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

security/lockdown/lockdown.c

@@ -31,12 +31,14 @@ static const char *const lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
 	[LOCKDOWN_MODULE_PARAMETERS] = "unsafe module parameters",
 	[LOCKDOWN_MMIOTRACE] = "unsafe mmio",
 	[LOCKDOWN_DEBUGFS] = "debugfs access",
+	[LOCKDOWN_XMON_WR] = "xmon write access",
 	[LOCKDOWN_INTEGRITY_MAX] = "integrity",
 	[LOCKDOWN_KCORE] = "/proc/kcore access",
 	[LOCKDOWN_KPROBES] = "use of kprobes",
 	[LOCKDOWN_BPF_READ] = "use of bpf to read kernel RAM",
 	[LOCKDOWN_PERF] = "unsafe use of perf",
 	[LOCKDOWN_TRACEFS] = "use of tracefs",
+	[LOCKDOWN_XMON_RW] = "xmon read and write access",
 	[LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
 };