From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-99127][MODERATE REGULAR] page_pool: keep frag_offset aligned for odd-sized requests Date: Wed, 28 Oct 2026 22:25:41 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-99127 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: 3 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 813a72beaa5d3f4bb135e0def9b3f07a3511ca0e List-Id: CVE: CVE-2026-99127 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: page_pool: keep frag_offset aligned for odd-sized requests Commit: 813a72beaa5d3f4bb135e0def9b3f07a3511ca0e Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=813a72beaa5d3f4bb135e0def9b3f07a3511ca0e Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99127 Analysis date: Wed, 28 Oct 2026 22:25:41 -0400 ActionableScore: 3 ActionableScore lower bound: unknown Actionable bucket: Borderline / Important-candidate manual review required Manual review required: YES Summary: An odd XDP fragment request can misalign shared page-pool state, causing a later SKB clone’s split atomic access to panic affected x86 kernels. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99127 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99127 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-2026-99127 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-99127 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=3 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC) 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-682;CWE-754;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'An odd sized request to page_pool_alloc_frag_netmem can leave the shared frag_offset misaligned when dma_get_cache_alignment returns 1. Later page pool users may receive a misaligned skb head, placing skb_shared_info dataref across a cache line. The atomic increment in skb_clone can then execute a split lock and panic x86 systems where kernel split locks are fatal. The demonstrated primitive is a kernel crash rather than UAF, OOB access, information disclosure, or an arbitrary write. For the CVSS the PR:L paranoid choice reflects containers, namespaces, or constrained services with delegated XDP and raw socket capabilities. Triggering also depends on generic XDP, the relevant skb allocation and clone paths, suitable packet sizes, and probabilistic allocator placement. Direct remote reachability is not established, although traffic may drive the final fault after a privileged local configuration exists. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 3 with recommendation Borderline / Important-candidate manual review required. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class 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 DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 4.1 and ActionableScore 3 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / Important-candidate manual review required (with actual score 3) MAYBE REMOTE OOB LEAK SKB DMAorINTERRUPT HARDWARE LINUS MEMORY PACKET DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 PrimitiveClass=CLASS_B RealUAF=NO HostAdminRequired=UNKNOWN NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=YES ManualReview=REQUIRED AutoDowngradeAllowed=NO RequiresHostUID0=UNKNOWN AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw base score: 3. * Conservative score: 3; no floor or cap applied. * Raw paranoid score: 3. * Paranoid score: 3; unsupported namespace crossing or broader corruption is not scored. * Final recommended bucket: **Borderline / Important-candidate manual review required**:: 2. Signal breakdown * +1 weak/indirect active-state corruption: an odd request advances the shared page-pool `frag_offset` without required alignment, causing later allocations to construct misaligned SKB state. * +1 reliable kernel crash: the demonstrated later `atomic_inc()` can straddle a cache line and panic x86 systems with fatal split-lock detection. * +1 common packet-processing consumer: the malformed SKB reaches cloning in local IP delivery after generic XDP setup. * No timing penalty: approximately one fault-capable placement per thousands of clones is probabilistic but cheaply repeatable and experimentally demonstrated. * No privilege penalty: the complete XDP-load/attach, raw-socket, and traffic privilege model is unresolved rather than proven host-admin-only. * No OOB-write, UAF, arbitrary write, disclosure, authorization bypass, or protected-resource modification is shown. * NEW52 B5-R does not apply: there is no confirmed retry-accumulated heap OOB write. * NEW53 B3-PENDING does not apply: no shared/file-backed storage write is shown. 3. Reachability analysis Generic-mode XDP must first be attached, and the demonstrated setup also opens a raw UDP socket. The evidence does not establish whether one below-host-admin actor can perform the complete load, attach, socket, and trigger sequence. Namespace-scoped XDP/BPF privileges are likewise unresolved. Once configured, ordinary traffic can repeatedly exercise the allocation and clone paths. No special hardware is required, but fatal split-lock handling is architecture/configuration dependent. 