CVE-2022-49075 is a vulnerability identified in the Linux kernel's Btrfs filesystem quota group (qgroup) management. The issue arises from an integer overflow in the qgroup reservation logic when handling large fallocate operations. Specifically, the variable extent_changeset->bytes_changed, which tracks the number of bytes reserved for the EXTENT_QGROUP_RESERVED state, is defined as an unsigned int (32-bit). This causes an overflow when attempting to fallocate a file larger than 4 GiB, resulting in an incorrect reservation of bytes and effectively breaking the qgroup limit enforcement. Unlike regular buffered or direct writes, which use multiple changesets capped at 256 MiB each, fallocate uses a single changeset for the entire range, bypassing this limit and triggering the overflow. The vulnerability allows a user to bypass quota restrictions by allocating files larger than the set qgroup limit, potentially leading to uncontrolled disk space usage within a quota-controlled environment. The provided test script demonstrates this by setting a 2 GiB quota limit and successfully fallocating files of 3 GiB and 5 GiB, which should have been blocked. The root cause is the 32-bit limitation of bytes_changed, and the recommended fix is to change this variable to a 64-bit unsigned integer (u64) to properly handle large allocations without overflow. This vulnerability affects Linux kernel versions identified by the given commit hash and is currently published without known exploits in the wild or an assigned CVSS score.

For European organizations, especially those relying on Linux servers with Btrfs filesystems and quota management enabled, this vulnerability could lead to quota enforcement bypass. This means that users or processes could allocate more disk space than allowed, potentially exhausting storage resources unexpectedly. This can disrupt services, cause denial of service due to lack of disk space, and complicate resource management. Organizations with multi-tenant environments, shared hosting, or cloud infrastructure using Btrfs quotas are particularly at risk. The integrity of quota enforcement is critical for operational stability and cost control. While this vulnerability does not directly lead to remote code execution or privilege escalation, the ability to bypass quotas can be leveraged in combination with other vulnerabilities or misconfigurations to impact availability and operational security. Additionally, uncontrolled disk usage can lead to data loss or corruption if the filesystem becomes unstable due to overcommitment.

1. Apply the official Linux kernel patch that changes extent_changeset->bytes_changed from a 32-bit unsigned int to a 64-bit unsigned integer to prevent overflow. 2. Upgrade Linux kernels to versions that include this fix as soon as they become available from trusted distributors. 3. Until patched, monitor disk usage closely on Btrfs filesystems with quota groups enabled, especially for unusually large fallocate operations. 4. Implement stricter access controls to limit which users or processes can perform large fallocate operations or modify quota settings. 5. Use filesystem auditing tools to detect quota limit breaches or anomalous allocation patterns. 6. Consider alternative quota enforcement mechanisms or filesystems if immediate patching is not feasible. 7. Educate system administrators about this vulnerability to ensure prompt detection and response.