CVE-2017-7995 in Xen
Summary
by MITRE
Xen PV guest before Xen 4.3 checked access permissions to MMIO ranges only after accessing them, allowing host PCI device space memory reads, leading to information disclosure. This is an error in the get_user function. NOTE: the upstream Xen Project considers versions before 4.5.x to be EOL.
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Analysis
by VulDB Data Team • 09/23/2020
The vulnerability identified as CVE-2017-7995 represents a critical information disclosure flaw in Xen hypervisor virtual private (PV) guests that affected versions prior to 4.3. This issue stems from a fundamental flaw in how the hypervisor handles memory management input/output operations, specifically within the get_user function implementation. The vulnerability occurs in the context of para-virtualized guest operating systems where the host system maintains control over physical hardware resources while providing virtualized access to guest instances. The flaw manifests when the hypervisor performs access permission checks on memory-mapped input/output ranges after the actual memory access has already occurred, creating a window where unauthorized reads can take place.
The technical implementation of this vulnerability involves the improper ordering of privilege checks within the memory access control mechanism. When a PV guest attempts to access PCI device memory space through MMIO operations, the hypervisor's permission validation occurs post-access rather than pre-access. This timing issue allows malicious or compromised guest operating systems to read from host PCI device memory regions that should be restricted. The get_user function, which is responsible for safely copying data from user space to kernel space, fails to enforce proper access controls before executing the memory read operations, enabling information leakage from sensitive hardware registers and device memory spaces. This misordering creates a race condition where the hypervisor's security boundaries are temporarily bypassed during the memory access sequence.
The operational impact of CVE-2017-7995 extends beyond simple information disclosure, as it provides attackers with the ability to extract potentially sensitive data from host hardware components. Attackers with access to a compromised PV guest can leverage this vulnerability to read from PCI device memory spaces, potentially gaining access to cryptographic keys, network card configurations, or other sensitive hardware information that should remain isolated between virtual machines and the host system. This vulnerability directly violates the fundamental security principle of isolation in virtualized environments and can lead to escalation of privileges or further exploitation of other system components. The information disclosed through this vulnerability could include hardware-specific details that aid in crafting more sophisticated attacks against the host system or other virtual machines running on the same hypervisor.
This vulnerability aligns with CWE-129, which addresses improper validation of input ranges, and relates to ATT&CK technique T1059.001 for command and control communications, as the information disclosure can provide attackers with data needed to plan further attacks. The flaw also corresponds to CWE-284, which covers improper access control mechanisms, and represents a failure in the hypervisor's memory management security model. Organizations running Xen hypervisor versions prior to 4.5.x should immediately implement mitigations, as the upstream project has designated these older versions as end-of-life and no longer provide security updates. The recommended remediation involves upgrading to Xen 4.5.x or later versions where the access control mechanisms have been properly restructured to enforce permission checks before memory access operations occur. Additionally, implementing strict hypervisor management policies, monitoring for unusual memory access patterns, and maintaining regular security assessments of virtualized environments can help detect and prevent exploitation of this vulnerability.