From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-97556][LOW] smb: client: avoid leaking refcount when cifs_sb_tlink() fails Date: Sat, 26 Sep 2026 14:43:33 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-97556 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: 1 X-AL-KERNEL-Commit: 9ec991e148368e3207e98134c509a625097f7693 List-Id: CVE: CVE-2026-97556 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: smb: client: avoid leaking refcount when cifs_sb_tlink() fails Commit: 9ec991e148368e3207e98134c509a625097f7693 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9ec991e148368e3207e98134c509a625097f7693 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97556 Analysis date: Sat, 26 Sep 2026 14:43:33 -0400 ActionableScore: 2 ActionableScore lower bound: 1 Actionable bucket: Ordinary Moderate Manual review required: NO Summary: No one-sentence report phrase was found. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-97556 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97556 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-2026-97556 LOW https://www.kernelcve.org/list/?q=CVE-2026-97556 CHECK WITH IMPACT FROM ORIG NN LOW 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-401;*CWE-772;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A missing cifs file reference release in cifs_oplock_break can occur when cifs_sb_tlink fails on a CIFS mount using the multiuser option. The leaked reference can keep CIFS inodes busy and cause unmount failures or VFS Busy inodes warnings after an oplock break is processed. For the CVSS the PR:L is selected because a local user or process must access an existing multiuser CIFS mount and participate in the affected client workflow. The issue is not directly reachable by an unauthenticated network attacker. A remote SMB server can participate in the oplock break flow, but practical triggering also depends on the local multiuser mount configuration and the cifs_sb_tlink failure condition. Impact is primarily local denial of service through resource retention and impaired unmount. No memory corruption primitive or privilege escalation path is indicated. NO MANUAL CHECK REQUIRED COULD BE AUTOCLOSED.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) YES DISK LEAK SERVERTOCLIENT LINUS YES NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=2 ActionableScoreLower=1 PrimitiveClass=NONE RealUAF=NO HostAdminRequired=UNKNOWN NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=NO ManualReview=NO AutoDowngradeAllowed=YES 1. ActionableScore * Raw conservative score: 1 * Conservative score: 1 * Raw paranoid score: 2 * Paranoid score: 2 * Final recommended bucket: **Ordinary Moderate**:: No floor or cap was applied. 2. Signal breakdown * +1 Concrete persistent kernel resource/reference leak. The queued oplock break owns a cifsFileInfo reference, but cifs_oplock_break did not call _cifsFileInfo_put when cifs_sb_tlink failed. The missing put is explicit in the patch and commit message. * +1 Paranoid only. The commit reports busy inodes after unmount and describes the resulting VFS BUG as a crash. This is treated as a plausible availability consequence, but not as a conservative reliable host crash because the failure path was found by code review and was not exercised at runtime. Important non-awarded signals: * No Real UAF. The bug is a missing reference drop, not a release followed by a stale use or a second release through a dangling reference. * No concrete repeated release/refcount mutation. The patch adds the missing first and only required put on the error path. It does not reveal two puts on the same potentially freed reference. * No controlled reclaim, object replacement, arbitrary write, type confusion, or callback control is established. * No confidentiality or integrity signal is awarded. Retaining a reference can keep an inode busy but does not expose data or modify protected state. * No local unprivileged trigger bonus is awarded. The supplied context does not prove that an ordinary unprivileged user can complete the full sequence involving an existing CIFS multiuser mount, an oplock break, cifs_sb_tlink failure, and unmount. * No remote reachability bonus is awarded. An SMB server may participate in oplock handling, but the vulnerable condition also requires local CIFS multiuser configuration and a local error path. This is not a directly network-triggerable vulnerability in the CVSS or ActionableScore sense. * No host-admin penalty is applied. The complete privilege boundary for establishing and using the affected mount is not fully demonstrated. * No hard race penalty is applied. This is an ordered error path, not a demonstrated timing race. * No niche-subsystem penalty is applied. CIFS multiuser configuration is a prerequisite, but the supplied evidence does not establish that it is attacker-inaccessible or otherwise justifies a negative score. 