CVE-2021-47007 is a vulnerability identified in the Linux kernel's f2fs (Flash-Friendly File System) implementation, specifically within the f2fs_resize_fs() function. The vulnerability arises due to a missing check for sufficient space in the resized filesystem before migrating valid data blocks during a filesystem resize operation. When resizing the filesystem to a smaller size, the allocator may run out of space during block migration in the free_segment_range() function, leading to a kernel panic or system hang. The issue manifests under a specific sequence of operations: creating a 16GB f2fs image, mounting it, writing two 8GB files, syncing, deleting one file, syncing again, and then resizing the filesystem to 8GB. The root cause is the failure to verify that the resized filesystem can accommodate all valid blocks from the original size, causing the allocator to exhaust available space and triggering a kernel BUG at segment.c:2484. This vulnerability can cause denial of service (DoS) by crashing or hanging the system during filesystem resize operations. No known exploits are reported in the wild, and the vulnerability requires specific conditions and operations to be triggered, typically involving administrative privileges to perform filesystem resizing. The vulnerability affects Linux kernel versions identified by the commit hash b4b10061ef98c583bcf82a4200703fbaa98c18dc and likely related versions around that commit. No CVSS score is assigned yet, and no patches or exploit code are currently publicly available.

For European organizations, this vulnerability primarily poses a risk of denial of service on systems using the f2fs filesystem, which is optimized for flash storage devices. Organizations relying on Linux servers or embedded systems with f2fs, especially those performing dynamic filesystem resizing, could experience system crashes or hangs, leading to service interruptions. This could impact data availability and operational continuity, particularly in environments with flash-based storage such as IoT devices, edge computing nodes, or specialized storage appliances. While the vulnerability does not directly expose confidentiality or integrity risks, the resulting system instability could disrupt critical services or maintenance operations. The requirement for administrative privileges to trigger the vulnerability limits the risk of remote exploitation but does not eliminate the threat from insider misuse or misconfigured automated processes. European organizations with Linux-based infrastructure should be aware of this vulnerability to avoid unexpected downtime and ensure system reliability.

To mitigate this vulnerability, organizations should: 1) Avoid resizing f2fs filesystems on affected Linux kernel versions until a patch is applied. 2) Monitor Linux kernel updates and apply security patches promptly once available from trusted sources or Linux distributions. 3) Implement strict access controls to limit filesystem resize operations to trusted administrators only. 4) Use alternative filesystems for critical systems where dynamic resizing is required and f2fs is not essential. 5) Test filesystem resize operations in controlled environments before deploying in production to detect potential issues. 6) Maintain regular backups of critical data to enable recovery in case of system crashes caused by this vulnerability. 7) Monitor system logs for kernel panics or unusual filesystem errors that may indicate attempted exploitation or accidental triggering of the vulnerability.