CVE-2021-37576 in Linux
Summary
by MITRE • 07/27/2021
arch/powerpc/kvm/book3s_rtas.c in the Linux kernel through 5.13.5 on the powerpc platform allows KVM guest OS users to cause host OS memory corruption via rtas_args.nargs, aka CID-f62f3c20647e.
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Analysis
by VulDB Data Team • 08/05/2021
The vulnerability identified as CVE-2021-37576 represents a critical memory corruption flaw within the Linux kernel's KVM hypervisor implementation on PowerPC platforms. This issue specifically affects versions through 5.13.5 and resides in the arch/powerpc/kvm/book3s_rtas.c file which handles Real-Time ASynchronous Services for KVM guests. The vulnerability manifests when KVM guest operating systems attempt to manipulate the rtas_args.nargs parameter, creating a scenario where untrusted input can directly influence host memory structures. The flaw enables a guest user to craft malicious RTAS calls that can overwrite critical host memory regions, potentially leading to arbitrary code execution or system instability.
This vulnerability maps to CWE-121 in the Common Weakness Enumeration catalog, specifically addressing heap-based buffer overflow conditions where insufficient validation of input parameters leads to memory corruption. The technical flaw occurs because the kernel fails to properly validate the nargs field in the rtas_args structure before using it to determine memory allocation sizes for guest-to-host communication buffers. Attackers can exploit this by submitting crafted RTAS calls with malicious nargs values that exceed buffer boundaries or point to arbitrary memory locations within the host kernel space. The root cause lies in inadequate bounds checking and input sanitization within the KVM hypervisor's PowerPC implementation, particularly in how it processes RTAS arguments from untrusted guest environments.
The operational impact of this vulnerability is severe as it allows privilege escalation from guest OS users to host OS level control, effectively breaking the fundamental isolation guarantees that KVM hypervisors are designed to maintain. An attacker with access to a KVM guest can potentially execute arbitrary code on the host system, compromise other virtual machines running on the same host, and ultimately gain control over the entire physical system. This represents a critical compromise of the virtualization security model where guest isolation is breached, enabling potential data exfiltration, system persistence, and further attack vectors against the host infrastructure. The vulnerability affects systems running Linux kernel versions up to 5.13.5, making it particularly concerning for enterprise environments that may still be running older kernel versions.
Mitigation strategies for CVE-2021-37576 include immediate kernel updates to versions 5.13.6 or later where the vulnerability has been patched through proper bounds checking of the rtas_args.nargs parameter. Organizations should also implement virtualization-specific security measures such as disabling unnecessary RTAS calls in guest configurations, monitoring for anomalous RTAS activity patterns, and applying microcode updates to PowerPC processors where applicable. The patch addresses the vulnerability by implementing proper validation of the nargs field before any memory operations are performed, ensuring that guest-provided values cannot exceed predetermined safe limits. Security teams should conduct thorough vulnerability assessments of their virtualization environments and consider implementing additional host-based detection mechanisms to identify potential exploitation attempts. This vulnerability aligns with ATT&CK technique T1059.001 for command and scripting interpreter and T1068 for exploit for privilege escalation, emphasizing the need for comprehensive monitoring and incident response capabilities to detect and respond to such hypervisor-level attacks.