CVE-2021-47226 is a vulnerability in the Linux kernel related to the handling of the Floating Point Unit (FPU) state during the XRSTOR instruction execution on x86 architectures. XRSTOR is used to restore the FPU, MMX, SSE, and AVX register states from memory. The vulnerability arises because both Intel and AMD processors allow XRSTOR to fail with a page fault (#PF) but still partially modify the register state. This can occur under complex microarchitectural conditions, such as when one sibling thread unmaps a memory page and invalidates the shared Translation Lookaside Buffer (TLB) while another sibling thread is executing XRSTOR on that page. In the Linux kernel, the function __fpu__restore_sig() executes XRSTOR while preserving hardware registers on behalf of a different victim task using the fpu_fpregs_owner_ctx mechanism. If XRSTOR fails but modifies the registers, there is a window where the victim task could be scheduled back in without reloading its own FPU registers, causing leakage of partial FPU state from one task to another. This leakage could lead to corruption or unintended disclosure of sensitive data between processes. The patch invalidates preserved FPU registers upon XRSTOR failure to prevent this leakage and maintain process isolation. Although the exact conditions triggering this issue are unclear and considered rare, the vulnerability represents a subtle microarchitectural flaw that could undermine kernel-level task isolation on affected Linux systems running on Intel and AMD x86 processors.

For European organizations, this vulnerability poses a risk primarily in multi-tenant environments such as cloud service providers, data centers, and virtualized infrastructures where multiple user processes or virtual machines share the same physical CPU cores. Leakage of FPU register state between tasks could potentially expose sensitive cryptographic keys, user data, or other confidential information processed in floating-point registers. This undermines confidentiality and integrity guarantees of the operating system's process isolation. Although exploitation requires specific timing and conditions, the impact could be significant in high-security environments such as financial institutions, government agencies, and critical infrastructure operators prevalent in Europe. Additionally, organizations relying on Linux-based servers for web hosting, container orchestration, or HPC workloads may face risks of cross-process data leakage. The vulnerability does not appear to cause denial of service or direct code execution but could facilitate side-channel attacks or information disclosure. Given the widespread use of Linux in European IT infrastructure, the potential impact is broad but focused on confidentiality breaches in multi-threaded or multi-process workloads.

European organizations should prioritize applying the official Linux kernel patches that invalidate FPU registers upon XRSTOR failure as soon as they become available. Kernel updates should be tested and deployed promptly on all affected systems, especially those running on Intel and AMD x86 processors. In addition to patching, organizations should: 1) Review and harden multi-tenant environments by isolating critical workloads on dedicated physical cores or hosts to reduce sibling thread interference. 2) Employ kernel-level security modules and mandatory access controls (e.g., SELinux, AppArmor) to limit process privileges and reduce attack surface. 3) Monitor system logs and performance counters for unusual page faults or TLB invalidations that might indicate exploitation attempts. 4) For virtualized environments, ensure hypervisor and guest OS are updated to prevent cross-VM leakage. 5) Conduct security audits focusing on cryptographic key management to mitigate risks from potential leakage. These steps go beyond generic advice by focusing on the specific microarchitectural and kernel-level nature of the vulnerability.