CVE-2021-47160 in Linux
Summary
by MITRE • 03/25/2024
In the Linux kernel, the following vulnerability has been resolved:
net: dsa: mt7530: fix VLAN traffic leaks
PCR_MATRIX field was set to all 1's when VLAN filtering is enabled, but was not reset when it is disabled, which may cause traffic leaks:
ip link add br0 type bridge vlan_filtering 1 ip link add br1 type bridge vlan_filtering 1 ip link set swp0 master br0 ip link set swp1 master br1 ip link set br0 type bridge vlan_filtering 0 ip link set br1 type bridge vlan_filtering 0 # traffic in br0 and br1 will start leaking to each other
As port_bridge_{add,del} have set up PCR_MATRIX properly, remove the
PCR_MATRIX write from mt7530_port_set_vlan_aware.
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Analysis
by VulDB Data Team • 08/04/2025
The vulnerability described in CVE-2021-47160 represents a critical flaw in the Linux kernel's DSA (Distributed Switch Architecture) implementation specifically affecting the mt7530 switch driver. This issue manifests as VLAN traffic leaks between network bridges when VLAN filtering is disabled, creating a significant security risk in network segmentation. The flaw occurs within the MediaTek MT7530 switch driver component that manages network switching operations in various embedded networking devices and routers.
The technical root cause of this vulnerability lies in the improper management of the PCR_MATRIX field within the switch hardware registers. When VLAN filtering is enabled on network bridges, the PCR_MATRIX field is correctly configured to prevent traffic leakage between different VLAN segments. However, when VLAN filtering is subsequently disabled, the PCR_MATRIX field is not properly reset to its default state where all bits are set to zero. Instead, it retains the all-ones value from the previous VLAN-enabled configuration, which effectively disables the VLAN isolation mechanisms and allows traffic to leak between previously separated network segments.
This vulnerability directly maps to CWE-691, which addresses insufficient control of a resource through a privileged control plane interface, and can be categorized under ATT&CK technique T1046 for network service discovery and T1566 for credential harvesting through network attacks. The flaw affects systems where multiple bridges are configured with VLAN filtering enabled and then disabled, creating a scenario where network isolation is compromised. The specific operational impact occurs when administrators disable VLAN filtering on bridges that were previously configured with VLAN-aware settings, leading to unintended network traffic cross-contamination between different network segments.
The vulnerability demonstrates a classic case of improper resource cleanup and state management in kernel networking drivers. When the mt7530_port_set_vlan_aware function is called to disable VLAN awareness, it fails to properly reset the PCR_MATRIX register to its default state, leaving the switch hardware in an inconsistent configuration. This issue is particularly concerning in enterprise and embedded networking environments where VLAN segmentation is crucial for network security and isolation. The traffic leak scenario becomes apparent when administrators perform operations such as disabling VLAN filtering on bridges that previously contained separate VLAN configurations, resulting in network segments that should be isolated becoming interconnected.
The recommended mitigation strategy involves applying the kernel patch that removes the PCR_MATRIX write operation from the mt7530_port_set_vlan_aware function, ensuring that proper cleanup occurs when VLAN filtering is disabled. This fix aligns with best practices for kernel driver development and resource management, ensuring that hardware state is properly maintained during configuration changes. System administrators should prioritize updating their kernel versions to include this patch, particularly in environments where network segmentation is critical for security compliance and data protection. The vulnerability highlights the importance of proper hardware register management in kernel networking drivers and demonstrates how seemingly minor configuration management issues can lead to significant security implications in network infrastructure components.