From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-53191][MODERATE REGULAR] io_uring/net: inherit IORING_CQE_F_BUF_MORE across bundle recv retries Date: Thu, 25 Jun 2026 06:37:15 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-53191 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: f40570fda3f3a1f96aeaa4aef665ba274b2810b5 List-Id: CVE: CVE-2026-53191 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: io_uring/net: inherit IORING_CQE_F_BUF_MORE across bundle recv retries Commit: f40570fda3f3a1f96aeaa4aef665ba274b2810b5 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f40570fda3f3a1f96aeaa4aef665ba274b2810b5 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53191 Analysis date: Thu, 25 Jun 2026 06:37:15 -0400 ActionableScore: 4 ActionableScore lower bound: 3 Actionable bucket: Borderline manual review recommended / Actionable Moderate not proven Manual review required: YES Summary: An io_uring bundle recv flag propagation bug can cause userspace to advance a provided buffer ring past a buffer entry still used by the kernel, leading to buffer ownership desynchronization and possible application-level data corruption or DoS. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-53191 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53191 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-2026-53191 MODERATE CHECK Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-670;*CWE-664;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.8';DESCR 'io_uring bundle recv can lose IORING_CQE_F_BUF_MORE across retry completion flag merging when provided buffer rings are used in incremental mode. Userspace may then believe that a partially consumed buffer is fully released and may advance the buffer ring head past an entry that the kernel still uses. This can desynchronize kernel and userspace buffer ownership and may cause data corruption or application level denial of service in software that relies on this io_uring mode. For the CVSS the PR:L is used because a local process must create the io_uring receive request with provided buffer rings, incremental consumption, and bundle receive enabled. The issue is not a direct network reachable kernel bug, although remote traffic may help trigger symptoms in a service that already uses this specific receive configuration. Impact is mainly integrity and availability at the userspace buffer management level, with no clear evidence of kernel memory corruption or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Actionable Moderate not proven (with actual score 4) YES IO_URING DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 YES NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=4 ActionableScoreLower=3 ## 1. ActionableScore * Conservative score: 3 * Paranoid score: 4 * Final recommended bucket: **Borderline manual review recommended / Actionable Moderate not proven** ## 2. Signal breakdown Triggered signals: * Local unprivileged trigger: +1 A local process can create the relevant io_uring recv operation with provided buffer rings, incremental consumption, and bundle recv. * Local unprivileged high-control kernel data-plane API: +1 io_uring with provided buffer rings gives userspace fine-grained control over buffer ownership and completion semantics. * Integrity impact plausible: +1 Userspace may advance the provided buffer ring head past an entry that the kernel still considers partially consumed, causing application-level buffer ownership desynchronization and possible data corruption. Paranoid-only signal: * Weak/indirect corruption candidate: +1 The bug creates a stale or incorrect ownership signal for a buffer still used by the kernel. This is not a proven kernel memory corruption primitive, but it is subtle enough to justify manual review. Not counted: * Remote reachable: +0 Network traffic may help trigger symptoms in an affected service, but the vulnerable setup must already exist locally. This is not directly remote-triggerable as a kernel interface. * Generic memory corruption: +0 The patch does not show UAF, OOB write, double free, arbitrary write, type confusion, or reclaimable stale kernel object reuse. * Reliable kernel crash / strong DoS: +0 The described impact is buffer state desynchronization, not a demonstrated kernel panic. * Privilege escalation plausible: +0 No concrete LPE primitive is shown. ## 3. Reachability analysis The bug is reachable by a local user or local process using io_uring recv with provided buffer rings in incremental mode and bundle recv retries. Namespaces may matter because io_uring availability and restrictions vary by distribution and container policy, but the interface is generally a local user-accessible kernel API when not administratively restricted. The issue is not directly network reachable. A remote peer could influence packet sizes, timing, or retry behavior only if a local service already uses this specific io_uring receive configuration. The path is not a generic TCP receive bug. ## 4. Severity interpretation This behaves more like a borderline actionable Moderate than an Important-class kernel vulnerability. The realistic impact is application-level data corruption or denial of service due to incorrect buffer ownership reporting. Theoretical concern exists because io_uring buffer lifetime and ownership bugs can be subtle, but the patch does not demonstrate kernel memory corruption, privilege escalation, or a direct kernel crash. ## 5. One-sentence report phrase An io_uring bundle recv flag propagation bug can cause userspace to advance a provided buffer ring past a buffer entry still used by the kernel, leading to buffer ownership desynchronization and possible application-level data corruption or DoS. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED Manual review is recommended because this is a subtle io_uring provided-buffer ownership bug. It should not be treated as Important without a stronger kernel memory corruption primitive, but it also should not be blindly auto-closed without checking real affected service patterns. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: io_uring/net: inherit IORING_CQE_F_BUF_MORE across bundle recv retries Commit: f40570fda3f3a1f96aeaa4aef665ba274b2810b5 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f40570fda3f3a1f96aeaa4aef665ba274b2810b5 Commit description: When a bundle recv retries inside io_recv_finish(), the merge logic OR the saved cflags from the previous iteration with the cflags returned by the new iteration: cflags = req->cqe.flags | (cflags & CQE_F_MASK); Bits listed in CQE_F_MASK are inherited from the new iteration, and all other bits (notably IORING_CQE_F_BUFFER and the buffer ID) come from the saved cflags. Before this change CQE_F_MASK covered only IORING_CQE_F_SOCK_NONEMPTY and IORING_CQE_F_MORE. When using provided buffer rings (IOU_PBUF_RING_INC) with incremental mode, and bundle recv, io_kbuf_inc_commit() can leave the head ring entry partially consumed, __io_put_kbufs() then sets IORING_CQE_F_BUF_MORE on the returned cflags so userspace knows the buffer ID will be reused for subsequent completions. Because IORING_CQE_F_BUF_MORE was not in CQE_F_MASK, the merge above silently dropped it whenever the final retry iteration partially consumed the buffer, and the subsequent req->cqe.flags = cflags & ~CQE_F_MASK save would have left a stale IORING_CQE_F_BUF_MORE in the carried-over cflags had one been present. Userspace would then wrongfully advance it ring head past an entry the kernel still uses. Add IORING_CQE_F_BUF_MORE to CQE_F_MASK so it is both inherited from the new iteration into the user-visible CQE and stripped from the saved cflags between iterations. Cc: stable@vger.kernel.org Signed-off-by: Clément Léger Assisted-by: Claude:claude-opus-4.6 Fixes: ae98dbf ("io_uring/kbuf: add support for incremental buffer consumption") Link: https://patch.msgid.link/20260604160715.2482972-1-cleger@meta.com Signed-off-by: Jens Axboe Signed-off-by: Greg Kroah-Hartman Changed files: io_uring/net.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=f40570fda3f3a1f96aeaa4aef665ba274b2810b5 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-53191 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53191 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:L/A:L The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H The Best / paranoid CVSS score: 5.8 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).