CVE-2016-7421 in QEMU
Summary
by MITRE
The pvscsi_ring_pop_req_descr function in hw/scsi/vmw_pvscsi.c in QEMU (aka Quick Emulator) allows local guest OS administrators to cause a denial of service (infinite loop and QEMU process crash) by leveraging failure to limit process IO loop to the ring size.
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Analysis
by VulDB Data Team • 10/05/2022
The vulnerability identified as CVE-2016-7421 resides within the paravirtualized scsi subsystem of QEMU, specifically in the pvscsi_ring_pop_req_descr function located in hw/scsi/vmw_pvscsi.c. This issue affects virtualized environments where guest operating systems utilize the paravirtualized SCSI controller for storage I/O operations. The vulnerability represents a critical design flaw that enables local guest OS administrators to manipulate the virtualized storage subsystem in a manner that can bring the entire QEMU process to a halt. The root cause lies in the function's failure to properly enforce bounds checking against the ring buffer size, which is a fundamental requirement for preventing buffer overflow conditions and maintaining system stability in virtualized environments.
The technical exploitation of this vulnerability occurs when a malicious guest administrator manipulates the storage I/O operations to create an infinite loop within the ring buffer management logic. Specifically, the pvscsi_ring_pop_req_descr function does not validate that the I/O processing loop remains within the allocated ring size boundaries, allowing an attacker to craft specific I/O requests that cause the function to repeatedly process the same ring descriptors without proper termination conditions. This flaw directly violates the principles of safe buffer management and can be categorized under CWE-129 as "Improper Validation of Array Index" and CWE-131 as "Incorrect Calculation of Buffer Size". The vulnerability operates at the intersection of virtualization security and storage subsystem reliability, where guest-level manipulation can directly impact the host system's stability.
The operational impact of this vulnerability extends beyond simple denial of service, as it can lead to complete system crashes and potential data loss in virtualized environments. When the infinite loop occurs within the QEMU process, it consumes excessive CPU resources and can cause the virtual machine to become unresponsive, effectively rendering the guest OS unusable. This type of attack can be particularly devastating in production environments where multiple virtual machines share the same host resources, potentially causing cascading failures and service disruptions. The vulnerability demonstrates a classic example of how virtualization security flaws can be exploited to compromise system availability, aligning with ATT&CK technique T1499.004 for "Network Denial of Service" and T1566.001 for "Phishing with Malicious Attachments" when considering the potential for exploitation through crafted I/O operations.
Mitigation strategies for this vulnerability require both immediate patching and architectural improvements to prevent similar issues in the future. The primary solution involves implementing proper bounds checking within the pvscsi_ring_pop_req_descr function to ensure that I/O processing loops respect the ring buffer size limitations and include proper termination conditions. System administrators should also implement monitoring solutions to detect anomalous I/O patterns that might indicate exploitation attempts, as well as enforce strict resource limits on virtual machines to prevent single guests from consuming excessive host resources. Additionally, virtualization platforms should consider implementing more robust isolation mechanisms between guest operating systems and hypervisor components, particularly for storage I/O subsystems, to prevent local privilege escalation attacks that could lead to such critical system failures. The vulnerability highlights the importance of thorough security testing for virtualization components and demonstrates the need for comprehensive security assurance processes that validate the resilience of hypervisor subsystems against malicious input manipulation.