FreeBSD: VID-13d37672-9791-11eb-b87a-901b0ef719ab (CVE-2021-29626): FreeBSD -- Memory disclosure by stale virtual memory mapping

Problem Description: A particular case of memory sharing is mishandled in the virtual memory system. It is possible and legal to establish a relationship where multiple descendant processes share a mapping which shadows memory of an ancestor process. In this scenario, when one process modifies memory through such a mapping, the copy-on-write logic fails to invalidate other mappings of the source page. These stale mappings may remain even after the mapped pages have been reused for another purpose. Impact: An unprivileged local user process can maintain a mapping of a page after it is freed, allowing that process to read private data belonging to other processes or the kernel.