CVE-2021-47507

In the Linux kernel, the following vulnerability has been resolved: nfsd:
Fix nsfd startup race (again) Commit bd5ae9288d64 (“nfsd: register pernet
ops last, unregister first”) has re-opened rpc_pipefs_event() race against
nfsd_net_id registration (register_pernet_subsys()) which has been fixed by
commit bb7ffbf29e76 (“nfsd: fix nsfd startup race triggering BUG_ON”).
Restore the order of register_pernet_subsys() vs register_cld_notifier().
Add WARN_ON() to prevent a future regression. Crash info: Unable to handle
kernel NULL pointer dereference at virtual address 0000000000000012 CPU: 8
PID: 345 Comm: mount Not tainted 5.4.144-… #1 pc :
rpc_pipefs_event+0x54/0x120 [nfsd] lr : rpc_pipefs_event+0x48/0x120 [nfsd]
Call trace: rpc_pipefs_event+0x54/0x120 [nfsd] blocking_notifier_call_chain
rpc_fill_super get_tree_keyed rpc_fs_get_tree vfs_get_tree do_mount
ksys_mount __arm64_sys_mount el0_svc_handler el0_svc