CVE-2017-6899 in LineageOS
Summary
by MITRE
The msm_bus_dbg_update_request_write function in drivers/platform/msm/msm_bus/msm_bus_dbg.c in android_kernel_huawei_msm8916 through 2017-06-16 in LineageOS, and possibly other kernels for MSM devices, allows attackers to cause a denial of service (NULL pointer dereference and device crash) via a crafted /sys/kernel/debug/msm-bus-dbg/client-data/update-request write request.
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Analysis
by VulDB Data Team • 12/29/2020
The vulnerability identified as CVE-2017-6899 represents a critical null pointer dereference flaw within the Android kernel's MSM bus debugging subsystem. This issue affects the msm_bus_dbg_update_request_write function located in drivers/platform/msm/msm_bus/msm_bus_dbg.c, specifically impacting kernel versions through the 2017-06-16 LineageOS release and potentially other MSM device kernels. The vulnerability manifests when attackers exploit a crafted write request to the /sys/kernel/debug/msm-bus-dbg/client-data/update-request interface, which serves as a debugging mechanism for the MSM bus subsystem responsible for managing bandwidth allocation between various hardware components in Qualcomm MSM processors.
The technical exploitation of this vulnerability occurs through improper input validation within the msm_bus_dbg_update_request_write function, which fails to adequately check for null pointer references before attempting to dereference pointers during the update-request processing. This flaw directly maps to CWE-476, which categorizes null pointer dereference as a common programming error that can lead to system crashes and denial of service conditions. When an attacker submits malicious data through the debug interface, the function attempts to access memory locations that have not been properly initialized, causing the kernel to crash and resulting in a complete system freeze or reboot. The vulnerability's impact extends beyond simple denial of service as it can potentially be leveraged for more sophisticated attacks that exploit the kernel's instability.
The operational implications of this vulnerability are significant for devices running affected kernel versions, particularly smartphones and tablets based on Qualcomm MSM processors such as those found in Huawei devices. The attack surface is relatively narrow since it requires access to the debug interface, which typically requires root privileges or physical access to the device. However, in environments where untrusted users have access to device debugging interfaces, or when the kernel is configured with debug features enabled in production, this vulnerability can be exploited to cause persistent service disruption. The flaw particularly affects systems where the MSM bus debugging functionality is enabled, making it a concern for both consumer devices and enterprise deployments where kernel debugging features might be enabled for troubleshooting purposes. According to ATT&CK framework, this vulnerability aligns with T1059.007 (Command and Scripting Interpreter: PowerShell) and T1499.004 (Domain: Endpoint Denial of Service) techniques, as it enables an attacker to cause system instability through kernel-level manipulation.
Mitigation strategies for CVE-2017-6899 should focus on both immediate patching and operational security measures. Organizations should prioritize applying kernel updates that address this specific vulnerability, particularly those containing fixes for the msm_bus_dbg_update_request_write function's input validation. The most effective long-term solution involves disabling the debug interface on production devices unless absolutely necessary for troubleshooting, as this eliminates the attack vector entirely. Additionally, implementing proper access controls and privilege separation can prevent unauthorized users from submitting malicious write requests to the debug interface. System administrators should consider disabling kernel debugging features in production environments and regularly audit kernel configurations to ensure that unnecessary debug interfaces remain closed. The vulnerability serves as a reminder of the importance of proper input validation in kernel space code and highlights the need for comprehensive security testing of kernel modules, particularly those providing debug interfaces that can be exposed to untrusted users or processes.