CVE-2019-20172 in SerenityOS
Summary
by MITRE
Kernel/VM/MemoryManager.cpp in SerenityOS before 2019-12-30 does not reject syscalls with pointers into the kernel-only virtual address space, which allows local users to gain privileges by overwriting a return address that was found on the kernel stack.
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Analysis
by VulDB Data Team • 03/18/2024
The vulnerability identified as CVE-2019-20172 resides within the kernel memory management subsystem of SerenityOS, specifically in the Kernel/VM/MemoryManager.cpp file. This flaw represents a critical privilege escalation vulnerability that affects versions of the operating system prior to the 2019-12-30 release. The vulnerability stems from inadequate validation of system call parameters, particularly concerning virtual memory address space boundaries. When a local user executes a crafted system call, the kernel fails to properly validate that pointer arguments do not reference kernel-only virtual address space regions, creating a pathway for malicious exploitation.
The technical implementation of this vulnerability involves a failure in kernel memory validation mechanisms that should prevent user-space processes from directly manipulating kernel memory regions. Specifically, the memory manager does not reject system calls that contain pointers pointing to kernel-only virtual address space locations. This oversight allows an attacker to craft system calls with carefully positioned pointer arguments that target kernel stack memory locations. When the kernel processes these invalid system calls, it can be induced to overwrite critical kernel data structures, including return addresses stored on the kernel stack. The vulnerability directly maps to CWE-787, which describes out-of-bounds writes, and more specifically to CWE-121, which covers stack-based buffer overflow conditions.
The operational impact of this vulnerability is severe and affects the fundamental security model of the operating system. Local users who can execute system calls can leverage this flaw to escalate their privileges from user level to kernel level execution. This privilege escalation allows attackers to bypass all kernel security controls, potentially gaining complete system control. The attack vector requires local access and the ability to execute system calls, but does not require network connectivity or complex external conditions. The vulnerability essentially allows a malicious user to directly modify kernel execution flow by overwriting return addresses, effectively enabling code execution within kernel space. This type of attack aligns with ATT&CK technique T1068, which covers local privilege escalation through kernel exploits.
The exploitation process involves crafting a system call that includes a pointer argument referencing kernel memory space, then triggering a kernel function that uses this pointer in a way that results in stack corruption. The kernel stack's return address becomes overwritten, allowing the attacker to redirect execution flow to arbitrary kernel code or manipulate kernel data structures. This vulnerability represents a classic case of insufficient input validation combined with improper memory management, where the kernel fails to enforce proper address space boundaries between user and kernel modes. The fix for this vulnerability required implementing proper validation checks in the kernel's system call handling code to ensure that all pointer arguments are validated against kernel memory regions and rejected if they point to protected virtual address spaces. The remediation approach aligns with defensive programming principles and memory safety best practices as recommended by industry standards and security frameworks.