CVE-2021-46927 is a vulnerability identified in the Linux kernel related to the nitro_enclaves subsystem, which is used for creating isolated execution environments. The issue arises from the improper use of the get_user_pages() function in conjunction with mmap locking mechanisms. Specifically, after the introduction of the mmap_assert_locked() annotation in commit 5b78ed24e8ec, the call to get_user_pages() triggers an mmap assertion failure because it expects the mmap_lock to be held, which is not the case in the vulnerable code path. This leads to a kernel BUG triggered in mmap_lock.h, causing a kernel panic or crash. The nitro_enclaves code was using get_user_pages() without holding the mmap_lock, violating the locking expectations introduced by the kernel change. The fix involves replacing get_user_pages() with get_user_pages_unlocked() when setting enclave memory regions, aligning with the locking semantics and preventing the assertion failure. This vulnerability can cause denial of service (DoS) by crashing the kernel when the affected ioctl calls are invoked, potentially impacting systems running the vulnerable Linux kernel versions with nitro_enclaves enabled. No known exploits are reported in the wild, and the vulnerability requires local access to invoke the ioctl interface. The vulnerability highlights the importance of correct locking discipline in kernel memory management functions to maintain system stability and security.

For European organizations, the primary impact of CVE-2021-46927 is the potential for local denial of service on Linux systems running vulnerable kernel versions with nitro_enclaves enabled. Nitro Enclaves are primarily used in cloud and virtualization environments to create isolated compute environments, so organizations leveraging such technologies for sensitive workloads could face service disruptions. This could affect cloud service providers, data centers, and enterprises using Linux-based virtualization or container platforms that incorporate nitro_enclaves or similar enclave technologies. The vulnerability does not appear to allow privilege escalation or remote code execution, limiting its impact to availability concerns. However, any kernel panic or crash in production environments can lead to downtime, data loss, or interruption of critical services. Given the widespread use of Linux in European IT infrastructure, especially in cloud and hosting providers, the risk of operational disruption is significant if patches are not applied. Organizations handling sensitive or regulated data must ensure system stability to comply with data protection regulations such as GDPR, making mitigation of this vulnerability important.

1. Apply the official Linux kernel patches that replace get_user_pages() with get_user_pages_unlocked() in the nitro_enclaves code to ensure proper locking semantics and prevent kernel panics. 2. Identify and inventory all Linux systems running kernel versions affected by this vulnerability, particularly those using nitro_enclaves or similar enclave technologies. 3. For systems where immediate patching is not feasible, restrict access to the ioctl interfaces related to nitro_enclaves to trusted administrators only, minimizing the risk of accidental or malicious triggering. 4. Monitor kernel logs for mmap assertion failures or kernel BUG messages indicating attempts to exploit this vulnerability. 5. Coordinate with cloud service providers to confirm that their infrastructure is patched or not affected. 6. Implement robust system monitoring and automated reboot procedures to minimize downtime in case of kernel crashes. 7. Conduct thorough testing of patches in staging environments to ensure compatibility and stability before deployment in production. 8. Educate system administrators about the importance of kernel locking mechanisms and the risks of improper memory management calls in kernel modules.