GHSA-7884-373w-qqhx: Zephyr virtio driver calls an arbitrary function pointer from an out-of-range used-ring descriptor id

The Zephyr virtio driver does not validate the descriptor-chain head id that the virtio device writes into the used ring. In virtioisr() (drivers/virtio/virtiocommon.c), the device-written vq->used->ring[idx].id is used directly as an index into vq->recvcbs[] and vq->desc[], which are both allocated with exactly vq->num entries. recvcbs[] holds {cb, opaque} callback entries, and the indexed callback pointer is then invoked as cbe.cb(cbe.opaque, usedlen).

Because the id is consumed as a 16-bit value with no bound check, a malicious or compromised virtio backend (an untrusted hypervisor, or an untrusted hardware/peer-processor virtio device on a PCI or MMIO transport) can supply an id far beyond vq->num. This causes an out-of-bounds read of a {function pointer, argument} pair from heap memory beyond recvcbs[], after which the driver calls that attacker-shaped pointer in the guest's interrupt context. No guest privileges or user interaction are required; the backend triggers it by writing the shared used ring and raising the queue interrupt.

The result is an arbitrary / attacker-influenced function-pointer call in the Zephyr guest, i.e. a control-flow-hijack primitive that can lead to code execution or, at minimum, a reliable crash. The fix rejects any used-ring id >= vq->num before indexing recvcbs[]/desc[] or invoking the callback. This affects builds using CONFIGVIRTIO with the PCI or MMIO transport.