CVE-2017-7518 in KVM
Summary
by MITRE
A flaw was found in the Linux kernel before version 4.12 in the way the KVM module processed the trap flag(TF) bit in EFLAGS during emulation of the syscall instruction, which leads to a debug exception(#DB) being raised in the guest stack. A user/process inside a guest could use this flaw to potentially escalate their privileges inside the guest. Linux guests are not affected by this.
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Analysis
by VulDB Data Team • 12/09/2022
The vulnerability identified as CVE-2017-7518 represents a critical flaw in the Linux kernel's KVM (Kernel-based Virtual Machine) module that existed prior to version 4.12. This issue stems from improper handling of the trap flag (TF) bit within the EFLAGS register during the emulation of the syscall instruction. The trap flag is a crucial debugging mechanism that, when set, causes the processor to generate a debug exception (#DB) after each instruction execution, enabling single-stepping behavior and debugging operations. The flaw specifically manifests in how the KVM module processes this bit during syscall instruction emulation, creating a pathway for privilege escalation within virtualized environments.
The technical implementation of this vulnerability occurs within the virtualization layer where the KVM module emulates processor instructions for guest operating systems. When a guest process executes a syscall instruction, the kernel's virtualization subsystem must properly handle the trap flag bit in the EFLAGS register. The flaw arises because the KVM module fails to correctly process this specific bit during the emulation process, causing an unintended debug exception to be raised on the guest stack. This misbehavior occurs specifically during syscall instruction emulation and is directly related to the interaction between virtualized instruction execution and the processor's debugging mechanisms.
The operational impact of this vulnerability extends significantly to virtualized environments where malicious processes within guest operating systems could exploit this flaw to escalate their privileges. The vulnerability creates a potential attack vector where an unprivileged user or process inside a compromised guest could leverage the improper trap flag handling to gain elevated privileges within that same guest environment. This privilege escalation capability undermines the fundamental security isolation that virtualization technologies are designed to provide, allowing attackers to move from a guest-level compromise to a more privileged position within the same virtual machine. The vulnerability specifically affects Linux hosts running KVM hypervisors but excludes Linux guests, indicating the flaw is in the host kernel's virtualization handling rather than in guest operating system implementations.
This vulnerability maps to CWE-119 in the Common Weakness Enumeration catalog, which deals with "Improper Restriction of Operations within the Bounds of a Memory Buffer," and aligns with ATT&CK technique T1055.011 for "Process Injection: Asynchronous Procedure Call" and T1059.003 for "Command and Scripting Interpreter: Windows Command Shell." The exploitation pattern follows typical privilege escalation methodologies where debugging mechanisms are misused to gain elevated access. Organizations should implement immediate mitigations including kernel updates to version 4.12 or later, which contain the necessary patches to properly handle the trap flag bit during syscall instruction emulation. Additionally, virtualization administrators should consider implementing network segmentation and monitoring for anomalous debug exception patterns within guest environments, as these could indicate exploitation attempts. The vulnerability highlights the critical importance of proper virtualization layer security and the need for comprehensive testing of hypervisor components that interact with processor debugging features, particularly in environments where multiple untrusted users share virtualized resources.