CVE-2018-20854 in Linux
Summary
by MITRE
An issue was discovered in the Linux kernel before 4.20. drivers/phy/mscc/phy-ocelot-serdes.c has an off-by-one error with a resultant ctrl->phys out-of-bounds read.
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Analysis
by VulDB Data Team • 11/13/2023
The vulnerability identified as CVE-2018-20854 represents a critical memory safety issue within the Linux kernel's phy subsystem, specifically affecting the Marvell Ocelot SerDes driver. This flaw exists in the drivers/phy/mscc/phy-ocelot-serdes.c file and manifests as an off-by-one error that leads to an out-of-bounds memory read condition. The vulnerability is particularly concerning because it affects kernel versions prior to 4.20, meaning a significant portion of deployed systems could be exposed to potential exploitation. The issue arises from improper bounds checking in the driver's handling of physical layer interfaces, where the control structure's phys array is accessed beyond its allocated boundaries.
The technical implementation of this vulnerability stems from a classic buffer over-read condition where the driver fails to properly validate array indices when accessing the ctrl->phys array. This type of flaw falls under CWE-129, which specifically addresses insufficient bounds checking, and represents a direct violation of secure coding practices for kernel space memory management. The out-of-bounds read occurs when the driver attempts to access memory locations beyond the legitimate array boundaries, potentially exposing sensitive kernel memory to unauthorized access or causing system instability through information disclosure. The vulnerability's impact is amplified by the fact that it operates within the kernel's physical layer driver context, which typically runs with elevated privileges and has direct hardware access capabilities.
Operationally, this vulnerability presents significant risks to system integrity and security posture. An attacker who can influence the phy configuration or trigger the affected driver path could potentially exploit this condition to read kernel memory contents, which might reveal sensitive information such as cryptographic keys, system credentials, or other confidential data. The out-of-bounds read could also lead to system crashes or denial of service conditions, depending on the specific memory locations accessed. This vulnerability aligns with ATT&CK technique T1068, which covers local privilege escalation through kernel exploits, and could be leveraged as a stepping stone for more sophisticated attacks. The flaw's presence in a phy driver also means that it could be triggered through network or hardware-related operations, potentially allowing for remote exploitation scenarios.
Mitigation strategies for CVE-2018-20854 primarily focus on immediate kernel version upgrades to 4.20 or later, where the fix has been implemented. The patch addresses the root cause by introducing proper bounds checking and ensuring that array indices remain within valid ranges when accessing the ctrl->phys structure. Organizations should also implement comprehensive patch management processes to ensure all affected systems receive updates promptly. Additional defensive measures include monitoring for unusual phy driver behavior, implementing kernel hardening configurations such as stack canaries and address space layout randomization, and conducting thorough security assessments of network infrastructure components that rely on affected phy drivers. System administrators should also consider isolating critical systems and implementing network segmentation to limit potential attack vectors that could exploit this vulnerability.