CVE-2021-47032 in Linux
Summary
by MITRE • 02/28/2024
In the Linux kernel, the following vulnerability has been resolved:
mt76: mt7915: fix tx skb dma unmap
The first pointer in the txp needs to be unmapped as well, otherwise it will leak DMA mapping entries
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Analysis
by VulDB Data Team • 12/13/2024
The vulnerability identified as CVE-2021-47032 affects the Linux kernel's wireless networking subsystem, specifically within the mt76 driver family that supports MediaTek wireless chipsets including the mt7915 model. This issue represents a critical memory management flaw that occurs during the transmission process of wireless packets, where the kernel fails to properly handle DMA (Direct Memory Access) mapping operations for transmitted sk_buff structures. The problem manifests in the transmit path where the driver maintains a transmit packet (txp) structure containing multiple pointers to memory buffers that need to be mapped for hardware access. When processing transmitted packets, the driver correctly unmaps most of the DMA mappings but neglects to unmap the first pointer in the txp structure, creating a memory leak scenario.
The technical root cause of this vulnerability lies in improper resource management within the DMA mapping subsystem of the Linux kernel. DMA mappings are essential for hardware devices to access kernel memory directly without CPU intervention, and each mapping consumes system resources that must be properly accounted for and released. The flaw occurs because the driver implementation follows an incomplete cleanup pattern where it processes all but the first element in a linked list or array of DMA mappings, leaving the initial mapping entry unreleased. This creates a gradual accumulation of unreleased DMA mapping entries that can eventually exhaust system resources and lead to system instability or denial of service conditions. The vulnerability directly maps to CWE-404, which describes improper resource management where resources are not properly released or recycled, and more specifically to CWE-129, which addresses improper validation of array indices that can lead to memory access violations.
The operational impact of CVE-2021-47032 extends beyond simple resource leakage to potentially compromise system availability and stability in wireless networking environments. When the DMA mapping table becomes exhausted due to leaked entries, the wireless subsystem may fail to process new transmissions, resulting in network connectivity issues that affect both client devices and access points. The vulnerability is particularly concerning in high-throughput wireless environments where continuous packet transmission occurs, as the leak accumulates rapidly and can lead to complete wireless interface failure. Attackers could potentially exploit this vulnerability by triggering sustained wireless transmission activity to force resource exhaustion, though the attack surface is limited to systems running affected MediaTek wireless chipsets. The impact aligns with ATT&CK technique T1499.004, which covers network denial of service attacks, and represents a form of resource exhaustion attack that affects system availability rather than confidentiality or integrity.
Mitigation strategies for CVE-2021-47032 focus on applying the upstream kernel patch that ensures all DMA mappings in the txp structure are properly unmapped, including the first pointer that was previously overlooked. System administrators should prioritize updating their Linux kernel versions to include the fix, particularly in enterprise environments where wireless connectivity is critical. The patch implementation requires careful verification to ensure compatibility with existing wireless configurations and driver-specific optimizations. Organizations should also implement monitoring for DMA mapping resource usage in wireless subsystems to detect potential leakage patterns before they cause system instability. Additional defensive measures include implementing proper resource limits and monitoring for wireless subsystem performance degradation that could indicate resource exhaustion issues. The fix demonstrates the importance of thorough resource management practices in kernel-level code and highlights the need for comprehensive testing of DMA mapping operations, particularly in high-frequency transmission scenarios where resource leakage can accumulate rapidly and lead to system-wide availability issues.