CVE-2013-6375 in Xen
Summary
by MITRE
Xen 4.2.x and 4.3.x, when using Intel VT-d for PCI passthrough, does not properly flush the TLB after clearing a present translation table entry, which allows local guest administrators to cause a denial of service or gain privileges via unspecified vectors related to an "inverted boolean parameter."
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Analysis
by VulDB Data Team • 06/02/2021
The vulnerability identified as CVE-2013-6375 affects Xen hypervisor versions 4.2.x and 4.3.x when utilizing Intel VT-d for PCI passthrough functionality. This issue represents a critical flaw in the memory management subsystem that directly impacts the security and stability of virtualized environments. The vulnerability stems from improper handling of Translation Lookaside Buffer (TLB) management within the hypervisor's virtualization layer, specifically when dealing with PCI device passthrough operations that rely on Intel's Virtualization Technology for Directed I/O.
The technical root cause involves a flaw in the TLB flushing mechanism that occurs when the hypervisor clears present translation table entries. During PCI passthrough operations, the hypervisor maintains translation tables that map guest physical addresses to host physical addresses. When these translation entries are cleared or modified, the hypervisor must flush the TLB to ensure that subsequent memory accesses use the updated mappings. However, in affected versions, the hypervisor fails to properly flush the TLB after clearing translation table entries, creating a race condition that can be exploited by malicious actors.
This vulnerability manifests through what is described as an "inverted boolean parameter" which fundamentally alters the control flow logic within the hypervisor's memory management code. The inverted parameter likely affects the conditional logic that determines when TLB flushing should occur, causing the system to skip necessary flushing operations when translation table entries are cleared. This creates a scenario where stale translation entries remain in the TLB cache, potentially allowing unauthorized memory access patterns.
The operational impact of this vulnerability extends beyond simple denial of service to potentially enable privilege escalation attacks. Local guest administrators with access to virtual machines can exploit this flaw to cause system instability through denial of service conditions or more severely compromise system security by gaining elevated privileges. The vulnerability is particularly dangerous in multi-tenant cloud environments where guest administrators might attempt to exploit this weakness to attack other virtual machines or the host system itself.
From a cybersecurity perspective, this vulnerability aligns with CWE-128, which addresses "Wrap-around or Wrap-under Error" and CWE-119, which covers "Improper Access Control". The attack vector follows patterns consistent with ATT&CK technique T1068, "Exploitation for Privilege Escalation," and T1499, "Endpoint Denial of Service." The vulnerability demonstrates how hypervisor-level flaws can create cascading security issues that affect the entire virtualization infrastructure.
Mitigation strategies for this vulnerability require immediate patching of Xen hypervisor installations to versions that properly address the TLB flushing logic. Organizations should also implement additional monitoring for unusual memory access patterns and TLB activity that might indicate exploitation attempts. Virtualization administrators should consider implementing stricter access controls for guest administrators and regularly audit PCI passthrough configurations to minimize exposure. The fix typically involves correcting the boolean parameter logic that controls TLB flushing operations and ensuring that all translation table modifications trigger proper TLB invalidation across all relevant CPU cores and memory domains.