From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-98989][LOW] namespace: check a recursive bind mount for mount namespace loops Date: Mon, 26 Oct 2026 22:01:16 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-98989 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: 2 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: a0ba48256f5dff6f75a45f8ce39fe57f21df1022 List-Id: CVE: CVE-2026-98989 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: namespace: check a recursive bind mount for mount namespace loops Commit: a0ba48256f5dff6f75a45f8ce39fe57f21df1022 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a0ba48256f5dff6f75a45f8ce39fe57f21df1022 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98989 Analysis date: Mon, 26 Oct 2026 22:01:16 -0400 ActionableScore: 2 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate Manual review required: NO Summary: A namespace-capable local process can repeatedly create recursive bind-mount cycles that retain mount namespaces and consume unbounded Shmem. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98989 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98989 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: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-98989 LOW https://www.kernelcve.org/list/?q=CVE-2026-98989 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=2 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both 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-400;*CWE-772;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A recursive bind mount with MS_REC could copy nested nsfs or pidfs mount namespace files from another mount namespace without validating the complete copied tree. This can create a reference cycle that permanently retains the affected namespace and its mounts. Repeating the deterministic operation can grow Shmem without limit and may eventually exhaust host resources. For the CVSS the PR:L value reflects that local code execution and mount capability inside an available user namespace are required. The issue is not directly network reachable. No confidentiality or integrity impact is supported because the demonstrated primitive is resource retention rather than invalid memory access or memory corruption. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE_KPANIC to LOW because the ActionableScore threshold 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 INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3. The independent weaker Aegis shadow evaluated CVSS 4.4 and ActionableScore 2 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) MAYBE DISK LEAK IMPROVEONLY LINUS INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=2 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=YES ImportantCandidate=NO ManualReview=NO AutoDowngradeAllowed=YES RequiresHostUID0=NO AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw base score: 2. * Conservative score: 2; no floor or cap applied. * Raw paranoid score: 2. * Paranoid score: 2; no additional technically supported impact. * Final recommended bucket: **Ordinary Moderate**:: 2. Signal breakdown * +1 Concrete persistent kernel resource/reference leak: the recursive copy can create a mount-namespace reference cycle, permanently retaining the namespace and its mounts. * +1 Material resource exhaustion: the demonstrated operation is repeatable and grows Shmem without limit, supporting eventual host resource pressure beyond a one-time bookkeeping leak. * +0 Namespaced privilege: the supplied evidence places the required mount capability in an available user namespace, so no host-administrator penalty applies. * No memory corruption, UAF, OOB access, protected-state modification, confidentiality impact, or integrity impact is shown. * No separate crash point is awarded; eventual exhaustion is already represented by the resource-exhaustion signal. * No authorization-bypass score: `check_for_nsfs_mounts()` enforces a cycle/retention invariant, not a mandatory access-control decision changing DENY to ALLOW for a protected resource. * NEW52/B5-R and NEW53/B3-PENDING do not apply: there is no heap OOB write or shared-backing write. 3. Reachability analysis A local actor with mount capability in an available user namespace can construct the bind-mount stack and repeatedly invoke `MS_BIND|MS_REC`. The supplied evidence supports a namespace-scoped complete trigger below host-administrator power and does not require host UID 0, networking, special hardware, or a timing race. Availability of user namespaces remains a deployment prerequisite, but the described trigger is deterministic once available. 4. Primitive and severity reading The primitive is a persistent namespace/mount lifetime-retention cycle causing repeatable kernel resource consumption. It is CLASS_C because the demonstrated security effect is resource exhaustion, without memory corruption or another useful exploitation primitive. The patch validates the copied recursive tree before grafting it. ImportantCandidate is NO and auto-downgrade is allowed because no unresolved corruption, authorization, ownership, or cross-authority integrity primitive remains. 5. One-sentence report phrase A namespace-capable local process can repeatedly create recursive bind-mount cycles that retain mount namespaces and consume unbounded Shmem. 6. Manual review advice NO MANUAL CHECK NEEDED The deterministic resource-retention impact is adequately established and no Important-preserving primitive is shown. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: namespace: check a recursive bind mount for mount namespace loops Commit: a0ba48256f5dff6f75a45f8ce39fe57f21df1022 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a0ba48256f5dff6f75a45f8ce39fe57f21df1022 Commit description: do_loopback() refuses to bind mount the file of a mount namespace that is as old as the caller's or older because a mount namespace that holds a mount of its own file, or of an ancestor's, would cause a cycle. But it only checks the dentry the bind mount starts from. With MS_REC everything below it is copied and do_loopback() passes CL_COPY_MNT_NS_FILE so mount namespace files below the source are copied. That's fine for a source in the caller's own mount namespace. Every mount namespace file in there passed the same check when it was bind-mounted. It isn't fine for a source in another mount namespace. may_copy_tree() accepts a bind mount of any nsfs or pidfs file no matter what mount namespace it lives in so that /proc//ns/ and pidfds can be bind mounted. If we create a bind-mount stack of mount namespaces file descriptors the kernel will copy them irrespective of their ancestoral relationship to the mount namespace in question: 151 146 0:7 net:[4026531833] /tmp/nrc/y rw - nsfs nsfs rw 152 151 0:7 mnt:[4026532293] /tmp/nrc/y rw - nsfs nsfs rw Mount 152 is a mount of the mount namespace file of mount namespace 4026532293 inside mount namespace 4026532293. That namespace and every mount in it are leaked... All it takes is a process in an older mount namespace that stacks the file and repeating it leaks without limit: after control: Shmem: 380 kB child: mount(/proc/self/fd/6, MS_BIND|MS_REC): ok after cycle: Shmem: 65916 kB do_move_mount() runs check_for_nsfs_mounts() over a detached tree for exactly that reason. Do the same for the copy before it is grafted. Fixes: e149ed2 ("take the targets of /proc/*/ns/* symlinks to separate fs") Fixes: ef4144a ("pidfs: allow bind-mounts") Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260930-work-mount-fixes-3-v1-4-be34c83956ae@kernel.org Signed-off-by: Christian Brauner (Amutable) Signed-off-by: Greg Kroah-Hartman Changed files: fs/namespace.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=a0ba48256f5dff6f75a45f8ce39fe57f21df1022 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98989 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98989 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: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: 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: 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).