An unprivileged user can mount an ecryptfs over /proc/$pid because according to stat(), it is a normal directory and owned by the user. However, the user is not actually permitted to create arbitrary directory entries in /proc/$pid, and ecryptfs' behavior might be enabling privilege escalation attacks with the help of other programs that use procfs.
Upstream bug report with reproducer:
Proposed upstream patch:
Created ecryptfs-utils tracking bugs for this issue:
Affects: fedora-all [bug 1300595]