powerpc/xmon: add read-only mode

Operations which write to memory and special purpose registers should be
restricted on systems with integrity guarantees (such as Secure Boot)
and, optionally, to avoid self-destructive behaviors.

Add a config option, XMON_DEFAULT_RO_MODE, to set default xmon behavior.
The kernel cmdline options xmon=ro and xmon=rw override this default.

The following xmon operations are affected:
memops:
	disable memmove
	disable memset
	disable memzcan
memex:
	no-op'd mwrite
super_regs:
	no-op'd write_spr
bpt_cmds:
	disable
proc_call:
	disable

Signed-off-by: Christopher M. Riedl <cmr@informatik.wtf>
Reviewed-by: Oliver O'Halloran <oohall@gmail.com>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/Kconfig.debug

@@ -117,6 +117,14 @@ config XMON_DISASSEMBLY
 	  to say Y here, unless you're building for a memory-constrained
 	  system.
 
+config XMON_DEFAULT_RO_MODE
+	bool "Restrict xmon to read-only operations by default"
+	depends on XMON
+	default y
+	help
+          Operate xmon in read-only mode. The cmdline options 'xmon=rw' and
+          'xmon=ro' override this default.
+
 config DEBUGGER
 	bool
 	depends on KGDB || XMON

arch/powerpc/xmon/xmon.c

@@ -80,6 +80,7 @@ static int set_indicator_token = RTAS_UNKNOWN_SERVICE;
 #endif
 static unsigned long in_xmon __read_mostly = 0;
 static int xmon_on = IS_ENABLED(CONFIG_XMON_DEFAULT);
+static bool xmon_is_ro = IS_ENABLED(CONFIG_XMON_DEFAULT_RO_MODE);
 
 static unsigned long adrs;
 static int size = 1;
@@ -202,6 +203,8 @@ static void dump_tlb_book3e(void);
 #define GETWORD(v)	(((v)[0] << 24) + ((v)[1] << 16) + ((v)[2] << 8) + (v)[3])
 #endif
 
+static const char *xmon_ro_msg = "Operation disabled: xmon in read-only mode\n";
+
 static char *help_string = "\
 Commands:\n\
   b	show breakpoints\n\
@@ -989,6 +992,10 @@ cmds(struct pt_regs *excp)
 				memlocate();
 				break;
 			case 'z':
+				if (xmon_is_ro) {
+					printf(xmon_ro_msg);
+					break;
+				}
 				memzcan();
 				break;
 			case 'i':
@@ -1042,6 +1049,10 @@ cmds(struct pt_regs *excp)
 			set_lpp_cmd();
 			break;
 		case 'b':
+			if (xmon_is_ro) {
+				printf(xmon_ro_msg);
+				break;
+			}
 			bpt_cmds();
 			break;
 		case 'C':
@@ -1055,6 +1066,10 @@ cmds(struct pt_regs *excp)
 			bootcmds();
 			break;
 		case 'p':
+			if (xmon_is_ro) {
+				printf(xmon_ro_msg);
+				break;
+			}
 			proccall();
 			break;
 		case 'P':
@@ -1777,6 +1792,11 @@ read_spr(int n, unsigned long *vp)
 static void
 write_spr(int n, unsigned long val)
 {
+	if (xmon_is_ro) {
+		printf(xmon_ro_msg);
+		return;
+	}
+
 	if (setjmp(bus_error_jmp) == 0) {
 		catch_spr_faults = 1;
 		sync();
@@ -2016,6 +2036,12 @@ mwrite(unsigned long adrs, void *buf, int size)
 	char *p, *q;
 
 	n = 0;
+
+	if (xmon_is_ro) {
+		printf(xmon_ro_msg);
+		return n;
+	}
+
 	if (setjmp(bus_error_jmp) == 0) {
 		catch_memory_errors = 1;
 		sync();
@@ -2880,9 +2906,17 @@ memops(int cmd)
 	scanhex((void *)&mcount);
 	switch( cmd ){
 	case 'm':
+		if (xmon_is_ro) {
+			printf(xmon_ro_msg);
+			break;
+		}
 		memmove((void *)mdest, (void *)msrc, mcount);
 		break;
 	case 's':
+		if (xmon_is_ro) {
+			printf(xmon_ro_msg);
+			break;
+		}
 		memset((void *)mdest, mval, mcount);
 		break;
 	case 'd':
@@ -3792,6 +3826,14 @@ static int __init early_parse_xmon(char *p)
 	} else if (strncmp(p, "on", 2) == 0) {
 		xmon_init(1);
 		xmon_on = 1;
+	} else if (strncmp(p, "rw", 2) == 0) {
+		xmon_init(1);
+		xmon_on = 1;
+		xmon_is_ro = false;
+	} else if (strncmp(p, "ro", 2) == 0) {
+		xmon_init(1);
+		xmon_on = 1;
+		xmon_is_ro = true;
 	} else if (strncmp(p, "off", 3) == 0)
 		xmon_on = 0;
 	else