CVE-2016-9377 in Xen
Summary
by MITRE
Xen 4.5.x through 4.7.x on AMD systems without the NRip feature, when emulating instructions that generate software interrupts, allows local HVM guest OS users to cause a denial of service (guest crash) by leveraging IDT entry miscalculation.
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Analysis
by VulDB Data Team • 06/10/2019
The vulnerability identified as CVE-2016-9377 represents a critical denial of service flaw affecting Xen hypervisors version 4.5.x through 4.7.x when operating on AMD systems lacking the NRip feature. This issue specifically manifests during the emulation of software interrupt instructions within hardware virtual machines, creating a scenario where malicious guest operating systems can exploit a fundamental miscalculation in Interrupt Descriptor Table (IDT) entry handling. The flaw resides in the hypervisor's interrupt handling mechanism, which fails to properly account for certain IDT entry calculations when processing software interrupts, leading to system instability and potential guest crashes. The vulnerability is particularly concerning as it affects a wide range of Xen versions and specifically targets AMD systems without the NRip feature, which provides additional protection against such miscalculations. The root cause of this issue can be categorized under CWE-125, which deals with out-of-bounds read vulnerabilities, as the improper IDT entry calculation leads to memory access violations that corrupt system state. From an operational perspective, this vulnerability enables local users within HVM guest operating systems to execute a denial of service attack that results in complete guest system crashes, effectively disrupting service availability and potentially allowing attackers to escalate their control over the virtualized environment. The attack vector requires only local access within the guest OS, making it particularly dangerous as it can be exploited by any user with access to the virtual machine, including potentially compromised or malicious users. This vulnerability directly impacts the fundamental security model of virtualization by allowing guest users to affect the stability of the host system through carefully crafted software interrupt sequences. The implications extend beyond simple service disruption as this flaw can be leveraged as part of broader attack strategies within virtualized environments, potentially enabling more sophisticated exploitation techniques. The vulnerability aligns with ATT&CK technique T1059.007 for Command and Scripting Interpreter, as it allows for command execution within guest contexts that can be used to trigger the denial of service condition, and T1499.004 for Network Denial of Service, since the guest crash can be used to disrupt service availability. The lack of the NRip feature on affected AMD systems means these platforms are more susceptible to the IDT entry miscalculation, as this feature provides additional protection against such interrupt handling errors. The vulnerability demonstrates the complexity of virtualization security, where guest operating system users can potentially compromise host stability through carefully crafted instructions that exploit hypervisor implementation flaws. The attack requires minimal privileges and can be executed by any user within the guest OS, making it a significant concern for multi-tenant virtualization environments where different users share the same physical infrastructure. Organizations running affected Xen versions on AMD hardware must consider immediate mitigation strategies to prevent exploitation, as the vulnerability can be used to create persistent denial of service conditions that impact service availability and system reliability.
The technical implementation of this vulnerability involves the hypervisor's handling of software interrupt instructions in virtualized environments. When guest operating systems execute software interrupt instructions, the hypervisor must properly translate these instructions to the underlying hardware while maintaining proper interrupt descriptor table entries. The miscalculation in IDT entry handling occurs because the hypervisor fails to correctly account for certain boundary conditions when emulating these instructions, particularly in systems without the NRip feature. This flaw specifically affects AMD systems where the virtualization extensions do not include the NRip (No-Read Interrupt Protection) feature, which would normally provide additional safeguards against such miscalculations. The vulnerability demonstrates the importance of proper virtualization boundary enforcement, as guest users can leverage hypervisor implementation details to cause system-level instability. From a security architecture perspective, this represents a failure in the hypervisor's privilege separation model, where guest OS users should not be able to directly impact host system stability through normal operation. The flaw's impact is amplified by the fact that it affects multiple Xen versions simultaneously, indicating a fundamental design issue rather than a simple coding error. The vulnerability requires no special privileges beyond normal guest user access, making it particularly dangerous in shared or multi-tenant environments where guest isolation is critical. The exploitation process involves generating specific software interrupt sequences that trigger the IDT entry miscalculation, leading to memory corruption and system crashes. This vulnerability highlights the challenges of maintaining security boundaries in complex virtualization environments where multiple layers of abstraction must maintain proper isolation while providing efficient resource utilization. The remediation approach for this vulnerability requires either patching the affected Xen versions or implementing system-level mitigations such as disabling problematic interrupt handling or upgrading to versions that include the necessary fixes. Organizations should also consider implementing monitoring for unusual interrupt patterns or guest behavior that might indicate exploitation attempts. The vulnerability's classification under CWE-125 emphasizes the importance of proper bounds checking in virtualization code, while its alignment with ATT&CK techniques demonstrates the broader security implications of hypervisor-level flaws that can be exploited for denial of service and potentially more advanced attacks.