3. Reachability analysis The affected path requires a CIFS client mount using the multiuser option. An oplock break must be queued for a CIFS file, and cifs_sb_tlink must subsequently fail while cifs_oplock_break processes that work item. The leaked file reference may then keep an inode or dentry busy until unmount. A remote SMB server can participate in generating the oplock break, but a network attacker cannot generally reach the vulnerable state without an existing client connection and the required local mount configuration. The supplied material does not establish whether the remote server is attacker controlled, whether an unprivileged local account can use the particular multiuser mount, or whether that account can cause or participate in the required tlink failure. Host-admin-equivalent privilege is not proven as required for the complete trigger, so HostAdminRequired is UNKNOWN. Below-host-admin reachability is also not fully established, so it is not scored as an ordinary local-unprivileged trigger. No namespace-scoped privilege path is shown. No special hardware, boot option, sysctl, or race condition is involved. The multiuser mount option is a functional prerequisite. The commit explicitly states that this failure path was found by code review and was not exercised at runtime, reducing confidence in practical trigger reliability. 4. Primitive and severity interpretation The exact primitive is a persistent CIFS file-reference leak on an error path. cifs_queue_oplock_break acquires a cifsFileInfo reference when it queues work. cifs_oplock_break normally consumes and releases that reference through _cifsFileInfo_put. When cifs_sb_tlink fails, the old code returned through the out path without dropping the reference. This is not an active stale-object use. It is not a UAF, double free, double put, refcount underflow, or post-lifetime refcount mutation. The patch adds a missing release before the error exit and does not establish that the object is accessed after it is freed. The demonstrated consequence is resource retention and a busy-inode condition during unmount. The commit reports a VFS BUG and describes a crash after unmount, but the practical reliability and host-wide effect are not demonstrated because the error branch was not runtime-tested. Resource exhaustion or repeated reference accumulation beyond the reported unmount condition is not established. PrimitiveClass is NONE because no meaningful kernel corruption, stale use, authorization bypass, ownership violation, or security-boundary primitive is established. ImportantCandidate is NO because the available evidence supports a constrained resource leak and local availability issue only. AutoDowngradeAllowed is YES because no unresolved corruption or security-boundary primitive would be discarded by ordinary low-priority handling. Likely operational severity is Low-like or ordinary Moderate at most. Patch deployment can be deferred relative to memory-corruption, authorization-bypass, and remotely reachable kernel issues, while still being included in normal stable-kernel maintenance. 5. One-sentence report phrase A missing cifsFileInfo reference release after cifs_sb_tlink failure on a CIFS multiuser mount can retain busy inodes and impair unmount operations, with no demonstrated memory-corruption or privilege-escalation primitive. 6. Manual review recommendation NO MANUAL CHECK NEEDED The patch establishes a missing reference release and resulting resource retention, but does not establish a UAF, double release, kernel-memory corruption, authorization bypass, or meaningful unresolved exploitation leverage. 7. Call-site confidence Call-site confidence: medium ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: smb: client: avoid leaking refcount when cifs_sb_tlink() fails Commit: 9ec991e148368e3207e98134c509a625097f7693 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9ec991e148368e3207e98134c509a625097f7693 Commit description: cifs_oplock_break() takes over the reference that cifs_queue_oplock_break() acquired when it queued the work, and drops it with _cifsFileInfo_put() once the break has been processed. Only in setups with "-o multiuser", cifs_sb_tlink() may fail, at which point cifs_oplock_break() returns without putting the file reference, mirroring the reference leak we already fixed in the companion patch to cifs_queue_oplock_break(). This would trigger a crash due to busy inodes on the next unmount: BUG: Dentry ... still in use (1) [unmount of cifs cifs] VFS: Busy inodes after unmount of cifs (cifs) Drop the reference on that path as well. Doing so before the out label mirrors the normal path, which also puts the reference before cifs_done_oplock_break(). Found by Sashiko code review. The failure path was not exercised at runtime. Fixes: e8f5f84 ("cifs: fix potential oops in cifs_oplock_break") Cc: stable@vger.kernel.org Assisted-by: Kiro:claude-opus-5 Signed-off-by: Bjoern Doebel Reviewed-by: Namjae Jeon Signed-off-by: Paulo Alcantara Signed-off-by: Greg Kroah-Hartman Changed files: fs/smb/client/file.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=9ec991e148368e3207e98134c509a625097f7693 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-97556 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97556 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: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: 1 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).