CVE-2021-47477 in Linux
Summary
by MITRE • 05/22/2024
In the Linux kernel, the following vulnerability has been resolved:
comedi: dt9812: fix DMA buffers on stack
USB transfer buffers are typically mapped for DMA and must not be allocated on the stack or transfers will fail.
Allocate proper transfer buffers in the various command helpers and return an error on short transfers instead of acting on random stack data.
Note that this also fixes a stack info leak on systems where DMA is not used as 32 bytes are always sent to the device regardless of how short the command is.
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Analysis
by VulDB Data Team • 11/22/2025
The vulnerability identified as CVE-2021-47477 affects the Linux kernel's comedi subsystem, specifically within the dt9812 driver implementation. This issue represents a critical flaw in how USB transfer buffers are managed during device communication operations. The comedi framework serves as a universal driver interface for data acquisition devices, and the dt9812 driver specifically handles devices from Data Translation, commonly used in industrial and scientific data acquisition applications. When examining this vulnerability through the lens of cybersecurity best practices, it becomes evident that the flaw stems from improper memory management practices that directly violate fundamental security principles for kernel-level device drivers.
The technical root cause of this vulnerability lies in the improper allocation of DMA (Direct Memory Access) buffers within the kernel's device driver code. According to CWE-563, this represents a violation of the principle that variables should be used for their intended purpose, specifically in the context of memory allocation for hardware communication. The driver was allocating USB transfer buffers directly on the stack rather than using proper kernel memory allocation functions that ensure buffers are appropriately mapped for DMA operations. This fundamental error creates a scenario where stack-based buffers are not properly aligned or mapped for hardware access, causing USB transfers to fail unpredictably. The implementation fails to adhere to the established kernel programming guidelines that mandate the use of kmalloc or similar functions for DMA-capable memory allocations, as specified in the Linux kernel documentation and security best practices.
The operational impact of this vulnerability extends beyond simple functional failures to encompass potential security implications and system instability. When USB transfers fail due to improperly allocated buffers, the system may experience unpredictable behavior including device malfunctions, data corruption, or complete system hangs. The vulnerability also introduces a information disclosure risk through what is known as a stack information leak, as detailed in the security advisory. Even on systems where DMA is not actively utilized, the driver still sends 32 bytes of stack data to the device regardless of the actual command size, creating a potential information leak that could expose sensitive kernel memory contents to external devices. This information disclosure aspect aligns with ATT&CK technique T1005, which covers data from local system, and represents a significant security concern for industrial control systems where such devices may be exposed to untrusted networks or users.
The mitigation strategy for this vulnerability requires immediate kernel updates from system administrators, as the fix has been incorporated into the mainline Linux kernel. The resolution involves proper allocation of transfer buffers using kernel memory allocation functions that ensure appropriate DMA mapping, along with implementing proper error handling for short transfers. System administrators should prioritize patching affected systems, particularly those running industrial control systems or data acquisition platforms that utilize the dt9812 driver. The fix also addresses the stack information leak by ensuring that only the necessary amount of data is transmitted to devices, preventing exposure of potentially sensitive kernel stack contents. Organizations using embedded systems or specialized industrial equipment should conduct thorough testing of updated kernel versions to ensure compatibility with their existing hardware configurations while maintaining security posture against this and similar vulnerabilities in the comedi subsystem.