Subject: CVE-2021-47507: nfsd: Fix nsfd startup race (again)

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

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

The Linux kernel CVE team has assigned CVE-2021-47507 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 5.4.102 with commit 8677e99150b0 and fixed in 5.4.165 with commit f5734b1714ca
	Issue introduced in 5.10.20 with commit 7c7cb07d4aff and fixed in 5.10.85 with commit c520943a00ad
	Issue introduced in 5.12 with commit bd5ae9288d64 and fixed in 5.15.8 with commit 8bf902fee589
	Issue introduced in 5.12 with commit bd5ae9288d64 and fixed in 5.16 with commit b10252c7ae9c
	Issue introduced in 5.11.3 with commit 4d41f65efeec

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2021-47507
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	fs/nfsd/nfs4recover.c
	fs/nfsd/nfsctl.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/f5734b1714ca355703e9ea8fb61d04beff1790b9
	https://git.kernel.org/stable/c/c520943a00ad5015704969ad3304c956bcd49d25
	https://git.kernel.org/stable/c/8bf902fee5893cfc2f04a698abab47629699ae9a
	https://git.kernel.org/stable/c/b10252c7ae9c9d7c90552f88b544a44ee773af64