From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-93208][LOW] kasan: fix cache shrink race with CPU hotplug Date: Thu, 24 Sep 2026 14:06:14 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-93208 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: 7cd164f0e1bab0f8dd125c92ed4f19bbd6b92a4a List-Id: CVE: CVE-2026-93208 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: kasan: fix cache shrink race with CPU hotplug Commit: 7cd164f0e1bab0f8dd125c92ed4f19bbd6b92a4a Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7cd164f0e1bab0f8dd125c92ed4f19bbd6b92a4a Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93208 Analysis date: Thu, 24 Sep 2026 14:06:14 -0400 ActionableScore: 0 ActionableScore lower bound: unknown 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-2026-93208 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93208 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-2026-93208 LOW https://www.kernelcve.org/list/?q=CVE-2026-93208 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';CWE-362;*CWE-401;**CWE-400;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.1';DESCR 'A race between KASAN cache shrinking and CPU hotplug can leave quarantined slab objects on the shrink list of a CPU that goes offline after the per CPU removal step. The affected list is skipped by the later online CPU scan, so objects may not be returned to the slab allocator and cache destruction can report remaining objects. This is a local resource retention issue that may cause denial of service through accumulated unreclaimed memory under repeated cache shrink and CPU hotplug activity. For the CVSS the PR:H because reliable triggering normally requires administrative control of CPU hotplug. The issue is not network reachable. No confidentiality or integrity impact is indicated because the patch describes retained objects and incomplete cleanup rather than a use after free or memory corruption primitive. The issue is limited to CONFIG_KASAN_GENERIC kernels.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0) YES INIT RACE UAF HARDWARE MEMORY DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 YES NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=0 PrimitiveClass=NONE RealUAF=NO HostAdminRequired=YES NamespaceScopedPrivilege=NO ImportantCandidate=NO ManualReview=NO AutoDowngradeAllowed=YES 1. ActionableScore * Raw conservative score: -2 * Conservative score: 0 * Raw paranoid score: -2 * Paranoid score: 0 * Score normalization: negative raw scores are normalized to 0. * Final recommended bucket: **Ordinary Moderate**:: 2. Signal breakdown * +1 Concrete persistent kernel resource retention. Objects moved to an offline CPUs shrink_qlist are skipped and not returned to the slab allocator. The later cache destruction can report remaining objects. * -2 Host-admin-equivalent privilege requirement. Reliable triggering requires control over CPU hotplug, which is a host-global administrative operation. * -1 Rare configuration exposure. The affected path is limited to CONFIG_KASAN_GENERIC kernels. This is a debug-oriented build configuration that an attacker normally cannot enable. * 0 Race penalty not applied. The bug is a race, but the supplied evidence does not establish an externally uncontrollable or one-shot timing window for an attacker already able to coordinate CPU hotplug. * 0 Resource exhaustion escalation not awarded. The patch establishes retained objects and incomplete cache reclamation, but does not establish host-wide exhaustion, cgroup isolation bypass, persistent cross-container impact, or a reliable system-wide DoS. * 0 Kernel crash signal not awarded. The observed result is a SLUB warning about objects remaining during cache shutdown, not a kernel panic or demonstrated fatal fault. * 0 Confidentiality and integrity signals not awarded. No use-after-free, stale dereference, out-of-bounds access, write primitive, information disclosure, ownership violation, or authorization bypass is established. * 0 Paranoid increase not awarded. Repeated leakage could theoretically increase retained memory, but the supplied evidence does not establish a realistic accumulation mechanism sufficient for host-wide resource exhaustion. 3. Reachability analysis The complete reliable trigger requires a KASAN_GENERIC kernel, cache shrink or destruction activity, and CPU offlining in the interval after on_each_cpu returns and before the subsequent CPU-list scan completes. CPU hotplug is a host-global administrative operation. A normal local user cannot normally offline CPUs through the relevant sysfs control path. Therefore HostAdminRequired is YES. The operation is not namespace-scoped. A network namespace, container, or namespace-local filesystem activity does not grant authority to offline host CPUs. NamespaceScopedPrivilege is NO. The virtio-9p mount and unmount activity shown in the report can help expose the symptom, but it does not itself establish a below-host-admin complete trigger sequence. The vulnerable code is limited to CONFIG_KASAN_GENERIC, which is not a typical production-kernel configuration. The issue is not network reachable. No special physical hardware is required, but CPU hotplug control and the KASAN_GENERIC configuration are material prerequisites. 