CVE-2021-47191 is a vulnerability identified in the Linux kernel's SCSI debug driver (scsi_debug), specifically in the resp_readcap16() function. The issue arises due to an out-of-bounds (OOB) read caused by improper handling of the alloc_len variable, which is incorrectly defined as a signed integer (int) rather than an unsigned 32-bit integer (u32). This misdefinition leads to a negative or extremely large value being interpreted as the length of data to be copied, resulting in an out-of-bounds read in the sg_copy_buffer() function. The vulnerability was discovered through fuzz testing with syzkaller, which generated kernel warnings and KASAN (Kernel Address Sanitizer) reports indicating stack-out-of-bounds reads. The root cause is that resp_readcap16() calculates an alloc_len value that can be negative (e.g., -1104926854), which is then passed as a large unsigned value to fill_from_dev_buffer(), causing it to read beyond the allocated buffer size (only 32 bytes). This can lead to kernel memory corruption or information disclosure. The vulnerability affects the Linux kernel versions containing the faulty commit (1da177e4c3f41524e886b7f1b8a0c1fc7321cac2) and was publicly disclosed in April 2024. The fix involves redefining alloc_len as an unsigned 32-bit integer to prevent negative values and ensure proper bounds checking. No known exploits are reported in the wild, and the vulnerability requires local code execution or kernel interaction, typically through the scsi_debug driver interface, which is often used for testing and development rather than production environments.

For European organizations, the impact of CVE-2021-47191 depends largely on the deployment of vulnerable Linux kernels with the scsi_debug driver enabled. While scsi_debug is primarily a testing and development driver, some environments may have it enabled for diagnostic or virtualization purposes. Exploitation could lead to kernel memory corruption, causing system instability, crashes, or potential information disclosure. This could disrupt critical infrastructure, especially in sectors relying on Linux-based servers and embedded systems. Although no known exploits exist in the wild, the vulnerability could be leveraged by attackers with local access or through compromised containers or virtual machines to escalate privileges or cause denial of service. European organizations with high reliance on Linux servers, cloud infrastructure, or embedded Linux devices should consider this vulnerability seriously, particularly those in finance, telecommunications, manufacturing, and government sectors where system availability and data integrity are paramount.

1. Apply the official Linux kernel patches that redefine alloc_len as u32 in the scsi_debug driver to eliminate the out-of-bounds read. 2. Audit and disable the scsi_debug driver on production systems unless explicitly required for testing or diagnostics. 3. Implement strict access controls to limit local user access to systems running vulnerable kernels, reducing the risk of exploitation. 4. Employ kernel hardening techniques such as Kernel Address Sanitizer (KASAN) and other runtime protections to detect and prevent memory corruption. 5. Monitor kernel logs for unusual SCSI debug driver activity or kernel warnings indicative of exploitation attempts. 6. For virtualized environments, ensure hypervisor and guest OS isolation to prevent lateral movement if the vulnerability is exploited within a guest. 7. Regularly update Linux distributions to incorporate security patches promptly, especially for kernels used in critical infrastructure.