CVE-2021-47434 in Linux
Summary
by MITRE • 05/22/2024
In the Linux kernel, the following vulnerability has been resolved:
xhci: Fix command ring pointer corruption while aborting a command
The command ring pointer is located at [6:63] bits of the command
ring control register (CRCR). All the control bits like command stop, abort are located at [0:3] bits. While aborting a command, we read the
CRCR and set the abort bit and write to the CRCR. The read will always give command ring pointer as all zeros. So we essentially write only the control bits. Since we split the 64 bit write into two 32 bit writes, there is a possibility of xHC command ring stopped before the upper dword (all zeros) is written. If that happens, xHC updates the upper dword of its internal command ring pointer with all zeros. Next time, when the command ring is restarted, we see xHC memory access failures. Fix this issue by only writing to the lower dword of CRCR where all control bits are located.
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Analysis
by VulDB Data Team • 09/25/2025
The vulnerability described in CVE-2021-47434 represents a critical flaw in the Linux kernel's USB xHCI (eXtensible Host Controller Interface) driver implementation that specifically affects the handling of command ring pointer corruption during command abort operations. This issue stems from a fundamental race condition and improper memory management within the USB host controller's control register operations, creating a potential pathway for system instability and memory access violations.
The technical flaw manifests in the command ring control register (CRCR) handling where the command ring pointer occupies bits 6:63 while control bits including stop and abort functionality reside in bits 0:3. During command abort procedures, the system reads the CRCR register, sets the abort bit in the control portion, and writes back to the register. However, due to the 64-bit register being split into two 32-bit write operations, there exists a window where the USB host controller may interpret an intermediate state where the command ring has been stopped before the upper 32 bits are properly written. This results in the upper dword of the internal command ring pointer being updated with zeros, effectively corrupting the pointer structure.
This vulnerability directly relates to CWE-367 which addresses Time-of-Check to Time-of-Use (TOCTOU) errors, specifically manifesting as a race condition during register manipulation. The flaw creates a scenario where the system's memory management becomes inconsistent, leading to potential memory access failures when the command ring is restarted. The improper handling of register writes during abort operations violates fundamental principles of atomic operations and concurrent access control that are essential for reliable hardware interface management.
The operational impact of this vulnerability extends beyond simple system instability to potentially compromise the entire USB subsystem functionality. When the command ring pointer becomes corrupted, subsequent USB operations may fail with memory access violations, leading to device disconnections, system hangs, or complete system crashes. This represents a significant threat to system reliability, particularly in environments where USB devices are critical for system operation or where continuous operation is required. The vulnerability affects systems running Linux kernel versions that include the affected xHCI driver code, potentially impacting servers, desktop systems, and embedded devices that rely on USB connectivity.
The mitigation strategy for this vulnerability involves modifying the xHCI driver implementation to ensure that only the lower 32 bits of the CRCR register are written during abort operations, specifically targeting the control bits that are located in the lower portion of the register. This approach prevents the race condition by eliminating the split write operation that causes the intermediate state corruption. The fix aligns with ATT&CK framework technique T1547.001 which addresses kernel mode rootkits and system-level persistence mechanisms, as it addresses a fundamental kernel-level memory corruption issue that could potentially be exploited for privilege escalation or system compromise. Organizations should immediately apply the kernel patches that implement this fix to prevent exploitation and maintain system stability. The vulnerability highlights the importance of proper register handling and atomic operations in kernel-level drivers, particularly those managing hardware interfaces with complex state management requirements.