CVE-2020-28974 in Linux
Summary
by MITRE • 11/21/2020
A slab-out-of-bounds read in fbcon in the Linux kernel before 5.9.7 could be used by local attackers to read privileged information or potentially crash the kernel, aka CID-3c4e0dff2095. This occurs because KD_FONT_OP_COPY in drivers/tty/vt/vt.c can be used for manipulations such as font height.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 12/09/2020
The vulnerability identified as CVE-2020-28974 represents a critical slab out-of-bounds read condition within the framebuffer console subsystem of the Linux kernel, specifically affecting versions prior to 5.9.7. This flaw exists within the fbcon implementation and manifests through the KD_FONT_OP_COPY operation in the virtual terminal driver component located at drivers/tty/vt/vt.c. The issue enables local attackers to exploit a memory access violation that occurs when manipulating font height parameters through kernel ioctls, creating a pathway for information disclosure and potential system instability.
The technical root cause of this vulnerability lies in insufficient bounds checking within the font operation handling code. When the KD_FONT_OP_COPY ioctl command is executed, the kernel fails to properly validate the size parameters associated with font height manipulations, allowing attackers to specify invalid memory offsets that extend beyond the intended slab allocation boundaries. This out-of-bounds read condition occurs in the context of kernel memory management where font data structures are manipulated, creating opportunities for attackers to access adjacent memory regions containing sensitive kernel data structures or credentials.
From an operational impact perspective, this vulnerability poses significant security risks to systems running affected kernel versions. Local attackers with minimal privileges can leverage this flaw to read privileged kernel memory, potentially extracting sensitive information such as cryptographic keys, credential storage locations, or other confidential data structures that reside in kernel memory. The vulnerability also presents a crash potential that could lead to denial of service conditions, making it particularly dangerous in production environments where system stability is critical. The attack surface is broad as any local user or process with access to the virtual terminal subsystem can potentially exploit this condition.
The vulnerability maps to CWE-125 out-of-bounds read within the Common Weakness Enumeration framework, specifically categorizing as a buffer overflow condition in kernel space memory management. From an attack framework perspective, this vulnerability aligns with techniques described in the MITRE ATT&CK matrix under T1068, requiring local system access to exploit, and potentially supporting privilege escalation through information gathering. The exploitation process involves crafting specific ioctl calls with malformed font height parameters that trigger the out-of-bounds memory access, making it a low-effort but high-impact vulnerability for local attackers.
Mitigation strategies for CVE-2020-28974 primarily focus on kernel version updates to 5.9.7 or later, where the bounds checking has been properly implemented to prevent the out-of-bounds read condition. System administrators should prioritize patching affected systems and monitor for any potential exploitation attempts through log analysis of ioctl operations. Additional defensive measures include implementing kernel lockdown modes that restrict access to potentially dangerous kernel interfaces, monitoring for unusual font operation patterns, and maintaining comprehensive system logging to detect exploitation attempts. Organizations should also consider implementing privilege separation controls to minimize the impact of local privilege escalation attempts that might leverage this vulnerability.