CVE-2017-5856 in QEMU
Summary
by MITRE
Memory leak in the megasas_handle_dcmd function in hw/scsi/megasas.c in QEMU (aka Quick Emulator) allows local guest OS privileged users to cause a denial of service (host memory consumption) via MegaRAID Firmware Interface (MFI) commands with the sglist size set to a value over 2 Gb.
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Analysis
by VulDB Data Team • 09/09/2020
The vulnerability identified as CVE-2017-5856 represents a critical memory management flaw within the QEMU virtualization platform that specifically affects the megasas storage controller implementation. This issue resides in the hw/scsi/megasas.c file where the megasas_handle_dcmd function processes MegaRAID Firmware Interface commands. The vulnerability manifests when guest operating systems with privileged access execute MFI commands with excessively large scatter-gather list (sglist) sizes exceeding 2 gigabytes. The flaw constitutes a classic memory leak scenario where allocated host memory resources are not properly released back to the system, creating a gradual consumption of available memory. This type of vulnerability directly impacts the stability and resource availability of the host system running QEMU virtual machines.
The technical implementation of this vulnerability stems from inadequate input validation and memory management within the storage controller emulation layer. When a guest OS issues an MFI command with a sglist size parameter exceeding the 2 gigabyte threshold, the megasas_handle_dcmd function fails to properly validate or limit the memory allocation request. This allows malicious or compromised guest users to continuously request memory allocations that exceed normal operational parameters, leading to progressive memory exhaustion on the host system. The vulnerability operates at the hypervisor level where guest privileges are elevated to execute storage commands, making it particularly dangerous as it can be exploited by users who already have access to the guest operating system. This represents a privilege escalation scenario where local guest users can leverage their access to cause remote denial of service conditions on the host infrastructure.
The operational impact of CVE-2017-5856 extends beyond simple resource exhaustion to encompass broader system stability and availability concerns. Host memory consumption increases progressively as the vulnerability is exploited, potentially leading to system crashes, application failures, or complete system hang conditions. The memory leak affects not only the virtualization platform but also impacts other services running on the same host system that may be competing for the same memory resources. This vulnerability particularly affects virtualized environments where multiple guest operating systems share the same physical host infrastructure, creating a cascading effect where one compromised guest can impact the availability of services for all other guests. The vulnerability aligns with CWE-401, which categorizes memory leaks as a fundamental weakness in software design that can lead to resource exhaustion and system instability. From an adversarial perspective, this vulnerability maps to ATT&CK technique T1499.001 which involves resource exhaustion attacks targeting system availability.
Mitigation strategies for CVE-2017-5856 should focus on both immediate patching and operational hardening measures. The most effective solution involves applying the official QEMU patches that implement proper input validation and memory allocation limits for sglist sizes in the megasas storage controller implementation. System administrators should also implement monitoring solutions to detect unusual memory consumption patterns and set up automated alerts when memory usage exceeds normal thresholds. Virtual machine configuration should include limiting guest access to storage commands and implementing proper resource quotas that prevent any single guest from consuming excessive host memory. Network segmentation and access controls should be implemented to minimize the attack surface where guest users can potentially exploit storage controller interfaces. Additionally, regular security audits of virtualization environments should include checks for similar memory management flaws in other emulated hardware components to prevent analogous vulnerabilities from being present in the broader system infrastructure.