sleepable BPF LSM program attached to either hook mutates the victim's xattrs without the lock held. Drop the two path hooks from d_inode_locked_hooks so that the verifier keeps the locking bpf_[set|remove]_dentry_xattr() variants, which take the lock themselves. Fixes: 5646729 ("bpf: fs/xattr: Add BPF kfuncs to set and remove xattrs") Cc: stable@vger.kernel.org Signed-off-by: Andrea Parri Link: https://patch.msgid.link/20260922145530.369775-1-parri.andrea@gmail.com Signed-off-by: Christian Brauner (Amutable) Signed-off-by: Greg Kroah-Hartman 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=7504b677c4370c9e4b153c553ff8831c093b1ff6 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98543 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98543 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:H/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:L/I:L/A:H The Best / paranoid CVSS score: 5.8 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: 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).