CVE-2021-29155

What it actually does

The Linux kernel BPF verifier in kernel/bpf/verifier.c fails to correctly account for pointer modifications from initial arithmetic operations when validating subsequent operations, enabling out-of-bounds speculative execution that bypasses Spectre mitigations and leaks kernel memory via side channels.

Why it matters to the business

Local attackers on affected Linux systems can extract sensitive kernel data, increasing risk of credential theft or privilege escalation on servers, containers, and endpoints; widespread kernel prevalence amplifies exposure across distributions despite low exploitation signals.

How an attack unfolds

An attacker compiles and loads a malicious BPF program containing crafted sequences of pointer arithmetic operations that trigger speculative out-of-bounds accesses; the speculative execution leaks kernel memory bytes through cache side channels, allowing reconstruction of sensitive data without triggering standard mitigations.

How to fix it

Upgrade the Linux kernel to a version containing commit 073815b756c51ba9d8384d924c5d1c03ca3d1ae4 or later; rebuild and reboot with the patched kernel; apply vendor-specific backports where available for older distribution releases.

How to confirm the fix worked

Confirm the running kernel includes commit 073815b756c51ba9d8384d924c5d1c03ca3d1ae4 via git log or version string; test with known BPF programs that previously triggered the speculation; verify Spectre-BPF mitigations are active post-upgrade.

Typical attack path

1. Obtain local user access on a vulnerable Linux system
2. Craft and load a BPF program with specific pointer arithmetic sequences via bpf() syscall
3. Trigger speculative execution paths in the verifier
4. Measure cache timing or other side-channel artifacts to exfiltrate kernel memory contents
5. Repeat to reconstruct targeted sensitive information

How to detect it

Review system logs for unusual BPF program loads or verifier warnings; inspect for anomalous cache side-channel activity or unexplained kernel memory access patterns; correlate with process execution of bpftool or custom loaders in high-privilege contexts.

What your security team should do

• Prioritize kernel updates on internet-facing and multi-tenant systems within standard patching cycles
• Review and restrict unprivileged BPF program loading via sysctl or LSM policies where feasible
• Monitor upstream kernel changelogs for related verifier hardening
• Maintain inventory of Linux kernel versions across the fleet to track exposure