The validator always returned true due to an incorrect check.

Reviewed by:	mhorne, imp
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D36125

jhb@ already fixed this in a different way

Reported by: andrew

This reverts commit 3810b37903220af1a369d3c4032ae25fb2d7949d.

Reported by:	kib
Sponsored by:	Juniper Networks, Inc.
Sponsored by:	Klara, Inc.

This fixes the build of at least i386 MINIMAL which was failing with
the error:

sys/security/mac_ddb/mac_ddb.c:200:15: error: use of undeclared identifier 'vnet'; did you mean 'int'?
                if ((void *)vnet == (void *)addr)
                            ^~~~
                            int

Sponsored by:	DARPA

These global objects are easy to validate, so provide the helper
functions to do so and include these commands in the allow lists.

Reviewed by:	markj
Sponsored by:	Juniper Networks, Inc.
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D35372

Generally, access to the kernel debugger is considered to be unsafe from
a security perspective since it presents an unrestricted interface to
inspect or modify the system state, including sensitive data such as
signing keys.

However, having some access to debugger functionality on production
systems may be useful in determining the cause of a panic or hang.
Therefore, it is desirable to have an optional policy which allows
limited use of ddb(4) while disabling the functionality which could
reveal system secrets.

This loadable MAC module allows for the use of some ddb(4) commands
while preventing the execution of others. The commands have been broadly
grouped into three categories:
 - Those which are 'safe' and will not emit sensitive data (e.g. trace).
   Generally, these commands are deterministic and don't accept
   arguments.
 - Those which are definitively unsafe (e.g. examine <addr>, search
   <addr> <value>)
 - Commands which may be safe to execute depending on the arguments
   provided (e.g. show thread <addr>).

Safe commands have been flagged as such with the DB_CMD_MEMSAFE flag.

Commands requiring extra validation can provide a function to do so.
For example, 'show thread <addr>' can be used as long as addr can be
checked against the system's list of process structures.

The policy also prevents debugger backends other than ddb(4) from
executing, for example gdb(4).

Reviewed by:	markj, pauamma_gundo.com (manpages)
Sponsored by:	Juniper Networks, Inc.
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D35371