CVE-2021-46909 is a vulnerability identified in the Linux kernel specifically affecting the ARM architecture's footbridge PCI interrupt mapping implementation. The root cause stems from a change introduced by commit 30fdfb929e82, which added a call to pci_assign_irq() during pci_device_probe(). This function is responsible for assigning IRQs (interrupt requests) to PCI devices when their drivers are probed. However, if the IRQ mapping functions are marked with the __init attribute, they are discarded after kernel initialization to free memory. Consequently, if a PCI driver is loaded or bound after the kernel has fully initialized, the kernel attempts to call these now non-existent IRQ mapping functions, leading to a kernel oops (a type of kernel panic or crash). This results in a denial of service condition as the kernel becomes unstable or crashes when handling late-bound PCI drivers on affected ARM platforms. The vulnerability is specific to certain Linux kernel versions containing the problematic commit and affects systems using the ARM footbridge PCI subsystem. There are no known exploits in the wild, and no CVSS score has been assigned yet. The issue has been publicly disclosed and patched in the Linux kernel source, although no direct patch links are provided in the data. The vulnerability primarily impacts system stability and availability rather than confidentiality or integrity.

For European organizations, the impact of CVE-2021-46909 is primarily related to system availability and operational continuity. Organizations running ARM-based Linux systems with PCI devices that rely on the footbridge subsystem could experience kernel crashes if PCI drivers are loaded dynamically after boot. This can disrupt critical services, especially in embedded systems, industrial control systems, telecommunications infrastructure, or ARM-based servers used in data centers. The denial of service caused by kernel oops could lead to unexpected downtime, impacting business operations, service delivery, and potentially causing financial losses. Since the vulnerability does not appear to allow privilege escalation or code execution, the confidentiality and integrity of data are less likely to be directly affected. However, repeated crashes could be exploited by attackers to cause persistent denial of service or to create conditions favorable for further attacks. The lack of known exploits reduces immediate risk, but organizations should not delay patching to avoid potential future exploitation.

To mitigate CVE-2021-46909, European organizations should: 1) Identify and inventory all ARM-based Linux systems using the footbridge PCI subsystem. 2) Apply the latest Linux kernel updates that include the fix for this vulnerability, ensuring that the pci_assign_irq() call and IRQ mapping functions are correctly handled without relying on __init-marked functions after initialization. 3) Avoid loading or binding PCI drivers dynamically after kernel initialization unless necessary, or ensure that such drivers are compatible with the patched kernel behavior. 4) Implement robust monitoring of kernel logs to detect oops or crashes related to PCI driver loading. 5) For critical systems, consider implementing redundancy and failover mechanisms to minimize downtime caused by potential kernel crashes. 6) Engage with Linux distribution vendors or maintainers to confirm that their kernel packages include the fix and to receive timely updates. 7) Test kernel updates in staging environments to ensure compatibility and stability before deployment in production.