CVE-2017-7889 in Linux
Summary
by MITRE
The mm subsystem in the Linux kernel through 4.10.10 does not properly enforce the CONFIG_STRICT_DEVMEM protection mechanism, which allows local users to read or write to kernel memory locations in the first megabyte (and bypass slab-allocation access restrictions) via an application that opens the /dev/mem file, related to arch/x86/mm/init.c and drivers/char/mem.c.
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Analysis
by VulDB Data Team • 11/29/2022
The vulnerability described in CVE-2017-7889 represents a critical flaw in the Linux kernel's memory management subsystem that undermines fundamental security protections designed to prevent unauthorized access to kernel memory space. This weakness exists within the mm subsystem of Linux kernel versions through 4.10.10 and specifically targets the CONFIG_STRICT_DEVMEM protection mechanism that should normally restrict direct memory access operations. The flaw allows local attackers to bypass critical access controls by exploiting the /dev/mem device file interface, creating a pathway for unauthorized memory manipulation that could lead to privilege escalation and system compromise. The vulnerability is particularly concerning because it affects the most basic memory access protections that should prevent direct kernel memory access from user-space applications.
The technical implementation of this vulnerability stems from improper enforcement of kernel memory protection mechanisms within the x86 architecture memory management code paths. Specifically, the flaw is located in arch/x86/mm/init.c and drivers/char/mem.c files where the kernel fails to properly validate memory access requests when applications attempt to open and interact with the /dev/mem device. This protection bypass occurs because the kernel does not adequately check whether the requested memory access falls within protected kernel memory regions, particularly in the first megabyte of memory space where kernel data structures and code reside. The vulnerability is categorized under CWE-264, which represents "Permissions, Privileges, and Access Controls" issues, specifically involving insufficient protection mechanisms. The flaw allows attackers to circumvent the slab-allocation access restrictions that normally prevent direct kernel memory manipulation, enabling access to critical kernel data structures and potentially sensitive information.
The operational impact of CVE-2017-7889 is severe and multifaceted, as it provides local attackers with direct access to kernel memory that could be exploited for privilege escalation and system compromise. Attackers can leverage this vulnerability to read sensitive kernel data, modify kernel memory structures, and potentially execute arbitrary code with kernel privileges. The ability to bypass slab-allocation access restrictions means that attackers can access memory regions that should normally be protected from direct user-space access, potentially leading to complete system compromise. This vulnerability directly maps to ATT&CK technique T1068, which involves "Local Privilege Escalation" through kernel exploits, and T1083, which covers "File and Directory Discovery" when combined with memory access capabilities. The vulnerability affects systems where the CONFIG_STRICT_DEVMEM kernel configuration option is not properly enforced, which could include various Linux distributions and kernel builds that have not been updated to address this specific protection mechanism failure.
Mitigation strategies for CVE-2017-7889 should focus on both immediate patching and configuration hardening approaches. The primary solution involves updating to Linux kernel versions that contain the fix for this vulnerability, specifically kernel versions 4.11.0 and later where the protection mechanism has been properly implemented. System administrators should also consider disabling the /dev/mem device access when possible, though this may impact legitimate system administration tasks. Additional mitigations include implementing proper kernel configuration options that enforce strict device memory protection, monitoring for unauthorized access attempts to kernel memory regions, and ensuring that systems are running patched kernel versions. Organizations should also consider implementing runtime protection mechanisms such as kernel lockdown modes and enhanced access control policies that can detect and prevent unauthorized memory access attempts. The vulnerability highlights the importance of proper kernel memory protection enforcement and demonstrates how seemingly simple protection mechanisms can be bypassed when not properly implemented across all relevant code paths.