CVE-2017-5973 in QEMU
Summary
by MITRE
The xhci_kick_epctx function in hw/usb/hcd-xhci.c in QEMU (aka Quick Emulator) allows local guest OS privileged users to cause a denial of service (infinite loop and QEMU process crash) via vectors related to control transfer descriptor sequence.
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Analysis
by VulDB Data Team • 11/22/2022
The vulnerability identified as CVE-2017-5973 resides within the QEMU virtual machine emulator's implementation of the USB host controller interface, specifically in the xhci_kick_epctx function located in hw/usb/hcd-xhci.c. This flaw affects the xHCI (eXtensible Host Controller Interface) USB controller implementation that QEMU uses to emulate USB devices for guest operating systems. The vulnerability represents a critical security issue that demonstrates how improper handling of USB control transfer descriptors can lead to system instability and denial of service conditions.
The technical flaw manifests when local guest OS privileged users execute control transfer operations that manipulate descriptor sequences in ways that cause the xhci_kick_epctx function to enter an infinite loop. This occurs due to inadequate validation of control transfer descriptor parameters and improper state management during USB endpoint context processing. The function fails to properly handle certain sequence transitions in the USB control transfer protocol, leading to a condition where the emulator becomes trapped in a loop that consumes excessive CPU resources and eventually causes the QEMU process to crash. This represents a classic example of a resource exhaustion vulnerability that can be exploited by malicious users within the guest environment.
From an operational impact perspective, this vulnerability enables local privilege escalation attacks where a guest OS user with sufficient privileges can cause a denial of service against the entire virtual machine. The infinite loop consumes system resources and can potentially affect other virtual machines running on the same host if they share the same QEMU process or if the host system becomes overwhelmed by resource consumption. The vulnerability is particularly concerning in multi-tenant virtualization environments where guest users might attempt to disrupt services on other virtual machines or compromise the stability of the entire virtualization platform.
The vulnerability aligns with CWE-835, which describes the weakness of infinite loops in software implementations, and can be mapped to ATT&CK technique T1499.001, which covers network denial of service attacks. The attack vector requires local access within the guest operating system and leverages the guest user's ability to manipulate USB control transfers. This makes it particularly dangerous in environments where guest users have elevated privileges or where the virtualization platform is used for testing or development purposes where users might have access to USB device emulation capabilities.
Mitigation strategies for CVE-2017-5973 include applying the official QEMU patch that addresses the descriptor validation issue in the xhci_kick_epctx function, implementing proper bounds checking for USB control transfer descriptors, and restricting guest OS access to USB device emulation when such access is not strictly necessary. Organizations should also consider implementing monitoring solutions to detect anomalous CPU usage patterns that might indicate the exploitation of this vulnerability, as well as ensuring that virtualization platforms are regularly updated with the latest security patches from vendors. Additionally, network segmentation and proper access controls should be implemented to limit the potential impact of such vulnerabilities in multi-tenant environments.