CVE-2016-7156 in QEMU
Summary
by MITRE
The pvscsi_convert_sglist 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 an incorrect cast.
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Analysis
by VulDB Data Team • 10/05/2022
The pvscsi_convert_sglist function in QEMU's VMware paravirtualized SCSI controller implementation contains a critical vulnerability that enables local guest OS administrators to trigger a denial of service condition. This flaw exists within the hw/scsi/vmw_pvscsi.c source file and represents a classic case of improper type handling that can be exploited to crash the hypervisor process. The vulnerability specifically manifests when the function processes scatter-gather lists, which are essential data structures used for managing I/O operations between guest operating systems and virtualized storage devices.
The technical root cause of this vulnerability stems from an incorrect cast operation within the pvscsi_convert_sglist function that fails to properly validate or handle the data type conversions required for scatter-gather list processing. When a malicious guest administrator manipulates the input parameters to this function, the improper cast causes the execution flow to enter an infinite loop, consuming excessive CPU resources and ultimately leading to the complete crash of the QEMU process. This occurs because the function does not properly validate the bounds or types of the scatter-gather list entries before performing the cast operation, creating a condition where the loop counter or iteration variables become corrupted and fail to progress toward termination.
From an operational perspective, this vulnerability presents a significant risk to virtualized environments as it allows local guest users with administrative privileges to perform a denial of service attack against the host system. The impact extends beyond simple service interruption since a crashed QEMU process can lead to complete virtual machine shutdowns, potential data loss, and disruption of critical services running within the virtualized infrastructure. The vulnerability is particularly concerning because it requires only local administrative access within the guest operating system, making it accessible to any user with sufficient privileges to manipulate the SCSI subsystem. This low attack vector combined with the high impact of a system crash makes this vulnerability especially dangerous in multi-tenant cloud environments where guest isolation is critical.
The vulnerability aligns with CWE-191, which describes an integer underflow or overflow condition, and can be mapped to ATT&CK technique T1499.001, which covers network denial of service attacks through resource exhaustion. Organizations using QEMU-based virtualization platforms should prioritize immediate patching of this vulnerability, as the fix typically involves correcting the type casting logic to ensure proper bounds checking and validation of scatter-gather list parameters. Additionally, implementing proper input validation and boundary checks within the SCSI controller implementation would prevent similar issues from occurring in other parts of the virtualization stack. The recommended mitigation strategy includes applying the latest QEMU security patches, monitoring for unusual CPU consumption patterns that might indicate exploitation attempts, and ensuring proper guest isolation mechanisms are in place to limit the potential impact of local privilege escalation within virtualized environments.