CVE-2021-47170 in Linux
Summary
by MITRE • 03/25/2024
In the Linux kernel, the following vulnerability has been resolved:
USB: usbfs: Don't WARN about excessively large memory allocations
Syzbot found that the kernel generates a WARNing if the user tries to submit a bulk transfer through usbfs with a buffer that is way too large. This isn't a bug in the kernel; it's merely an invalid request from the user and the usbfs code does handle it correctly.
In theory the same thing can happen with async transfers, or with the packet descriptor table for isochronous transfers.
To prevent the MM subsystem from complaining about these bad allocation requests, add the __GFP_NOWARN flag to the kmalloc calls for these buffers.
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Analysis
by VulDB Data Team • 11/23/2025
The vulnerability identified as CVE-2021-47170 resides within the Linux kernel's USB subsystem, specifically affecting the usbfs (USB filesystem) interface that provides userspace access to USB devices. This issue represents a denial of service vulnerability that manifests through excessive kernel logging warnings during legitimate but resource-intensive USB transfer operations. The vulnerability was discovered through automated fuzzing by syzbot, which identified that the kernel's usbfs implementation generates unnecessary WARN messages when userspace applications attempt to submit bulk USB transfers with buffers that exceed reasonable memory allocation limits. While the kernel correctly handles these invalid requests without crashing or compromising security, the excessive warning messages can overwhelm system logs and potentially impact system performance through unnecessary kernel logging overhead.
The technical flaw stems from the kernel's memory allocation strategy within the usbfs subsystem where kmalloc calls for large buffer allocations trigger kernel memory management subsystem warnings even when the operations are valid user requests that the kernel properly handles. The vulnerability occurs during bulk USB transfers through the usbfs interface where userspace applications can request transfers with buffer sizes that are excessively large, causing the kernel to issue WARN messages that are technically unnecessary since the kernel code correctly processes these invalid requests. This behavior affects not only bulk transfers but also async transfers and isochronous transfers with packet descriptor tables, making it a broader class of issues within the USB subsystem's memory management approach. The root cause lies in the absence of the __GFP_NOWARN flag during memory allocation operations, which would prevent the kernel's memory management subsystem from generating these excessive warning messages for what are essentially valid but resource-intensive user requests.
The operational impact of this vulnerability manifests primarily as excessive logging and potential performance degradation in systems running USB-intensive applications or those subject to automated testing scenarios. While the kernel does not crash or expose security vulnerabilities, the continuous generation of WARN messages can fill system logs rapidly, making it difficult to identify genuine system issues and potentially causing log rotation or storage problems. In high-traffic environments or systems with extensive USB device usage, this could lead to significant logging overhead that impacts system responsiveness and monitoring capabilities. The vulnerability particularly affects systems where automated testing tools or malicious actors might attempt to stress test USB subsystems with oversized buffer requests, potentially leading to log flooding that could obscure real security events or system anomalies.
Mitigation strategies for this vulnerability involve modifying the kernel's usbfs implementation to add the __GFP_NOWARN flag to kmalloc calls when allocating buffers for USB transfers. This change prevents the kernel's memory management subsystem from generating unnecessary warnings while maintaining the existing validation and error handling behavior. System administrators should ensure their systems are updated with kernel versions containing this fix, typically found in kernel releases 5.13 and later. Organizations running USB-intensive workloads should monitor their system logs for unusual warning patterns that might indicate attempts to exploit this vulnerability, though such exploitation would be limited to denial of service rather than privilege escalation or data compromise. The fix aligns with security best practices for avoiding false positive warnings in system components while maintaining proper resource validation and error handling. This vulnerability maps to CWE-704 in the CWE taxonomy, which covers incorrect type conversion or cast, and relates to ATT&CK technique T1499.001 for resource exhaustion, though the impact is limited to logging overhead rather than complete system compromise.