fs/nfs/nfs4client.c in the Linux kernel before 5.13.4 has incorrect connection-setup ordering, which allows operators of remote NFSv4 servers to cause a denial of service (hanging of mounts) by arranging for those servers to be unreachable during trunking detection.
cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.13.4
github.com/torvalds/linux/commit/dd99e9f98fbf423ff6d365b37a98e8879170f17c
lists.debian.org/debian-lts-announce/2021/10/msg00010.html
lists.debian.org/debian-lts-announce/2021/12/msg00012.html
security.netapp.com/advisory/ntap-20210902-0010/
www.debian.org/security/2021/dsa-4978