powerpc/xmon: Restrict when kernel is locked down

Xmon should be either fully or partially disabled depending on the
kernel lockdown state.

Put xmon into read-only mode for lockdown=integrity and prevent user
entry into xmon when lockdown=confidentiality. Xmon checks the lockdown
state on every attempted entry:

 (1) during early xmon'ing

 (2) when triggered via sysrq

 (3) when toggled via debugfs

 (4) when triggered via a previously enabled breakpoint

The following lockdown state transitions are handled:

 (1) lockdown=none -> lockdown=integrity
     set xmon read-only mode

 (2) lockdown=none -> lockdown=confidentiality
     clear all breakpoints, set xmon read-only mode,
     prevent user re-entry into xmon

 (3) lockdown=integrity -> lockdown=confidentiality
     clear all breakpoints, set xmon read-only mode,
     prevent user re-entry into xmon

Suggested-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Christopher M. Riedl <cmr@informatik.wtf>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190907061124.1947-3-cmr@informatik.wtf

include/linux/security.h

@@ -116,12 +116,14 @@ enum lockdown_reason {
 	LOCKDOWN_MODULE_PARAMETERS,
 	LOCKDOWN_MMIOTRACE,
 	LOCKDOWN_DEBUGFS,
+	LOCKDOWN_XMON_WR,
 	LOCKDOWN_INTEGRITY_MAX,
 	LOCKDOWN_KCORE,
 	LOCKDOWN_KPROBES,
 	LOCKDOWN_BPF_READ,
 	LOCKDOWN_PERF,
 	LOCKDOWN_TRACEFS,
+	LOCKDOWN_XMON_RW,
 	LOCKDOWN_CONFIDENTIALITY_MAX,
 };