CVE-2026-80891 in Linux
Summary
by MITRE • 09/04/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: s390: pci: Validate AIBV and AISB before pinning guest pages
The AIBV holds one bit per MSI-X vector for a given function. The size of the bit vector is derived from the NOI and the AIBVO. If the size of the AIBV exceeds a single page boundary, then reject the request as we cannot safely pin the guest AIBV.
Similarly reject the request if the AISB address is not 8-byte aligned as the architecture requires doubleword alignment for the summary bit address. Since the AISBO can address up to 64 bits, the size of the AISB can only be 8 bytes for the function. This also ensures the AISB doesn't exceed a single page boundary.
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Analysis
by VulDB Data Team • 09/05/2026
The Linux kernel's KVM subsystem for s390 architecture contains a critical validation flaw within its PCI passthrough implementation that allows guest virtual machines to trigger invalid memory operations during MSI-X vector configuration. Specifically, the vulnerability arises when handling the Assignment Interrupt Vector (AIBV) and the Assignment Interrupt Summary Bit (AISB) structures provided by the guest operating system. These data structures are essential for managing interrupt routing in IBM Z systems, where precise alignment and size constraints are enforced by the hardware architecture to ensure stable operation of message-signaled interrupts. The kernel previously failed to adequately verify these parameters before attempting to pin guest physical pages into host memory, creating a pathway for potential denial of service or unstable system behavior due to improper resource allocation.
The technical root cause lies in insufficient boundary and alignment checks during the initialization phase of PCI device assignment. For the AIBV, which contains one bit per MSI-X vector determined by the Number Of Interrupts (NOI) and the Assignment Interrupt Vector Offset (AIBVO), the kernel did not verify whether the resulting size exceeded a single page boundary. If an oversized AIBV is provided, the subsequent attempt to pin these guest pages could fail or behave unpredictably because the underlying memory management logic assumes manageable, page-aligned chunks that fit within expected limits. Similarly, for the AISB address, which serves as a summary bit location requiring doubleword alignment per s390 architecture specifications, there was no check ensuring 8-byte alignment. The size of the AISB is strictly limited to eight bytes because the Assignment Interrupt Summary Bit Offset (AISBO) can only address up to sixty-four bits. Without these validations, misaligned or oversized addresses could lead to hardware faults or kernel panics when the system attempts to map and pin these invalid memory regions.
The operational impact of this vulnerability primarily centers on stability and availability within virtualized environments hosting s390 workloads. An attacker with control over a guest VM that has PCI passthrough capabilities could craft malicious configuration requests containing oversized AIBV structures or misaligned AISB addresses. Upon submission, these invalid parameters would cause the KVM subsystem to attempt unsafe memory pinning operations. This can result in immediate kernel crashes, leading to a denial of service for all virtual machines running on the affected host system. Furthermore, depending on the specific state of the hypervisor at the time of exploitation, there may be risks associated with memory corruption or inconsistent device states that could compromise the isolation between VMs, although the primary documented risk is stability degradation through crash conditions rather than direct privilege escalation.
To mitigate this vulnerability, it is imperative to apply the latest kernel updates provided by your distribution vendor, which include patches enforcing strict validation of AIBV size and AISB alignment before any guest page pinning occurs. Administrators should ensure that their KVM configurations for s390 PCI devices adhere strictly to architectural constraints, verifying that MSI-X vector counts do not result in bit vectors exceeding single-page boundaries and that summary bit addresses are properly aligned to eight-byte boundaries. Regular auditing of virtual machine configurations and keeping the host kernel up to date with security fixes is essential to prevent exploitation of this flaw. This issue aligns with CWE-20 Improper Input Validation, as it stems from a failure to correctly validate user-supplied data against expected structural constraints before processing, and relates to ATT&CK techniques involving resource hijacking or denial of service through system instability.