CVE-2021-47376 is a vulnerability identified in the Linux kernel related to the handling of memory allocation within the BPF (Berkeley Packet Filter) subsystem. Specifically, the issue arises from the lack of an oversize check before calling the kvcalloc() function, which is used to allocate zeroed memory arrays. The vulnerability was addressed by adding an oversize check in the kernel's memory management code (commit 7661809d493b), preventing allocations larger than what kmalloc() supports. Without this check, an attacker could potentially trigger oversized allocations leading to kernel warnings or crashes. The vulnerability is rooted in the kernel's memory allocator functions kvmalloc_node(), kvmalloc_array(), and kvcalloc(), which are used during BPF program verification and loading. The trace provided shows the vulnerability manifests during BPF program verification in the kernel's verifier code, which could be exploited by submitting crafted BPF programs. Although no known exploits are currently reported in the wild, the vulnerability could allow denial of service (DoS) conditions by causing kernel warnings or crashes due to improper memory allocation handling. The BPF subsystem is critical for packet filtering, tracing, and security monitoring, making this vulnerability significant in contexts where untrusted users can load BPF programs. The vulnerability affects Linux kernel versions prior to the patch and is relevant for systems running kernel version 5.14.0-syzkaller and similar. The fix involves adding a size check to prevent oversized allocations, thereby improving kernel stability and security during BPF program verification and execution.

For European organizations, this vulnerability poses a risk primarily in environments where unprivileged or semi-privileged users have the ability to load or verify BPF programs, such as multi-tenant cloud infrastructures, containerized environments, or systems using advanced networking and monitoring tools that leverage BPF. Exploitation could lead to denial of service by crashing the kernel or causing instability, impacting availability of critical systems. This could disrupt services, especially in sectors relying heavily on Linux-based infrastructure such as telecommunications, finance, and cloud service providers. While the vulnerability does not appear to allow privilege escalation or remote code execution directly, the resulting DoS could be leveraged as part of a broader attack chain. Additionally, given the widespread use of Linux in European data centers and cloud platforms, unpatched systems could be vulnerable to targeted attacks or accidental crashes caused by malformed BPF programs. The absence of known exploits reduces immediate risk but does not eliminate the potential for future weaponization, especially as BPF usage grows in security and networking applications.

European organizations should prioritize patching Linux kernels to versions that include the fix for CVE-2021-47376. Specifically, update to kernel versions incorporating commit 7661809d493b or later. For environments where immediate patching is challenging, consider restricting the ability to load or verify BPF programs to trusted users only, using Linux capabilities and seccomp filters to limit exposure. Employ kernel security modules (e.g., SELinux, AppArmor) to enforce strict policies around BPF program loading. Monitor kernel logs for warnings related to kvmalloc_node or kvcalloc to detect potential exploitation attempts. In containerized or cloud environments, isolate workloads and limit privileges to reduce the attack surface. Regularly audit and update kernel versions as part of vulnerability management programs. Additionally, educate system administrators about the risks associated with untrusted BPF programs and enforce strict code review and validation processes for any custom BPF code deployed in production.