From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-68173][MODERATE REGULAR] ublk: wait on ublk_dev_ready() instead of ub->completion Date: Mon, 10 Aug 2026 16:18:29 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-68173 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: 4 X-AL-KERNEL-ActionableScore-Lower: 3 X-AL-KERNEL-Commit: 7dd26adf7e7d482af524e3a0cca4a81ef7c159d0 List-Id: CVE: CVE-2026-68173 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: ublk: wait on ublk_dev_ready() instead of ub->completion Commit: 7dd26adf7e7d482af524e3a0cca4a81ef7c159d0 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7dd26adf7e7d482af524e3a0cca4a81ef7c159d0 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68173 Analysis date: Mon, 10 Aug 2026 16:18:29 -0400 ActionableScore: 4 ActionableScore lower bound: 3 Actionable bucket: Borderline, manual review recommended Manual review required: YES Summary: A stale completion in ublk recovery can mark a device LIVE before all queues are ready, stranding requeued requests and causing persistent local I/O hangs or teardown DoS when the ublk userspace recovery flow is misordered. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-68173 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68173 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-2026-68173 MODERATE CHECK WITH IMPACT FROM ORIG NN 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-362;CWE-667;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'ublk user recovery can use a stale completion as a proxy for device readiness, so END_USER_RECOVERY may mark the device LIVE before all queues have fetched their I/O commands. This can redispatch a requeued request while queue canceling is still active, causing the request to be parked again and then stranded after canceling is cleared. If the stranded request is the flush request, later fsync operations can pile up in uninterruptible sleep and device teardown can hang while draining tags. For the CVSS the PR:L is used because triggering requires local ability to run or control the ublk userspace server or recovery flow, but not necessarily full administrator privileges in deployments that allow ublk user daemons. The issue is not network reachable. Impact is denial of service through blocked I/O and hung teardown, with no evidence of confidentiality impact, integrity impact, UAF, OOB access, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4) SKIP CVE-2026-68173 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 728cbac5fe219d3b8a21a0688a08f2b7f8aeda2b YES NO NO unknown MAYBE LOCK DANGER DISK INIT DMAorINTERRUPT HARDWARE DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=4 ActionableScoreLower=3 ## 1. ActionableScore * Conservative score: 3 * Paranoid score: 4 * Final recommended bucket: **Borderline, manual review recommended** ## 2. Signal breakdown Conservative signals: * Local reduced privilege trigger: +1. Triggering requires local control of the ublk userspace server or recovery flow. This is not network reachable, but it may not require full host root in deployments that allow ublk user daemons. * Reliable kernel strong DoS: +1. A stale completion can mark the device LIVE too early, strand a requeued request, and make later fsync operations block in uninterruptible sleep. * Availability impact realistic: +1. The impact can persist as blocked I/O and teardown hang on tag draining, not only a transient warning. * Important filesystem/storage path: +1. The bug affects ublk block I/O recovery and was observed with ext4 PREFLUSH behavior. * Special recovery sequence required: -1. The trigger depends on daemon crash or recovery, failed START_USER_RECOVERY, ignored error, and END_USER_RECOVERY ordering. Paranoid difference: * The -1 special sequence penalty is not applied if the local attacker controls the ublk daemon and can reliably drive the recovery sequence. Not counted: * No remote reachable signal. * No generic memory corruption signal. * No real lifetime corruption signal. * No privilege escalation signal. * No confidentiality or integrity impact signal. * No Dirty Pipe like or page cache corruption primitive. Call-site confidence: high. The patch includes the relevant start recovery, end recovery, readiness, and wakeup paths. ## 3. Reachability analysis The bug is locally reachable through ublk device recovery control, especially by the userspace ublk server or a process controlling that server. It is not triggered by network traffic. Namespace and container impact depends on whether the product permits untrusted users or container workloads to create or manage ublk devices. If ublk