[BeeOnRope]

The kernel has a separate stack, inaccessible to user-space.

Otherwise, you'd be right: a shared stack would be a giant source of information leakage from kernel space to user-space unless it was very carefully managed (probably at a significant performance cost). Thus, separate stacks (it also has the advantage of not needing to make assumptions about how user-mode programs use their stack, e.g., if they are transiently using "unallocated" stack above rsp, etc).

Probably what happened here is that this structure was copied back to user space (e.g., as the result of a system call) exposing the kernel data.