CVE-2020-10767: Linux Kernel Privilege Escalation Flaw

CVE-2020-10767 Overview

A flaw was found in the Linux kernel before version 5.8-rc1 in the implementation of the Enhanced IBPB (Indirect Branch Prediction Barrier). The IBPB mitigation will be disabled when STIBP is not available or when the Enhanced Indirect Branch Restricted Speculation (IBRS) is available. This flaw allows a local attacker to perform a Spectre V2 style attack when this configuration is active. The highest threat from this vulnerability is to confidentiality.

Critical Impact

Local attackers can exploit this Enhanced IBPB implementation flaw to perform Spectre V2 side-channel attacks, potentially leaking sensitive data from kernel memory or other processes on systems where STIBP is unavailable or Enhanced IBRS is enabled.

Affected Products

• Linux Kernel versions before 5.8-rc1
• Systems where STIBP is not available
• Systems where Enhanced IBRS is enabled

Discovery Timeline

• 2020-09-15 - CVE-2020-10767 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-10767

Vulnerability Analysis

This vulnerability exists in the Linux kernel's implementation of Enhanced IBPB, which is a hardware mitigation designed to protect against Spectre V2 (branch target injection) attacks. The flaw occurs in the logic that determines when IBPB mitigations should be applied.

The issue stems from improper conditional logic that disables IBPB protections under two specific scenarios: when STIBP (Single Thread Indirect Branch Predictors) is not available on the system, or when Enhanced IBRS (Indirect Branch Restricted Speculation) is available. This creates a security gap where the intended Spectre V2 mitigations are not properly enforced.

When exploited, an attacker with local access can leverage speculative execution to read sensitive data from kernel memory or other processes running on the same system. The attack exploits the CPU's branch prediction mechanisms to speculatively execute instructions that access privileged memory, then uses side-channel techniques to extract the data before the speculative execution is rolled back.

Root Cause

The root cause of this vulnerability is a logic flaw in the Enhanced IBPB implementation within the Linux kernel. Specifically, the code incorrectly assumes that when Enhanced IBRS is available, IBPB protections are unnecessary, or that systems without STIBP support do not require IBPB mitigations. This assumption is flawed because IBPB provides complementary protections that are still needed even when other mitigations are present or absent.

The vulnerability is classified under CWE-440 (Expected Behavior Violation), as the mitigation mechanism does not behave as expected under certain hardware configurations, leaving systems vulnerable to speculative execution attacks.

Attack Vector

The attack requires local access to the vulnerable system. An attacker must be able to execute code on the target machine, either as an unprivileged user or through a compromised application. The attack exploits the disabled IBPB mitigations to perform Spectre V2 style branch target injection attacks.

The attacker can craft malicious code that manipulates branch prediction to speculatively execute privileged memory reads. By carefully timing cache access patterns, the attacker can infer the values read during speculative execution, effectively bypassing memory isolation protections. This technique can be used to leak sensitive information such as cryptographic keys, passwords, or other confidential data residing in kernel memory or other processes.

Detection Methods for CVE-2020-10767

Indicators of Compromise

• Unusual CPU performance counter activity related to speculative execution
• Suspicious local processes repeatedly accessing timing-sensitive operations
• Unexpected cache timing variations that may indicate side-channel attacks
• Processes attempting to probe kernel memory addresses through timing analysis

Detection Strategies

• Monitor for processes exhibiting unusual memory access patterns combined with high-precision timing measurements
• Implement kernel auditing to detect attempts to exploit speculative execution vulnerabilities
• Use hardware performance counters to identify anomalous branch prediction behavior
• Deploy endpoint detection solutions that can identify known Spectre attack patterns

Monitoring Recommendations

• Enable kernel auditing and logging of security-relevant system calls
• Monitor system logs for signs of privilege escalation attempts
• Implement continuous vulnerability scanning to identify unpatched systems
• Use SentinelOne's behavioral AI to detect anomalous process behavior indicative of side-channel attacks

How to Mitigate CVE-2020-10767

Immediate Actions Required

• Update the Linux kernel to version 5.8-rc1 or later, which contains the fix for this vulnerability
• Review system hardware configuration to understand STIBP and Enhanced IBRS availability
• Apply vendor-specific kernel patches from distribution maintainers (Red Hat, Ubuntu, SUSE, etc.)
• Consider enabling additional Spectre mitigations at the boot level if immediate patching is not possible

Patch Information

The vulnerability has been addressed in the Linux kernel commit 21998a351512eba4ed5969006f0c55882d995ada. This fix corrects the Enhanced IBPB implementation to ensure proper mitigation enforcement regardless of STIBP availability or Enhanced IBRS status. The patch is included in Linux kernel version 5.8-rc1 and has been backported to various distribution kernels.

For detailed patch information, refer to the Linux Kernel Commit and the Red Hat Bug Report.

Workarounds

• If immediate patching is not possible, consider enabling manual Spectre mitigations via kernel boot parameters
• Restrict local access to systems handling sensitive data to reduce the attack surface
• Implement process isolation using containers or VMs with separate kernel instances
• Monitor for and terminate suspicious processes that may be attempting side-channel attacks

# Check current Spectre mitigation status
cat /sys/devices/system/cpu/vulnerabilities/spectre_v2

# Verify kernel version
uname -r

# Review IBPB and STIBP status in kernel logs
dmesg | grep -i "spectre\|ibpb\|stibp\|ibrs"