CVE-2021-47300 is a vulnerability identified in the Linux kernel's eBPF (extended Berkeley Packet Filter) subsystem, specifically related to the handling of tail calls within BPF subprograms when the Just-In-Time (JIT) compiler fails. The eBPF subsystem allows for running sandboxed programs in the kernel, commonly used for networking, tracing, and security purposes. The vulnerability arises from a logic flaw in the kernel's verification and execution of BPF programs. When the JIT compiler fails to compile subprograms, the kernel falls back to interpreting the BPF bytecode. However, due to a bug in the tail call reachability tracking, the kernel incorrectly rejects certain programs or mishandles tail calls, leading to a use-after-free condition and slab-out-of-bounds memory access. This can cause kernel crashes (freezes) or potentially allow an attacker to execute arbitrary code within the kernel context. The root cause is tied to improper propagation of the tail_call_reachable flag across subprograms during verification and execution, introduced in commits related to tail call handling and JIT prologue/epilogue rework. The vulnerability manifests under specific failure conditions of the JIT compiler and affects Linux kernel versions containing the faulty commits. While no public exploits are known, the vulnerability could be triggered by maliciously crafted BPF programs, which are often loaded by privileged users or processes with BPF capabilities. The detailed kernel logs indicate a KASAN (Kernel Address Sanitizer) detected out-of-bounds read, confirming memory safety violations. This vulnerability is significant because eBPF is widely used in modern Linux systems for performance monitoring, security enforcement, and networking, making the kernel susceptible to stability and security issues if exploited.

For European organizations, the impact of CVE-2021-47300 can be substantial, especially for those relying on Linux-based infrastructure for critical services such as cloud computing, telecommunications, financial services, and industrial control systems. Exploitation could lead to kernel panics causing denial of service (DoS), disrupting business operations and service availability. More critically, if leveraged for arbitrary code execution, attackers could gain kernel-level privileges, bypassing security controls, potentially leading to full system compromise, data breaches, or lateral movement within networks. Given the prevalence of Linux in servers, containers, and embedded devices across Europe, this vulnerability poses a risk to a broad range of sectors. Organizations using eBPF-based security tools or network functions could be particularly exposed. The lack of known exploits reduces immediate risk but does not eliminate the threat, as attackers may develop exploits once the vulnerability details are public. The complexity of the vulnerability and requirement for crafted BPF programs suggest that exploitation may require privileged access or sophisticated attack vectors, somewhat limiting exposure to external attackers but increasing risk from insider threats or compromised accounts.

1. Apply kernel updates: Organizations should promptly apply Linux kernel patches that address CVE-2021-47300 once available from their distribution vendors. Since this vulnerability is in the kernel, updating to a fixed kernel version is the most effective mitigation. 2. Restrict BPF usage: Limit the ability to load and run BPF programs to trusted users and processes only. Use Linux capabilities (CAP_BPF and CAP_SYS_ADMIN) judiciously to reduce attack surface. 3. Harden container environments: For containerized workloads, ensure that container runtimes and orchestrators restrict BPF program loading and usage, preventing untrusted containers from exploiting this vulnerability. 4. Monitor kernel logs: Enable and monitor kernel logs for signs of unusual BPF activity or kernel crashes that may indicate exploitation attempts. 5. Use security modules: Employ Linux Security Modules (LSMs) like SELinux or AppArmor to enforce policies restricting BPF program loading and execution. 6. Network segmentation: Isolate critical Linux hosts running eBPF programs to limit exposure to potentially compromised systems. 7. Incident response readiness: Prepare for potential exploitation by having incident response plans that include kernel-level compromise scenarios and forensic capabilities to analyze kernel memory and logs.