CVE-2019-20636 in Linux
Summary
by MITRE
In the Linux kernel before 5.4.12, drivers/input/input.c has out-of-bounds writes via a crafted keycode table, as demonstrated by input_set_keycode, aka CID-cb222aed03d7.
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Analysis
by VulDB Data Team • 05/17/2024
The vulnerability identified as CVE-2019-20636 represents a critical out-of-bounds write flaw in the Linux kernel's input subsystem, specifically within the drivers/input/input.c file. This issue affects kernel versions prior to 5.4.12 and stems from improper validation of keycode table parameters during input device configuration operations. The vulnerability manifests when a crafted keycode table is processed through the input_set_keycode function, which serves as a core interface for configuring input device key mappings. The flaw allows for memory corruption that can potentially be exploited to achieve arbitrary code execution or system instability. This type of vulnerability falls under the CWE-787 category of out-of-bounds write conditions, which represents one of the most dangerous classes of memory safety issues in kernel space code.
The technical implementation of this vulnerability involves the input subsystem's handling of key mapping tables where insufficient bounds checking occurs when processing user-supplied keycode data. When input_set_keycode is invoked with maliciously crafted parameters, the kernel fails to validate the size or bounds of the provided keycode table, leading to writes beyond allocated memory regions. This memory corruption can overwrite adjacent data structures, potentially corrupting kernel metadata or even allowing attackers to overwrite function pointers or other critical kernel variables. The vulnerability is particularly concerning because it operates within the kernel's input handling layer, which is frequently accessed during normal system operation, making exploitation more likely and potentially persistent across system sessions.
The operational impact of CVE-2019-20636 extends beyond simple system instability to encompass potential privilege escalation and system compromise. Attackers who can manipulate input device configurations or inject malicious keycode tables through various attack vectors could leverage this vulnerability to execute arbitrary code with kernel privileges. This represents a significant threat to system security as it bypasses typical user-space protections and can lead to complete system compromise. The vulnerability is particularly dangerous in environments where untrusted users have access to input device configuration interfaces or where kernel modules can be loaded dynamically. From an ATT&CK framework perspective, this vulnerability maps to techniques involving privilege escalation through kernel exploits and can be classified under T1068 for locally executed malicious code and T1543 for kernel module loading.
Mitigation strategies for CVE-2019-20636 primarily focus on immediate kernel version upgrades to 5.4.12 or later, which contain the necessary patches to address the out-of-bounds write condition. System administrators should prioritize updating all affected systems, particularly those running older kernel versions or custom kernel builds that may not have received the patch. Additional defensive measures include implementing proper input device access controls, restricting user permissions for input device configuration, and monitoring for suspicious input device behavior or unauthorized keycode table modifications. The vulnerability also highlights the importance of kernel security hardening practices including stack canaries, kernel address space layout randomization, and other exploit mitigation techniques. Organizations should conduct comprehensive vulnerability assessments to identify systems running vulnerable kernel versions and ensure proper patch management procedures are in place to prevent similar issues from occurring in the future.