CVE-2021-47544 is a vulnerability in the Linux kernel's TCP stack that causes corruption of TCP streams under specific conditions involving the CIFS filesystem and memory-mapped files. The root cause lies in the handling of page fragments (page frags) during page faults triggered by memory-mapped user-space buffers backed by CIFS mounts. When an HTTP request is served by the Apache web server using a CIFS-mounted file that is memory-mapped, a page fault handler initiates an SMB transaction on a different socket within the same process context. This nested socket operation leads to recursive access and modification of the task's page fragment structure (task page frag) without proper protection, resulting in corruption of the TCP stream data being sent. The vulnerability is related to a previously addressed issue (commit 20eb4f29b602) involving recursion in sk_page_frag() during memory reclaim. The Linux kernel's CIFS filesystem sets socket allocations with GFP_NOFS to avoid filesystem recursion, but the nested allocation in this case lacks the __GFP_FS flag, allowing the corruption to occur. The patch involves stricter checks in sk_page_frag() usage to prevent nested allocations that modify the page frag in use. The vulnerability affects Linux kernel versions identified by the commit hash 5640f7685831e088fe6c2e1f863a6805962f8e81 and was published on May 24, 2024. No known exploits are reported in the wild. This vulnerability specifically impacts network communication integrity when using CIFS mounts with memory-mapped files in TCP streams, such as HTTP traffic served by Apache. The technical complexity and specific conditions required limit the attack surface but pose a risk of data corruption and potential denial of service in affected environments.

For European organizations, this vulnerability could disrupt critical services that rely on Linux servers using CIFS mounts with memory-mapped files, particularly those serving HTTP content via Apache. The corruption of TCP streams can lead to data integrity issues, causing application errors, corrupted responses, or dropped connections. This can degrade service availability and reliability, impacting business operations, customer experience, and potentially leading to data loss or retransmission overhead. Organizations using CIFS to integrate Windows file shares with Linux-based web servers or applications are most at risk. Given the prevalence of Linux in European data centers, cloud environments, and enterprise infrastructure, especially in sectors like finance, telecommunications, and public administration, the vulnerability could affect critical infrastructure. However, exploitation requires specific configurations and workloads, somewhat limiting widespread impact. Still, any disruption in web services or internal file sharing could have cascading effects on operational continuity and trust. Additionally, the vulnerability could be leveraged in targeted attacks aiming to cause denial of service or data corruption in sensitive environments.

European organizations should apply the Linux kernel patch that addresses CVE-2021-47544 as soon as it becomes available from their Linux distribution vendors. Until patched, organizations should audit their use of CIFS mounts combined with memory-mapped files, especially in web server contexts like Apache. Where possible, avoid memory-mapping files served over CIFS or disable CIFS mounts for critical HTTP content delivery. Network segmentation and strict access controls should be enforced to limit exposure of vulnerable services. Monitoring for unusual TCP stream errors or application-level corruption can help detect exploitation attempts. Additionally, organizations should review kernel configurations and socket allocation flags to ensure they align with best practices to prevent nested page frag allocations. Testing updates in staging environments before deployment is recommended to avoid service disruptions. Finally, maintain up-to-date backups and incident response plans to mitigate potential impacts from service interruptions or data corruption.