4. Primitive and severity reading The primitive is corruption of an active shared allocator alignment invariant, followed by construction and privileged consumption of a misaligned `skb_shared_info`. It is CLASS_B because one caller’s influenced size poisons shared active state used by later callers. The proven effect is host panic, not OOB memory corruption. A namespace/delegated-interface boundary is credible because the per-CPU pool is shared and later users pay for the earlier caller’s request, but the complete cross-boundary trigger is not proven. Under the active-state/trust-boundary rule, that unresolved chain requires Important-candidate review and prevents automatic downgrade. Non-crash integrity or exploitation remains unproven. 5. One-sentence report phrase An odd XDP fragment request can misalign shared page-pool state, causing a later SKB clone’s split atomic access to panic affected x86 kernels. 6. Manual review advice MANUAL CHECK REQUIRED Confirm BPF/XDP and raw-socket privilege scope, pool sharing across namespaces, and whether other consumers derive non-crash effects from the malformed SKB layout. 7. Call-site confidence Call-site confidence: medium ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: page_pool: keep frag_offset aligned for odd-sized requests Commit: 813a72beaa5d3f4bb135e0def9b3f07a3511ca0e Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=813a72beaa5d3f4bb135e0def9b3f07a3511ca0e Commit description: page_pool_alloc_frag_netmem() rounds the requested fragment size with size = ALIGN(size, dma_get_cache_alignment()); dma_get_cache_alignment() returns 1 unless the architecture defines ARCH_DMA_MINALIGN, which DMA-coherent architectures such as x86 do not. There the ALIGN() is a no-op and pool->frag_offset advances by the raw, unrounded size. A single caller asking for an odd size then leaves frag_offset misaligned for every fragment carved out of that page afterwards. The pool is shared, so the damage is not confined to the caller that caused it. The per-cpu system_page_pool used by generic XDP hits this. skb_pp_cow_data() allocates its fragments with the raw packet length: size = min_t(u32, len, PAGE_SIZE); truesize = size; page = page_pool_dev_alloc(pool, &page_off, &truesize); leaving frag_offset odd for whatever is carved out of that page next. Its own head allocation is already aligned -- SKB_HEAD_ALIGN(size) plus the XDP_PACKET_HEADROOM its callers pass -- so it is a later user of the shared pool that pays: page_pool_dev_alloc_va() returns a misaligned buffer, napi_build_skb() installs it as skb->head, and skb_shinfo(skb) == skb->head + skb->end is misaligned with it. skb_shinfo()->dataref is a 4-byte atomic_t at offset 0x20, so the atomic_inc() in __skb_clone() straddles a cache line. On x86 with split lock detection -- fatal for kernel split locks by default -- this panics the machine: Oops: Split lock detected RIP: 0010:skb_clone+0x154/0x1e0 Call Trace: raw_local_deliver+0x1ed/0x2c0 ip_protocol_deliver_rcu+0x54/0x1c0 ip_local_deliver_finish+0x85/0x100 ip_local_deliver+0x67/0x100 __netif_receive_skb_one_core+0x85/0xa0 process_backlog+0x87/0x130 Reproduced by attaching any generic-mode XDP program to loopback and opening a RAW IPPROTO_UDP socket, which makes raw_local_deliver() clone every locally delivered UDP packet; ordinary DNS traffic then triggers it, roughly once per 2500 clones. Observed on 6.12.101 and 7.1.8. Tracing page_pool_alloc_frag_netmem() over one such run shows the amplification -- two odd-sized requests, nine misaligned offsets: requested size & 7: 0: 17035 5: 1 7: 1 frag_offset & 7: 0: 17028 3: 1 4: 1 5: 1 6: 1 7: 5 and skb_pp_cow_data() returning heads that were aligned on entry: head 0xffff8f4c86aeac00 -> 0xffff8f4c53a9a9c4 (&7=4) head 0xffff8f4d6a8a42c0 -> 0xffff8f4c4f7b7a45 (&7=5) Round the fragment size up to at least the alignment struct skb_shared_info requires, so fragments are always suitably aligned for the objects callers build on them. Architectures needing a larger DMA alignment keep it. This also makes the remainder computed in page_pool_alloc_netmem(), *size = max_size - *offset; aligned, since max_size is a power of two -- which fixes the matching misalignment of skb->end. Verified with a controlled A/B under QEMU/KVM: same tree, same config, same compiler, same rootfs and identical traffic, differing only by this patch. A SEC("xdp.frags") XDP_PASS program on lo plus UDP datagrams larger than max_head_size drives skb_pp_cow_data()'s fragment loop, which passes raw packet lengths to the pool. Measured at the return of skb_pp_cow_data(): unpatched patched skb_pp_cow_data calls 40800 40800 misaligned skb->head 1120 0 [Commit description truncated; see the upstream URL below] Changed files: net/core/page_pool.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=813a72beaa5d3f4bb135e0def9b3f07a3511ca0e ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99127 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99127 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:H 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: unknown Paranoid ActionableScore: 3 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).