CVE-2021-47608 is a vulnerability in the Linux kernel's Berkeley Packet Filter (BPF) subsystem, specifically related to atomic fetch operations on the stack. The issue arises from a bug introduced in commit 37086bfdc737, which modified the handling of stack bounds propagation to registers during atomic fetch operations with BPF_FETCH. The vulnerability allows unprivileged users to leak kernel pointers due to improper type handling of spilled pointers on the stack. Normally, the kernel uses checks in check_mem_access() to prevent such leaks by distinguishing pointer values from scalars and disallowing arithmetic on pointers for unprivileged users. However, the buggy change caused spilled pointer registers to be treated as scalars during atomic fetch/and operations, enabling these pointers to be exported out of the kernel via BPF map values. This pointer leakage can reveal kernel memory layout information, which is valuable for attackers attempting to bypass kernel address space layout randomization (KASLR) and escalate privileges. The fix involves restoring proper memory access checks by first validating reads with a placeholder register value (-1) before performing the actual load, ensuring stack bounds and pointer types are correctly propagated and preventing leakage. No known exploits are reported in the wild as of the publication date. The vulnerability affects Linux kernel versions containing the faulty commit and requires unprivileged user access to the BPF subsystem to exploit. Since BPF is widely used for networking, tracing, and security monitoring, this vulnerability poses a significant risk if left unpatched.

For European organizations, this vulnerability poses a risk primarily to systems running vulnerable Linux kernel versions with BPF enabled, which is common in servers, cloud infrastructure, and embedded devices. Successful exploitation allows unprivileged users to leak kernel pointers, undermining kernel memory protection mechanisms such as KASLR. This information disclosure can facilitate further kernel exploits, potentially leading to privilege escalation and full system compromise. Critical infrastructure, financial institutions, and enterprises relying on Linux-based servers or cloud platforms could face increased risk of targeted attacks leveraging this vulnerability. The impact is heightened in multi-tenant environments like cloud providers or shared hosting, where unprivileged users have access to BPF features. Additionally, organizations using Linux for network appliances or security monitoring tools that rely on BPF may inadvertently expose sensitive kernel information. While no direct denial of service or remote code execution is indicated, the pointer leak significantly lowers the barrier for advanced kernel exploits, increasing the overall threat landscape for European entities.

European organizations should prioritize patching Linux kernels to versions that include the fix for CVE-2021-47608. This involves updating to kernel releases that revert the buggy commit or explicitly address the pointer leakage in BPF atomic fetch operations. System administrators should audit their environments to identify systems running vulnerable kernel versions, especially those exposing BPF functionality to unprivileged users. Where immediate patching is not feasible, consider restricting unprivileged access to BPF via kernel lockdown features or sysctl settings (e.g., disabling unprivileged BPF usage with 'kernel.unprivileged_bpf_disabled=1'). Employ kernel hardening techniques such as enabling KASLR, SELinux/AppArmor policies, and seccomp filters to reduce attack surface. Monitoring BPF map usage and unusual kernel pointer exposures can help detect exploitation attempts. For cloud and multi-tenant environments, enforce strict container and namespace isolation to limit unprivileged user capabilities. Regular vulnerability scanning and compliance checks should include verification of kernel patch levels addressing this issue.