CVE-2021-38166 Overview
CVE-2021-38166 is an integer overflow and out-of-bounds write vulnerability in the Linux kernel's BPF (Berkeley Packet Filter) hashtable implementation. The vulnerability exists in kernel/bpf/hashtab.c and can be triggered when many elements are placed in a single bucket, potentially leading to memory corruption and local privilege escalation.
Critical Impact
Local attackers with limited privileges may exploit this integer overflow to achieve out-of-bounds memory writes, potentially escalating privileges or causing system instability. While exploitation may be impractical without CAP_SYS_ADMIN capability, the vulnerability affects multiple Linux distributions including Fedora and Debian.
Affected Products
- Linux Kernel through version 5.13.8
- Fedora 33 and Fedora 34
- Debian Linux 11.0
Discovery Timeline
- 2021-08-07 - CVE-2021-38166 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2021-38166
Vulnerability Analysis
This vulnerability stems from improper handling of integer arithmetic within the BPF hashtable subsystem of the Linux kernel. The BPF subsystem allows user-space programs to run sandboxed code in kernel space for various purposes including packet filtering, tracing, and performance monitoring. The hashtable implementation in kernel/bpf/hashtab.c contains a flaw where adding numerous elements to a single hash bucket can trigger an integer overflow condition.
When the bucket element count exceeds the maximum value that can be represented by the integer type used, the value wraps around, leading to incorrect bounds calculations. This subsequently enables out-of-bounds memory writes, corrupting adjacent kernel memory regions. The impact includes potential arbitrary code execution in kernel context, privilege escalation, and denial of service through kernel panic.
The vulnerability requires local access to the system, and the official CVE notes indicate that practical exploitation may require the CAP_SYS_ADMIN capability, which limits the attack surface to privileged local users or containers with elevated capabilities.
Root Cause
The root cause is an integer overflow vulnerability (CWE-190) in the BPF hashtable element counting mechanism. When calculating the number of elements in a hash bucket, the code fails to properly validate that the count does not exceed integer bounds before using it in subsequent memory operations. This arithmetic oversight allows attackers to manipulate the bucket state in ways that bypass intended memory safety checks.
Attack Vector
The attack vector is local, requiring an attacker to have user-level access to the target system. The attacker must be able to interact with the BPF subsystem, which may involve creating BPF maps and manipulating their contents. The attack flow involves:
- Creating a BPF hashtable map with specific parameters
- Inserting a large number of elements designed to hash to the same bucket
- Triggering the integer overflow condition through continued element insertion
- Exploiting the resulting out-of-bounds write to corrupt kernel memory
The vulnerability mechanism involves manipulating BPF hashtable bucket operations to trigger integer overflow conditions. When a large number of elements are inserted into a single bucket, the internal counter can overflow, leading to incorrect memory boundary calculations and subsequent out-of-bounds writes. For detailed technical analysis, refer to the BPF Mailing List Discussion and the Linux Kernel BPF Commit.
Detection Methods for CVE-2021-38166
Indicators of Compromise
- Unusual BPF map creation activity with abnormally high element counts
- Kernel log messages indicating memory corruption or BPF subsystem errors
- Unexpected system crashes or kernel panics related to BPF operations
- Suspicious processes attempting to create or manipulate BPF hashtable maps
Detection Strategies
- Monitor for processes invoking BPF system calls with unusual parameters or frequencies
- Implement audit rules to track BPF map creation and manipulation operations
- Deploy kernel integrity monitoring to detect unexpected memory modifications
- Use SentinelOne's behavioral AI to identify anomalous BPF-related activity patterns
Monitoring Recommendations
- Enable kernel audit logging for BPF-related system calls (bpf() syscall)
- Monitor system logs for kernel warnings related to BPF or memory subsystems
- Track processes that request CAP_SYS_ADMIN or CAP_BPF capabilities
- Implement real-time alerting on kernel crash events or BPF-related anomalies
How to Mitigate CVE-2021-38166
Immediate Actions Required
- Update the Linux kernel to a patched version that addresses this vulnerability
- Review and restrict CAP_SYS_ADMIN and CAP_BPF capability grants to only essential processes
- Apply vendor-specific security patches from Fedora or Debian as applicable
- Consider disabling unprivileged BPF access via kernel.unprivileged_bpf_disabled sysctl
Patch Information
The Linux kernel maintainers have released a fix for this vulnerability. The patch is available in the official kernel BPF git repository with commit ID c4eb1f403243fc7bbb7de644db8587c03de36da6. Users should update to kernel versions containing this fix.
Distribution-specific patches are available:
Workarounds
- Disable unprivileged BPF access by setting kernel.unprivileged_bpf_disabled=1
- Restrict access to BPF subsystem through AppArmor or SELinux policies
- Limit CAP_SYS_ADMIN capability grants using capability bounding sets
- Isolate untrusted workloads in containers without BPF capabilities
# Disable unprivileged BPF access (temporary)
sysctl -w kernel.unprivileged_bpf_disabled=1
# Make the change persistent across reboots
echo "kernel.unprivileged_bpf_disabled = 1" >> /etc/sysctl.conf
sysctl -p
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
