CVE-2021-47430 addresses a vulnerability in the Linux kernel related to the handling of the Supervisor Mode Access Prevention (SMAP) feature on x86 architectures. SMAP is a CPU feature designed to prevent the kernel from inadvertently accessing user-space memory, thereby enhancing security by reducing the attack surface for privilege escalation exploits. The vulnerability arises because the kernel did not properly clear the X86_FEATURE_SMAP flag when the kernel configuration option CONFIG_X86_SMAP was disabled. This inconsistency could cause misleading warnings or improper behavior when SMAP is supported by the CPU but disabled in the kernel configuration or at boot time via the "nosmap" parameter. Specifically, the kernel's irqentry_enter_from_user_mode function could trigger warnings or potentially incorrect handling of access control checks due to the presence of the X86_FEATURE_SMAP flag despite SMAP being disabled. The issue was identified through fuzz testing (entry-fuzz + satrandconfig) and resolved by ensuring that the X86_FEATURE_SMAP flag is cleared when CONFIG_X86_SMAP is not enabled, aligning kernel behavior with the actual runtime configuration and boot parameters. Although the vulnerability does not directly describe an exploit or a method for privilege escalation, the improper handling of SMAP flags could potentially lead to kernel instability or security checks being bypassed under specific conditions. No known exploits are reported in the wild, and the vulnerability affects Linux kernel versions around the commit 3c73b81a9164d0c1b6379d6672d2772a9e95168e. This fix improves kernel robustness and consistency in enforcing SMAP-related protections on x86 systems.

For European organizations, the impact of this vulnerability is primarily related to the security and stability of Linux-based systems running on x86 hardware where SMAP is supported but disabled either by kernel configuration or boot parameters. Many European enterprises, government agencies, and critical infrastructure providers rely heavily on Linux servers and workstations. If left unpatched, this vulnerability could cause kernel warnings or unexpected behavior that might be leveraged in complex attack scenarios to bypass certain kernel memory protections, potentially leading to privilege escalation or denial of service. However, since no known exploits exist and the vulnerability requires specific kernel configurations and hardware support, the immediate risk is moderate. Organizations with high-security requirements, such as financial institutions, healthcare providers, and public sector entities, could face increased risk if attackers discover ways to exploit this inconsistency. Additionally, environments using customized or minimal kernel configurations disabling SMAP for compatibility or performance reasons should be particularly vigilant. The vulnerability also highlights the importance of kernel configuration management and consistent security feature enforcement, which is critical for maintaining the integrity and confidentiality of sensitive data processed on Linux systems.

European organizations should ensure that their Linux kernel versions are updated to include the patch that clears the X86_FEATURE_SMAP flag when CONFIG_X86_SMAP is disabled. Specifically, system administrators should: 1) Audit kernel configurations to verify whether SMAP is enabled or intentionally disabled; 2) Avoid disabling SMAP unless absolutely necessary, as it provides an important security boundary; 3) Apply the latest stable Linux kernel updates or backported patches that address CVE-2021-47430; 4) Review boot parameters to ensure that "nosmap" is not set unless required, and understand the security implications if it is; 5) Implement kernel self-tests and fuzz testing tools similar to entry-fuzz to detect anomalies in kernel entry points; 6) Monitor kernel logs for warnings related to irqentry_enter_from_user_mode and investigate any occurrences promptly; 7) For environments with strict security policies, consider enabling additional kernel hardening features and security modules to compensate for any potential gaps; 8) Engage with Linux distribution vendors to confirm that their kernels incorporate this fix and follow best practices for secure kernel configuration management.