From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-64328][MODERATE REGULAR] usb: gadget: f_fs: Fix DMA fence leak Date: Sat, 25 Jul 2026 06:50:43 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-64328 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: b7475b2dce5e121e687280ba5732ccefe77ffd2f List-Id: CVE: CVE-2026-64328 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: usb: gadget: f_fs: Fix DMA fence leak Commit: b7475b2dce5e121e687280ba5732ccefe77ffd2f Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b7475b2dce5e121e687280ba5732ccefe77ffd2f Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64328 Analysis date: Sat, 25 Jul 2026 06:50:43 -0400 ActionableScore: 3 ActionableScore lower bound: 2 Actionable bucket: Ordinary Moderate / Low-like, not Important Manual review required: NO Summary: A missing dma_fence_put in the USB gadget FunctionFS DMABUF transfer path leaks ffs_dma_fence objects across repeated transfers, allowing a local process with access to the configured gadget interface to cause gradual kernel memory exhaustion and denial of service. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-64328 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64328 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-2026-64328 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-401;*CWE-772;**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 'The USB/gadget FunctionFS DMABUF transfer path can leak ffs_dma_fence objects because the initial dma_fence reference is not released after dma_resv_add_fence takes its own reference. Repeated transfers can slowly consume kernel memory and may lead to denial of service through resource exhaustion. For the CVSS the PR:L is used because a local process or gadget service with access to the FunctionFS DMABUF transfer interface is needed to trigger the leak. The issue is not directly network reachable and there is no evidence of a memory corruption primitive beyond resource leakage. Impact is denial of service only in the realistic case.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like, not Important (with actual score 3) SKIP CVE-2026-64328 UNKNOWN SKIP No affected files built, so skip this CVE NO - - unknown MAYBE OOB USB SIMPLEFIX LEAK DMAorINTERRUPT IMPROVEONLY LINUS MEMORY DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 ActionableScoreLower=2 ## 1. ActionableScore * Conservative score: 2 * Paranoid score: 3 * Final recommended bucket: **Ordinary Moderate / Low-like, not Important**:: ## 2. Signal breakdown Conservative signals: * Local trigger: +1. Triggering requires a local process or gadget service with access to the FunctionFS DMABUF transfer path. * Real lifetime or refcount bug: +1. The initial dma_fence reference is not released after dma_resv_add_fence takes its own reference. * Availability impact realistic: +1. Repeated transfers can slowly leak kernel memory and may eventually cause resource exhaustion. * Rare or configuration-dependent subsystem: -1. USB gadget FunctionFS with DMABUF import is not a broad default server or desktop exposure. Paranoid additional signal: * Reliable resource-exhaustion DoS under sustained abuse: +1. If an attacker can repeatedly drive the transfer path, the leak can accumulate until memory pressure or failure. Not applied: * No remote reachable signal. * No generic memory corruption signal. This is a reference leak, not UAF, OOB write, double free, or type confusion. * No privilege escalation signal. * No confidentiality or integrity impact. * No Dirty-Pipe-like or page-cache corruption primitive. ## 3. Reachability analysis The bug is reachable only when USB gadget FunctionFS with the DMABUF import interface is configured and usable. In typical deployments, gadget setup is controlled by root or a privileged service, but the actual transfer path may be reachable by a local gadget process with delegated access to FunctionFS endpoint files. Namespaces do not by themselves make this broadly unprivileged unless the device and FunctionFS access are explicitly delegated. The path is not network reachable and is not enabled on most general-purpose systems. Call-site confidence: high. The patch and commit message directly describe the reference ownership sequence around dma_fence_init, dma_resv_add_fence, ffs_dmabuf_cleanup, and dma_fence_put. ## 4. Severity interpretation This behaves like an ordinary Moderate or Low-like kernel resource leak. The realistic impact is local denial of service through gradual kernel memory exhaustion. The paranoid score recognizes that sustained abuse could make availability impact high, but the patch does not support memory corruption, privilege escalation, confidentiality loss, or integrity impact. This should not be treated as an Important-class vulnerability. ## 5. One-sentence report phrase A missing dma_fence_put in the USB gadget FunctionFS DMABUF transfer path leaks ffs_dma_fence objects across repeated transfers, allowing a local process with access to the configured gadget interface to cause gradual kernel memory exhaustion and denial of service. ## 6. Manual review recommendation NO MANUAL CHECK NEEDED This is a clear resource leak with no concrete memory corruption primitive, no network exposure, and no plausible privilege escalation path shown by the patch. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: usb: gadget: f_fs: Fix DMA fence leak Commit: b7475b2dce5e121e687280ba5732ccefe77ffd2f Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b7475b2dce5e121e687280ba5732ccefe77ffd2f Commit description: In ffs_dmabuf_transfer(), a ffs_dma_fence object is kmalloc'd, with the underlying dma_fence later initialized by dma_fence_init(), which sets its kref counter to 1. Then, dma_resv_add_fence() gets a second reference, and a pointer to the ffs_dma_fence is passed as the usb_request's "context" field. The dma-resv mechanism will manage the second reference, but the first reference is never properly released; the ffs_dmabuf_cleanup() function decreases the reference count, but only to balance with the reference grab in ffs_dmabuf_signal_done(). The code will then slowly leak memory as more ffs_dma_fence objects are created without being ever freed. Address this issue by transferring ownership of the fence to the DMA reservation object, by calling dma_fence_put() right after dma_resv_add_fence(). The ffs_dma_fence then gets properly discarded after being signalled. Fixes: 7b07a2a ("usb: gadget: functionfs: Add DMABUF import interface") Cc: stable Signed-off-by: Paul Cercueil Tested-by: Nuno Sá Reviewed-by: Nuno Sá Link: https://patch.msgid.link/20260609152905.729328-1-paul@crapouillou.net Signed-off-by: Greg Kroah-Hartman Changed files: drivers/usb/gadget/function/f_fs.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=b7475b2dce5e121e687280ba5732ccefe77ffd2f ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-64328 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64328 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).