control is restricted to full host root, practical risk is lower. If unprivileged or reduced privilege ublk daemons are allowed, PR:L is a reasonable practical interpretation. ## 4. Severity interpretation This behaves like a storage subsystem denial of service rather than an Important class memory corruption issue. The realistic impact is blocked I/O, fsync hangs, and teardown hangs after recovery state desynchronization. There is no evidence in the patch of UAF, OOB access, arbitrary write, stale callback, refcount takeover, or privilege escalation. Because the affected path is storage and the DoS can be persistent for the affected device, it should not be treated as a trivial cleanup, but it remains below Actionable Moderate unless the deployment exposes ublk recovery control to untrusted local users. ## 5. One-sentence report phrase A stale completion in ublk recovery can mark a device LIVE before all queues are ready, stranding requeued requests and causing persistent local I/O hangs or teardown DoS when the ublk userspace recovery flow is misordered. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED Manual review should confirm whether affected systems allow untrusted or reduced privilege users to run ublk daemons or control ublk recovery. If only full host root can trigger the path, this can likely be handled as ordinary Moderate or lower. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: ublk: wait on ublk_dev_ready() instead of ub->completion Commit: 7dd26adf7e7d482af524e3a0cca4a81ef7c159d0 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7dd26adf7e7d482af524e3a0cca4a81ef7c159d0 Commit description: ub->completion is only re-armed by a successful START_USER_RECOVERY. If the ublk server sends END_USER_RECOVERY without one - e.g. its START failed with -EBUSY and the error was ignored - the wait is satisfied by the stale completion of the previous recovery cycle, and the device is marked LIVE and the requeue list kicked while the FETCH stream is still running and ubq->canceling is still set. The kick redispatches a previously requeued request, __ublk_queue_rq_common() sees ->canceling and parks it again via __ublk_abort_rq(), and after the last FETCH clears ->canceling nothing ever kicks the requeue list again: the request is stranded there while holding its tag. If it is the flush machinery's flush_rq, every subsequent fsync piles up in uninterruptible sleep and teardown hangs on tag draining. This matches a report of a lost PREFLUSH with ext4 on top of ublk after daemon crash recovery. ub->completion is an edge-triggered latch used as a proxy for the level condition "every queue has fetched all I/O commands", which can regress (F_BATCH's UNPREP, daemon death) and whose re-arm can be skipped. Drop it and wait on the real condition instead: the new helper ublk_wait_dev_ready_and_lock() waits on ublk_dev_ready() via wait_var_event_interruptible(), woken from ublk_mark_io_ready(), then re-checks it under ub->mutex, waiting again on regression, and returns with the mutex held and readiness guaranteed. Readiness becomes true in the same ub->mutex critical section that clears the last queue's ->canceling, so END_USER_RECOVERY marks the device LIVE and kicks the requeue list strictly after ->canceling clears. The wait stays interruptible, so a server whose daemon died can still be signalled out. For ublk_ctrl_start_dev() this replaces the fail-fast -EINVAL on an F_BATCH ready->UNPREP regression with waiting until the device is ready again. Reported-by: George Salisbury Fixes: 728cbac ("ublk: move device reset into ublk_ch_release()") Cc: stable@vger.kernel.org Signed-off-by: Ming Lei Link: https://patch.msgid.link/20260719134540.120269-1-tom.leiming@gmail.com Signed-off-by: Jens Axboe [ 6.18.y tracks readiness per fetched I/O command in ->nr_io_ready rather than per queue in ->nr_queue_ready, so wait and wake on that counter; ->canceling is cleared by ublk_reset_io_flags(), which stays in ublk_mark_io_ready(). UBLK_F_BATCH_IO does not exist here, so ublk_ctrl_start_dev() has no F_BATCH readiness re-check to replace, and readiness can only regress via ublk_reset_ch_dev() on daemon death. ] Signed-off-by: Sasha Levin Changed files: drivers/block/ublk_drv.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=7dd26adf7e7d482af524e3a0cca4a81ef7c159d0 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-68173 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68173 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:H 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: 3 Paranoid ActionableScore: 4 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).