From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-64426][MODERATE REGULAR] io_uring/nop: fix file reference leak with IOSQE_FIXED_FILE Date: Sat, 25 Jul 2026 10:22:32 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-64426 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: 2 X-AL-KERNEL-Commit: 722869fcff598fad20d5ab79c305897a7534708b List-Id: CVE: CVE-2026-64426 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: io_uring/nop: fix file reference leak with IOSQE_FIXED_FILE Commit: 722869fcff598fad20d5ab79c305897a7534708b Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=722869fcff598fad20d5ab79c305897a7534708b Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64426 Analysis date: Sat, 25 Jul 2026 10:22:32 -0400 ActionableScore: 3 ActionableScore lower bound: 2 Actionable bucket: Ordinary Moderate / Low-like Manual review required: NO Summary: A local io_uring NOP request with mismatched fixed-file flags can leak fget file references, allowing repeated local abuse to consume kernel resources and potentially cause denial of service. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-64426 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64426 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: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-64426 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-772;*CWE-401;CWE-404;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A file reference leak in io_uring NOP handling can occur when IOSQE_FIXED_FILE is set but IORING_NOP_FIXED_FILE is not set. The request is marked as fixed file during generic initialization, but io_nop can still acquire a normal fget reference and completion will not drop it because REQ_F_FIXED_FILE remains set. A local user can potentially repeat this pattern to accumulate unreleased file references and consume kernel resources. For the CVSS the PR:L is used because triggering requires a local process able to submit io_uring requests, not remote network access. The issue is not network reachable. Impact is denial of service through resource exhaustion. No confidentiality or integrity impact is supported because the primitive is a reference leak rather than UAF, OOB access, or arbitrary write.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 3) YES LEAK TEST IO_URING SYZBOT INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 YES NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 ActionableScoreLower=2 ## 1. ActionableScore * Conservative score: 2 * Paranoid score: 3 * Final recommended bucket: **Ordinary Moderate / Low-like**:: ## 2. Signal breakdown Triggered signals: * Local unprivileged trigger: +1 A local process able to submit io_uring NOP requests can trigger the mismatched IOSQE_FIXED_FILE and IORING_NOP_FIXED_FILE path. * Availability impact realistic: +1 Repeated submissions can leak file references and consume kernel resources. This is resource exhaustion, not immediate memory corruption. Paranoid-only signal: * Local unprivileged high-control kernel data-plane API refinement: +1 io_uring gives local users fine-grained request construction and lifetime control. This raises triage interest slightly, but here the primitive is still only a reference leak. Not applied: * Generic memory corruption: +0 The patch shows a file reference leak, not UAF, double-free, OOB write, type confusion, or arbitrary write. * Privilege escalation plausible: +0 No reclaim, stale callback, overwrite, refcount takeover, or object confusion primitive is shown. * Reliable kernel crash / strong DoS: +0 conservative The issue is a gradual leak. It may lead to DoS under sustained abuse, but not a single-shot reliable crash. * Confidentiality / Integrity impact: +0 No information disclosure or write/corruption primitive is supported. ## 3. Reachability analysis A local user with access to io_uring can submit the affected NOP request pattern. The issue is not network reachable. Namespaces or containers may matter only if io_uring is available to the contained workload, but the attacker still needs local code execution in that environment. The affected path is a local kernel API path and does not require CAP_SYS_ADMIN in the generic case, although some products may restrict io_uring for untrusted users. Call-site confidence: high. The patch and commit message directly show acquisition through io_file_get_normal() and missing release due to REQ_F_FIXED_FILE. ## 4. Severity interpretation This behaves like an ordinary Moderate or Low-like local resource exhaustion issue. The theoretical worst case is local DoS through repeated file reference leaks. Realistic exploitation evidence does not support memory corruption, privilege escalation, confidentiality impact, or integrity impact. It should not be treated as Important-class based on the provided patch. ## 5. One-sentence report phrase A local io_uring NOP request with mismatched fixed-file flags can leak fget file references, allowing repeated local abuse to consume kernel resources and potentially cause denial of service. ## 6. Manual review recommendation NO MANUAL CHECK NEEDED This is a local resource leak with no demonstrated memory corruption, no network reachability, and no plausible privilege escalation primitive in the patch context. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: io_uring/nop: fix file reference leak with IOSQE_FIXED_FILE Commit: 722869fcff598fad20d5ab79c305897a7534708b Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=722869fcff598fad20d5ab79c305897a7534708b Commit description: NOP file-acquisition support choses between a fixed (registered) file and a normal fget()'d file based on its own IORING_NOP_FIXED_FILE flag in sqe->nop_flags. However, a request's REQ_F_FIXED_FILE is set independently from the generic IOSQE_FIXED_FILE sqe flag during request init, before the issue handler runs. If a NOP is submitted with IOSQE_FIXED_FILE set (so REQ_F_FIXED_FILE is set) but without IORING_NOP_FIXED_FILE, io_nop() takes the normal path and grabs a real reference via io_file_get_normal(). On completion, io_put_file() only drops the reference when REQ_F_FIXED_FILE is clear, so the fget()'d file is never released and leaks: BUG: memory leak unreferenced object 0xffff88800f42c240 (size 176): kmem_cache_alloc_noprof+0x358/0x440 alloc_empty_file+0x57/0x180 path_openat+0x44/0x1e50 do_file_open+0x121/0x200 do_sys_openat2+0xa7/0x150 __x64_sys_openat+0x82/0xf0 Decide between fixed and normal file acquisition from REQ_F_FIXED_FILE, the same way io_assign_file() does for every other opcode, and fold IORING_NOP_FIXED_FILE into REQ_F_FIXED_FILE at prep time. Cc: stable@vger.kernel.org Fixes: a85f310 ("io_uring/nop: add support for testing registered files and buffers") Reported-by: syzbot+2cd473471e77bda12b0e@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?id=879092631b98f73a28ea405adacfa5bb34a14a25 Signed-off-by: Vasileios Almpanis Link: https://patch.msgid.link/20260615144619.482749-1-vasilisalmpanis@gmail.com Signed-off-by: Jens Axboe Signed-off-by: Greg Kroah-Hartman Changed files: io_uring/nop.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=722869fcff598fad20d5ab79c305897a7534708b ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-64426 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64426 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:L/UI:N/S:U/C:N/I:N/A:L 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: 2 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).