4. Primitive and severity interpretation The exact defect is a cleanup and reclamation race. Objects are correctly moved from a per-CPU quarantine list to a per-CPU shrink list, but the later cleanup scans only online CPUs. If a CPU becomes offline in between, its shrink list is omitted and objects remain retained. This is not an active stale-object use. No object is freed while still dereferenced, no dangling pointer is consumed, and no second release, put, free, or refcount mutation occurs through a potentially freed reference. RealUAF is NO. The retained objects are not an attacker-controlled replacement object or an active structured object being interpreted after its lifetime. The demonstrated consequence is incomplete slab-cache cleanup and a warning from __kmem_cache_shutdown. The patch does not establish a memory-corruption primitive, a security-boundary bypass, information disclosure, integrity impact, privilege escalation, or reliable host crash. PrimitiveClass is NONE because this is a resource-retention and cleanup-accounting defect rather than a demonstrated corruption or lifetime-use primitive. ImportantCandidate is NO. AutoDowngradeAllowed is YES because no meaningful unresolved exploitation leverage remains after excluding speculative resource-exhaustion scenarios. Likely operational severity is low. Systems using CONFIG_KASAN_GENERIC may retain allocations or emit warnings during cache destruction when CPU hotplug races with cache shrinking. Production kernels without KASAN_GENERIC are not affected by this path. 5. One-sentence report phrase A race between KASAN cache shrinking and CPU hotplug can retain quarantined slab objects on an offline CPU shrink list, causing incomplete cache cleanup and potential limited memory retention on CONFIG_KASAN_GENERIC kernels. 6. Manual review recommendation NO MANUAL CHECK NEEDED The patch establishes a bounded cleanup and resource-retention defect without a use-after-free, write primitive, information disclosure, privilege escalation path, host crash, or security-boundary bypass. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: kasan: fix cache shrink race with CPU hotplug Commit: 7cd164f0e1bab0f8dd125c92ed4f19bbd6b92a4a Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7cd164f0e1bab0f8dd125c92ed4f19bbd6b92a4a Commit description: kasan_quarantine_remove_cache() first invokes per_cpu_remove_cache() on all online CPUs. Each callback moves objects belonging to the cache from cpu_quarantine to the CPU's shrink_qlist, where they can later be freed from task context. kmem_cache_destroy() invokes the quarantine removal path while holding cpus_read_lock(), but kmem_cache_shrink() does not. The latter can therefore race with CPU offlining as follows: kmem_cache_shrink() CPU hotplug ------------------- ----------- on_each_cpu() CPU1 moves objects to CPU1's shrink_qlist on_each_cpu() returns CPU1 goes offline kasan_cpu_offline() drains cpu_quarantine leaves shrink_qlist untouched for_each_online_cpu() skips CPU1 The objects left on CPU1's shrink_qlist are not returned to the slab allocator. This may prevent kmem_cache_shrink() from releasing slabs that would otherwise become empty. If CPU1 remains offline, a later kmem_cache_destroy() also skips the list and can report that the cache still contains objects. An intermittent occurrence was observed with a virtio-9p filesystem. The mount and umount commands both returned 0, but the kernel logged the following during the userspace-triggered teardown: [ 2994.380134][ T111] BUG 9p-fcall-cache-1 (Tainted: G B ): Objects remaining on __kmem_cache_shutdown() [ 2994.381140][ T111] Object 0xff11000004361118 @offset =4376 [ 2994.381607][ T111] Allocated in p9_fcall_init+0x201/0x400 age=19564 cpu=1 pid=104 [ 2994.382591][ T111] p9_fcall_init+0x201/0x400 [ 2994.382810][ T111] p9_tag_alloc+0x12f/0x700 [ 2994.382982][ T111] p9_client_prepare_req+0x102/0x3e0 [ 2994.383165][ T111] p9_client_rpc+0x1ab/0xa50 [ 2994.383334][ T111] p9_client_getattr_dotl+0xb0/0x1a0 [ 2994.383515][ T111] v9fs_vfs_getattr_dotl+0x115/0x360 [ 2994.383719][ T111] vfs_getattr_nosec+0x22c/0x3a0 [ 2994.383910][ T111] vfs_statx+0xd7/0x170 [ 2994.384062][ T111] vfs_fstatat+0x45/0x80 [ 2994.384215][ T111] __do_sys_newfstatat+0x84/0xe0 [ 2994.384386][ T111] do_syscall_64+0x115/0x6a0 [ 2994.384566][ T111] entry_SYSCALL_64_after_hwframe+0x77/0x7f [ 2994.399720][ T111] WARNING: mm/slub.c:1244 at __kmem_cache_shutdown+0x363/0x500, CPU#0: busybox/111 [ 2994.405655][ T111] Call Trace: [ 2994.406325][ T111] kmem_cache_destroy+0x73/0x1b0 [ 2994.406630][ T111] p9_client_destroy+0x271/0x3c0 [ 2994.407210][ T111] v9fs_session_close+0x3c/0x260 [ 2994.407409][ T111] v9fs_kill_super+0x48/0x90 [ 2994.407584][ T111] deactivate_locked_super+0xa3/0x160 [ 2994.407778][ T111] cleanup_mnt+0x1dd/0x3e0 Thus, a successful umount left objects in the 9p fcall cache and prevented the cache from being destroyed cleanly. Per-CPU shrink_qlist storage exists for every possible CPU, and each list is protected by its own raw spinlock. Iterate over possible CPUs so that a list populated before its CPU went offline is drained as well. for_each_possible_cpu() can do more work than for_each_online_cpu(), but this change only affects CONFIG_KASAN_GENERIC kernels. The extra work is limited to cache shrink and cache destruction paths and does not affect the normal allocation/free fast path. It adds one raw-spinlock-protected scan of each possible CPU's shrink list. These lists are normally empty; a non-empty list is traversed to remove objects belonging to the cache being shrunk or destroyed. Link: https://lore.kernel.org/20260808031459.3032812-1-sh_def@163.com Fixes: 07d067e ("kasan: fix sleeping function called from invalid context on RT kernel") Signed-off-by: Hui Su Reviewed-by: Andrey Ryabinin Cc: Alexander Potapenko Cc: Andrey Konovalov Cc: Dmitry Vyukov Cc: Vincenzo Frascino Cc: "Zhang, Qiang1" [Commit description truncated; see the upstream URL below] Changed files: mm/slub.c mm/kasan/quarantine.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=7cd164f0e1bab0f8dd125c92ed4f19bbd6b92a4a ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-93208 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93208 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:N/A:L The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 4.1 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).