CVE-2020-27170: Linux Kernel Information Disclosure Flaw

CVE-2020-27170 Overview

An issue was discovered in the Linux kernel before 5.11.8 where kernel/bpf/verifier.c performs undesirable out-of-bounds speculation on pointer arithmetic. This vulnerability leads to side-channel attacks that defeat Spectre mitigations and obtain sensitive information from kernel memory. The flaw affects pointer types that do not define a ptr_limit, allowing attackers to bypass existing Spectre protections and leak confidential kernel data through timing-based side channels.

Critical Impact

This vulnerability enables local attackers with low privileges to bypass Spectre mitigations and extract sensitive kernel memory contents through side-channel attacks, potentially compromising system confidentiality.

Affected Products

• Linux Kernel (versions before 5.11.8)
• Fedora 32, 33, and 34
• Canonical Ubuntu Linux 14.04 ESM, 16.04 LTS, 18.04 LTS, 20.04 LTS
• Debian Linux 9.0

Discovery Timeline

• March 20, 2021 - CVE-2020-27170 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-27170

Vulnerability Analysis

This vulnerability exists within the BPF (Berkeley Packet Filter) verifier component of the Linux kernel, specifically in kernel/bpf/verifier.c. The BPF verifier is responsible for ensuring that BPF programs loaded into the kernel are safe to execute and do not violate security boundaries. However, the verifier fails to properly account for speculative execution when performing pointer arithmetic on certain pointer types.

The core issue stems from how the verifier handles pointer types that do not have a defined ptr_limit. When performing arithmetic operations on these pointers, the verifier allows out-of-bounds speculation to occur during speculative execution paths. This creates a timing-based side channel that attackers can exploit to infer the contents of kernel memory locations that should be inaccessible.

By crafting malicious BPF programs that leverage this speculative execution behavior, a local attacker can systematically leak sensitive information from kernel memory. This effectively defeats the Spectre mitigations that were implemented to prevent such speculative execution attacks, representing a significant regression in kernel security.

Root Cause

The root cause lies in the incomplete Spectre mitigation within the BPF verifier. Specifically, pointer types without a defined ptr_limit were not properly constrained during the verification process, allowing speculative out-of-bounds memory accesses. The verifier's analysis did not account for the microarchitectural side effects of speculative execution, permitting BPF programs to construct timing channels that leak memory contents. The vulnerable code path is identified by commit ID f232326f6966cf2a1d1db7bc917a4ce5f9f55f76.

Attack Vector

This is a local attack vector requiring an authenticated user with low privileges on the target system. The attacker must have the ability to load BPF programs into the kernel, which is typically available to unprivileged users on many Linux distributions. The attack follows this general pattern:

1. The attacker crafts a malicious BPF program that performs pointer arithmetic on pointer types lacking ptr_limit definitions
2. The BPF verifier accepts the program as safe, failing to detect the speculative execution vulnerability
3. During execution, the CPU speculatively accesses out-of-bounds kernel memory
4. The attacker uses cache timing measurements to infer the values of speculatively accessed memory
5. By repeating this process, the attacker can systematically extract sensitive kernel data

The attack requires no user interaction and the complexity is considered high due to the need for precise timing measurements and understanding of the CPU's speculative execution behavior. For detailed technical information about the vulnerability mechanism, refer to the Openwall OSS Security Notice and the Linux Kernel Commit Details.

Detection Methods for CVE-2020-27170

Indicators of Compromise

• Unusual BPF program loading activity from non-administrative users
• Anomalous cache timing patterns that may indicate side-channel exploitation attempts
• Unexpected access patterns to the BPF subsystem, particularly from processes that don't typically use BPF
• Evidence of BPF programs performing unusual pointer arithmetic operations

Detection Strategies

• Monitor bpf() system calls and log BPF program loading events using audit frameworks
• Implement kernel audit rules to track access to BPF-related operations: auditctl -a always,exit -F arch=b64 -S bpf -k bpf_activity
• Deploy runtime security monitoring to detect anomalous BPF program behavior
• Use SentinelOne's kernel-level monitoring to detect suspicious BPF activity and speculative execution exploitation attempts

Monitoring Recommendations

• Enable detailed logging of BPF program loads and rejections in kernel logs
• Configure alerts for unexpected BPF activity from unprivileged user contexts
• Implement baseline monitoring of cache timing patterns to detect potential side-channel attacks
• Review system logs for indicators of repeated BPF program loading attempts

How to Mitigate CVE-2020-27170

Immediate Actions Required

• Update the Linux kernel to version 5.11.8 or later immediately
• Restrict BPF program loading to privileged users only by setting kernel.unprivileged_bpf_disabled=1
• Apply vendor-provided security patches from your Linux distribution
• Review system access to identify potentially compromised sensitive data

Patch Information

The vulnerability was addressed in Linux kernel version 5.11.8 through commit f232326f6966cf2a1d1db7bc917a4ce5f9f55f76. The fix implements proper bounds checking for pointer types in the BPF verifier to prevent speculative out-of-bounds access. Detailed patch information is available in the Kernel ChangeLog for v5.11.8.

Distribution-specific patches are available from:

• Debian LTS Security Announcement
• Fedora Package Announcements
• Kernel Live Patch Security Notice LSN-0075-1

Workarounds

• Disable unprivileged BPF by setting kernel.unprivileged_bpf_disabled=1 in sysctl configuration
• Restrict access to the BPF subsystem using SELinux or AppArmor policies
• Limit user access to systems running vulnerable kernels until patching is completed
• Consider using kernel live patching if available to minimize downtime during remediation

# Configuration example
# Disable unprivileged BPF to mitigate the vulnerability
echo "kernel.unprivileged_bpf_disabled=1" >> /etc/sysctl.conf
sysctl -p

# Verify the setting is applied
sysctl kernel.unprivileged_bpf_disabled