CVE-2026-98390 in Linux
Summary
by MITRE • 10/15/2026
In the Linux kernel, the following vulnerability has been resolved:
x86/amd_node: Avoid divide by zero on virtualized systems
On a virtualized system, the number of nodes does not have a relationship to the number of roots. A Xen PVH dom0 can calculate roots_per_node as 0, which crashes with a divide by zero in:
if (count++ % roots_per_node)
because the underlying topology code on Xen ends up making num_nodes 2 and num_roots 1 and the integer division result is 0.
The issue is seen with Xen, but it could affect other systems.
Set roots_per_node to 1 in this case. Print a firmware bug when this is performed for non-virtualized systems.
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Analysis
by VulDB Data Team • 10/15/2026
A critical divide-by-zero vulnerability exists within the Linux kernel's x86 AMD node topology handling code, specifically affecting virtualized environments such as Xen PVH dom0 configurations. The flaw originates in the logic that calculates the ratio of roots per node to determine system topology structure. On certain virtualized platforms, the relationship between the number of nodes and the number of roots is not linear or predictable compared to bare-metal hardware. In specific scenarios involving Xen PVH dom0, the underlying firmware or hypervisor reports a configuration where num_nodes equals two while num_roots equals one. When the kernel attempts to compute roots_per_node by dividing num_roots by num_nodes, the result is zero due to integer division rules. This zero value is subsequently used as a divisor in a conditional statement that checks if count modulo roots_per_node evaluates to true. Since division by zero is undefined and triggers a general protection fault or similar exception on x86 architectures, this condition causes an immediate kernel panic, resulting in a complete system crash and denial of service for the virtual machine instance.
The technical root cause lies in the assumption that topology data provided by firmware adheres to standard physical hardware constraints where node counts typically correlate directly with root complex counts. However, virtualization layers often abstract or modify these hardware attributes to present a specific view to the guest operating system. The Xen PVH implementation can expose a topology state that violates this implicit assumption, leading to an arithmetic underflow in the logical calculation of roots_per_node. This vulnerability is classified as CWE-369: Divide By Zero and aligns with ATT&CK technique T1499: Endpoint Denial of Service, as it allows for local denial of service through a kernel crash triggered by specific hardware enumeration data rather than malicious code execution per se. While the issue has been explicitly observed in Xen environments, similar topology reporting anomalies could potentially affect other virtualization platforms or systems with non-standard ACPI SRAT tables, making this a broader compatibility and robustness concern within the x86 architecture support layer of the Linux kernel.
The operational impact of this vulnerability is severe for affected systems, as it prevents successful booting or causes runtime instability whenever the specific topology condition is met. For cloud providers running Xen PVH dom0 instances, this represents a significant availability risk if not patched, potentially leading to service interruptions across multiple tenants depending on how the hypervisor schedules and exposes hardware resources. The crash occurs during early kernel initialization when CPU hotplug and node management subsystems are being probed, meaning the system may fail to reach an operational state entirely or become unstable shortly after booting as topology information is accessed for scheduling decisions. This highlights a critical gap in input validation regarding firmware-provided data structures, where the operating system must defensively handle potentially malformed or unexpected hardware descriptions from virtualization layers.
To mitigate this vulnerability, kernel developers have implemented a fix that explicitly sets roots_per_node to one when the calculated value would be zero, thereby preventing the division by zero error and allowing the topology code to proceed with a safe default assumption. This patch also includes diagnostic logic to print a firmware bug warning if such an adjustment is made on non-virtualized systems, aiding in future debugging of hardware-specific issues where BIOS or UEFI implementations might incorrectly report node-root relationships. Administrators should ensure their Linux kernels are updated to include this fix, particularly those running Xen PVH dom0 configurations. Additionally, hypervisor administrators may need to review how topology information is exposed to guests to prevent similar anomalies in other virtualization scenarios. Regular patching of the kernel and monitoring for firmware updates that correct ACPI SRAT table reporting can further reduce the risk associated with hardware enumeration inconsistencies.