CVE-2026-49429: Heap-based Buffer Overflow

The ZFSIOCUSERSPACEMANY ioctl, used by zfs-userspace8, truncated a 64-bit output buffer size to a 32-bit integer for the kernel allocation, but used the original 64-bit size as the buffer limit when writing records. A local user with the "userused" delegated ZFS permission can trigger a kernel he...