CVE-2020-15852 in Linux
Summary
by MITRE
An issue was discovered in the Linux kernel 5.5 through 5.7.9, as used in Xen through 4.13.x for x86 PV guests. An attacker may be granted the I/O port permissions of an unrelated task. This occurs because tss_invalidate_io_bitmap mishandling causes a loss of synchronization between the I/O bitmaps of TSS and Xen, aka CID-cadfad870154.
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Analysis
by VulDB Data Team • 11/05/2020
The vulnerability CVE-2020-15852 represents a critical privilege escalation flaw affecting Linux kernel versions 5.5 through 5.7.9 when used in Xen virtualized environments with x86 PV (Paravirtualized) guests. This issue stems from improper handling of I/O port permissions within the Time Stamp Counter (TSS) mechanism, creating a dangerous synchronization gap between the kernel's TSS I/O bitmap and Xen's hypervisor-level I/O bitmap management. The flaw specifically manifests in the tss_invalidate_io_bitmap function which fails to properly invalidate I/O port permissions, allowing malicious code to exploit this inconsistency and gain unauthorized access to I/O port operations that should be restricted to other processes or tasks.
The technical root cause of this vulnerability lies in the mismatch between kernel-level TSS state management and hypervisor-level I/O bitmap synchronization within Xen's paravirtualized environment. When the kernel attempts to invalidate I/O bitmaps through the tss_invalidate_io_bitmap function, it fails to properly communicate or synchronize with Xen's I/O permission bitmap state, creating a window where an attacker can manipulate I/O port access permissions. This synchronization failure enables an attacker to effectively bypass the normal I/O port access controls that should prevent one process from accessing the I/O resources of another process, resulting in a privilege escalation scenario where guest processes can gain elevated permissions and access restricted I/O ports.
From an operational perspective, this vulnerability poses significant risks to virtualized environments where multiple guest VMs share the same host system, particularly affecting cloud computing platforms and server virtualization deployments. The impact extends beyond simple information disclosure as attackers can potentially access sensitive hardware resources, manipulate device drivers, and perform unauthorized I/O operations that could lead to complete system compromise. The vulnerability affects the fundamental security model of paravirtualized guests, where the isolation between processes should be maintained through proper I/O port permissions, making it particularly dangerous for multi-tenant cloud environments where guest isolation is paramount.
The security implications of CVE-2020-15852 align with CWE-284 Access Control Issues, specifically addressing insufficient access control mechanisms in virtualized environments. This vulnerability also maps to ATT&CK technique T1068, Privilege Escalation through exploitation of kernel vulnerabilities, and T1059, Command and Scripting Interpreter, as attackers could leverage the gained I/O port access to execute malicious code or establish persistence. Organizations utilizing Xen hypervisors with Linux kernel versions 5.5 through 5.7.9 should prioritize immediate patching and implementation of additional monitoring controls to detect unauthorized I/O port access patterns that could indicate exploitation attempts. The mitigation strategy requires updating both the Linux kernel to versions 5.7.10 or later and Xen hypervisor to version 4.13.1 or newer, while also implementing hypervisor-level monitoring for anomalous I/O bitmap synchronization behaviors.
This vulnerability demonstrates the complex security challenges inherent in virtualized environments where multiple abstraction layers must maintain consistent security policies, highlighting the importance of thorough testing and validation of security mechanisms across kernel and hypervisor boundaries. The flaw serves as a reminder that virtualization security models depend heavily on proper synchronization between different security domains, and that even subtle implementation errors in kernel-level state management can have severe consequences for overall system security in virtualized deployments.