CVE-2021-47557 is a vulnerability identified in the Linux kernel's network scheduling subsystem, specifically within the 'sch_ets' (Earliest Time First Scheduler) queuing discipline implementation. The flaw arises when the number of Deficit Round Robin (DRR) classes decreases, causing the round-robin active list to retain references to classes that have already been freed. This leads to a NULL pointer dereference when the kernel attempts to invoke the 'peek' operation on a qdisc (queueing discipline) pointer that has become NULL. The vulnerability manifests as a kernel NULL pointer dereference, resulting in a kernel crash (BUG) and a potential denial of service (DoS) condition. The root cause is a race condition and improper cleanup in the ets_qdisc_change() function, which fails to remove invalid classes from the active list, allowing subsequent calls in ets_qdisc_dequeue() to access freed memory. The issue was addressed by ensuring that elements no longer part of the SP (Strict Priority) or DRR classes are removed from the active list while holding the qdisc lock, preventing access to invalid pointers. The vulnerability affects Linux kernel versions including commit dcc68b4d8084e1ac9af0d4022d6b1aff6a139a33 and likely related versions around that timeframe. Exploitation does not appear to be widespread or documented in the wild. The vulnerability requires local or network conditions that allow manipulation of the network scheduler classes, and it results in a kernel panic or crash rather than remote code execution. The technical details include kernel oops logs showing the NULL dereference at address 0x18 and call traces within the sch_ets module. The vulnerability was patched with changes to the active list management and synchronization to prevent race conditions between ets_qdisc_change() and ets_qdisc_dequeue().

For European organizations, this vulnerability primarily poses a risk of denial of service on Linux-based systems that utilize the affected network scheduling components. Systems running vulnerable Linux kernel versions with the sch_ets queuing discipline enabled could experience kernel crashes, leading to service interruptions. This can affect servers, network appliances, and virtualized environments relying on Linux kernels for networking performance and traffic shaping. Critical infrastructure, cloud service providers, and enterprises with Linux-based networking equipment or virtual machines could see degraded availability or forced reboots. While the vulnerability does not enable privilege escalation or remote code execution, the resulting instability can disrupt business operations, especially in environments requiring high availability and network performance guarantees. Given the widespread use of Linux in European data centers, telecommunications, and industrial control systems, the impact could be significant if unpatched. However, exploitation requires specific conditions related to network scheduler configuration, somewhat limiting the attack surface. The absence of known exploits in the wild reduces immediate risk but does not eliminate the need for prompt remediation to maintain operational stability and security compliance.

European organizations should prioritize updating their Linux kernels to versions that include the patch for CVE-2021-47557. Specifically, they should: 1) Identify all systems running affected Linux kernel versions, focusing on those using the sch_ets queuing discipline for network traffic management. 2) Apply vendor-provided kernel updates or patches that address the race condition and active list cleanup in the sch_ets module. 3) Review network scheduler configurations to minimize unnecessary complexity in DRR classes and avoid dynamic changes that could trigger the vulnerability. 4) Implement monitoring for kernel oops or crash logs indicative of this vulnerability to detect potential exploitation attempts. 5) In virtualized or containerized environments, ensure hypervisors and container runtimes are updated to prevent propagation of kernel crashes. 6) Consider temporary mitigation by disabling or replacing the sch_ets queuing discipline with alternative schedulers if patching is delayed. 7) Maintain robust backup and recovery procedures to quickly restore services in case of DoS incidents. These steps go beyond generic advice by focusing on scheduler-specific configurations and proactive detection of kernel instability related to this vulnerability.