CVE-2020-27777: Linux Kernel Privilege Escalation Flaw

CVE-2020-27777 Overview

A flaw was found in the way RTAS (Run-Time Abstraction Services) handled memory accesses in userspace to kernel communication within the Linux kernel. On a locked down guest system (usually due to Secure Boot) running on top of PowerVM or KVM hypervisors (pseries platform), a root-like local user could exploit this vulnerability to further increase their privileges to that of a running kernel.

Critical Impact

This privilege escalation vulnerability allows a local attacker with root privileges on locked-down PowerPC guest systems to bypass Secure Boot protections and gain kernel-level privileges, potentially compromising the entire system's security posture.

Affected Products

• Linux Linux Kernel (multiple versions)
• Red Hat OpenShift Container Platform 4.4, 4.5, 4.6
• Red Hat Enterprise Linux 5.0, 6.0, 7.0, 8.0

Discovery Timeline

• 2020-12-15 - CVE-2020-27777 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-27777

Vulnerability Analysis

This vulnerability exists in the Linux kernel's implementation of RTAS on PowerPC pseries platforms. RTAS provides firmware services to the operating system and serves as an abstraction layer between the kernel and platform firmware. The flaw stems from missing authorization checks (CWE-862) when handling memory accesses during userspace to kernel communication.

On systems with kernel lockdown enabled (typically enforced when Secure Boot is active), the kernel should prevent privileged users from modifying kernel memory or executing arbitrary code. However, this vulnerability allows bypassing these restrictions through improper handling of RTAS calls.

The attack requires local access with high privileges (root or equivalent), making it a local privilege escalation vector. An attacker who has already compromised a guest system with root access can leverage this flaw to escalate to kernel-level privileges, effectively defeating Secure Boot protections.

Root Cause

The root cause is missing authorization checks (CWE-862) in the RTAS interface implementation for PowerPC pseries platforms. The kernel failed to properly restrict memory access operations through RTAS calls, allowing privileged userspace processes to interact with kernel memory in ways that should be blocked under kernel lockdown mode.

Attack Vector

The attack requires local access to a PowerPC pseries guest system running on PowerVM or KVM hypervisors. The attacker must already possess root-level privileges on the guest system. By exploiting the improper memory access handling in RTAS, the attacker can manipulate kernel memory and bypass the kernel lockdown restrictions, effectively escalating from root privileges to kernel-level access.

This vulnerability is particularly concerning in multi-tenant virtualized environments where guest isolation and Secure Boot are relied upon as security boundaries. An attacker with root access in a guest could potentially compromise the hypervisor or other guests.

Detection Methods for CVE-2020-27777

Indicators of Compromise

• Unexpected RTAS calls originating from userspace processes on PowerPC pseries systems
• Unusual kernel memory access patterns detected in system logs
• Modifications to kernel code or data structures on locked-down systems
• Secure Boot bypass indicators or integrity check failures

Detection Strategies

• Monitor for suspicious RTAS-related system calls on PowerPC pseries platforms
• Implement kernel integrity monitoring to detect unauthorized modifications
• Review system logs for signs of privilege escalation attempts from root users
• Deploy endpoint detection solutions capable of monitoring kernel-level activities

Monitoring Recommendations

• Enable comprehensive audit logging for privileged user activities on affected systems
• Implement real-time monitoring of RTAS interface usage
• Configure alerting for any attempts to modify kernel memory on locked-down systems
• Utilize SentinelOne's behavioral AI to detect anomalous privilege escalation patterns

How to Mitigate CVE-2020-27777

Immediate Actions Required

• Apply the kernel patch from the Kernel Commit for PowerPC
• Update to patched versions of Red Hat Enterprise Linux and OpenShift Container Platform
• Review and restrict root access on PowerPC pseries guest systems
• Monitor for exploitation attempts while patching is in progress

Patch Information

The vulnerability has been addressed through a kernel patch available in the PowerPC Linux tree. The fix implements proper authorization checks for RTAS memory access operations, preventing userspace processes from bypassing kernel lockdown restrictions.

For Red Hat systems, refer to the Red Hat Bug Report for specific package updates. Additional technical details are available in the OpenWall OSS Security Notice.

Workarounds

• Restrict root access to only trusted administrators on affected PowerPC pseries guest systems
• Implement strict access controls and privilege separation to limit potential attack surface
• Consider enabling additional kernel hardening options where available
• Monitor RTAS interface usage and implement detection rules for suspicious activity

# Verify kernel version and check for patch status
uname -r

# Check if system is running on pseries platform
cat /proc/cpuinfo | grep -i platform

# Review kernel lockdown status
cat /sys/kernel/security/lockdown