From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-99256][LOW] dm cache: fix issue with background work locking Date: Tue, 03 Nov 2026 22:36:19 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-99256 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: 0 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 0c7a8cad7c96012a9157a189649e569d0390f384 List-Id: CVE: CVE-2026-99256 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: dm cache: fix issue with background work locking Commit: 0c7a8cad7c96012a9157a189649e569d0390f384 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0c7a8cad7c96012a9157a189649e569d0390f384 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99256 Analysis date: Tue, 03 Nov 2026 22:36:19 -0400 ActionableScore: 0 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate Manual review required: NO Summary: A host storage administrator can trigger a DEBUG_RWSEMS ownership warning by suspending and resuming a dm-cache target, with no demonstrated memory corruption or authorization bypass. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99256 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99256 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-2026-99256 LOW https://www.kernelcve.org/list/?q=CVE-2026-99256 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=0 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:H/UI:N/S:U/C:N/I:N/A:H';CWE-667;CWE-662;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.4';DESCR 'dm cache used a write locked rw semaphore across operations that could run in different process contexts. The task calling up_write was therefore not guaranteed to own the write lock acquired by the earlier task, which violates rw semaphore owner rules and triggers a DEBUG_RWSEMS warning during dmsetup suspend followed by resume. For the CVSS the PR:H value is used because reliable triggering normally requires administrative control of the device mapper cache target through CAP_SYS_ADMIN or equivalent host storage management access. The demonstrated primitive is improper synchronization and a kernel warning rather than UAF, OOB access, or another memory corruption primitive. Availability impact is normally limited, although panic_on_warn can turn the warning into a host crash. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.4 and the production ActionableScore returned 0 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. 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. 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 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 0) MAYBE LOCK DISK INIT WARNONLY LEAK HARDWARE LINUS - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=0 PrimitiveClass=NONE RealUAF=NO HostAdminRequired=YES NamespaceScopedPrivilege=NO ImportantCandidate=NO ManualReview=NO AutoDowngradeAllowed=YES RequiresHostUID0=NO AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw base score: -2. * Conservative score: 0, bounded at the minimum score of 0. * Raw paranoid score: -2. * Paranoid score: 0, bounded at the minimum score of 0. * Final recommended bucket: **Ordinary Moderate**:: 2. Signal breakdown * -2 — Host-administrator-equivalent privilege: the demonstrated trigger requires control of a device-mapper cache target through `dmsetup suspend` and `resume`, normally requiring host `CAP_SYS_ADMIN` or equivalent storage-management authority. * +0 — The shown result is a `CONFIG_DEBUG_RWSEMS` ownership warning. Under the strict WARN rule, this is not a reliable host crash merely because `panic_on_warn` could convert it into one. * +0 — The patch replaces cross-task rwsem ownership with a spinlock, counter, and wait queue; it does not show UAF, OOB access, stale lifetime use, or attacker-controlled memory corruption. * No confidentiality, integrity, authorization-bypass, protected-resource, or persistent availability signal is shown. * B5-R and B3-PENDING do not apply: there is no heap OOB write or shared-backing write. 3. Reachability analysis The evidenced trigger is a local device-mapper administrative operation against an already configured dm-cache device. Numeric UID 0 is not intrinsically required if equivalent host capabilities are held, but effective host-administrator storage authority is required. No user-namespace-scoped complete trigger, ordinary-user path, special hardware prerequisite, or race is shown. 4. Primitive and severity reading The defect is improper synchronization: one task may acquire the rwsem write lock and another release it, violating strict owner semantics. The demonstrated effect is a debug ownership warning, not active-state corruption or a lifetime primitive. `PrimitiveClass=NONE`; panic under special WARN policy is conditional and unproven as the normal outcome. There is no Important-preserving primitive, so automatic downgrade is safe. 5. One-sentence report phrase A host storage administrator can trigger a DEBUG_RWSEMS ownership warning by suspending and resuming a dm-cache target, with no demonstrated memory corruption or authorization bypass. 6. Manual review advice NO MANUAL CHECK NEEDED The patch and reproducer establish a privileged synchronization warning only. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: dm cache: fix issue with background work locking Commit: 0c7a8cad7c96012a9157a189649e569d0390f384 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0c7a8cad7c96012a9157a189649e569d0390f384 Commit description: dm cache used a rw_semaphore for background_work_lock. Write locks on rw_semaphores have strict owner semantics, but there was no guarantee that the process that locked background_work_lock was the same process that unlocked it. This can be easily seen using a kernel compiled with CONFIG_DEBUG_RWSEMS. Given a dm cache device , run: 'dmsetup suspend && dmsetup resume '. This will trigger a kernel warning: DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) && !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE)) triggered by cache_resume(). To fix this, switch from a rw_semaphore to a spinlock and a wait queue. dm cache already has a wait queue and associated counter, migration_wait and nr_allocated_migrations, that was getting woken up when background work was getting completed, but wasn't actually used by anything. This is replaced by the background_work queue and counter. Fixes: b29d498 ("dm cache: significant rework to leverage dm-bio-prison-v2") Cc: stable@vger.kernel.org Signed-off-by: Benjamin Marzinski Reviewed-by: Matthew Sakai Reviewed-by: Ming-Hung Tsai Signed-off-by: Mikulas Patocka Signed-off-by: Greg Kroah-Hartman Changed files: drivers/md/dm-cache-target.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=0c7a8cad7c96012a9157a189649e569d0390f384 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99256 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99256 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:L The Best / paranoid CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 4.4 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: 0 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).