[PATCH] powerpc/xmon: add read-only mode

[cmr]

Operations which write to memory should be restricted on secure systems
and optionally to avoid self-destructive behaviors.

Add a config option, XMON_RO, to control default xmon behavior along
with kernel cmdline options xmon=ro and xmon=rw for explicit control.
The default is to enable read-only mode.

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

Signed-off-by: cmr <cmr at informatik.wtf>
---
 arch/powerpc/Kconfig.debug |  7 +++++++
 arch/powerpc/xmon/xmon.c   | 24 ++++++++++++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
index 4e00cb0a5464..33cc01adf4cb 100644
--- a/arch/powerpc/Kconfig.debug
+++ b/arch/powerpc/Kconfig.debug
@@ -117,6 +117,13 @@ config XMON_DISASSEMBLY
 	  to say Y here, unless you're building for a memory-constrained
 	  system.
 
+config XMON_RO
+	bool "Set xmon read-only mode"
+	depends on XMON
+	default y
+	help
+	  Disable state- and memory-altering write operations in xmon.
+
 config DEBUGGER
 	bool
 	depends on KGDB || XMON
diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index a0f44f992360..c13ee73cdfd4 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/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 int xmon_ro = IS_ENABLED(CONFIG_XMON_RO);
 
 static unsigned long adrs;
 static int size = 1;
@@ -1042,6 +1043,8 @@ cmds(struct pt_regs *excp)
 			set_lpp_cmd();
 			break;
 		case 'b':
+			if (xmon_ro == 1)
+				break;
 			bpt_cmds();
 			break;
 		case 'C':
@@ -1055,6 +1058,8 @@ cmds(struct pt_regs *excp)
 			bootcmds();
 			break;
 		case 'p':
+			if (xmon_ro == 1)
+				break;
 			proccall();
 			break;
 		case 'P':
@@ -1777,6 +1782,9 @@ read_spr(int n, unsigned long *vp)
 static void
 write_spr(int n, unsigned long val)
 {
+	if (xmon_ro == 1)
+		return;
+
 	if (setjmp(bus_error_jmp) == 0) {
 		catch_spr_faults = 1;
 		sync();
@@ -2016,6 +2024,10 @@ mwrite(unsigned long adrs, void *buf, int size)
 	char *p, *q;
 
 	n = 0;
+
+	if (xmon_ro == 1)
+		return n;
+
 	if (setjmp(bus_error_jmp) == 0) {
 		catch_memory_errors = 1;
 		sync();
@@ -2884,9 +2896,13 @@ memops(int cmd)
 	scanhex((void *)&mcount);
 	switch( cmd ){
 	case 'm':
+		if (xmon_ro == 1)
+			break;
 		memmove((void *)mdest, (void *)msrc, mcount);
 		break;
 	case 's':
+		if (xmon_ro == 1)
+			break;
 		memset((void *)mdest, mval, mcount);
 		break;
 	case 'd':
@@ -3796,6 +3812,14 @@ 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_ro = 0;
+	} else if (strncmp(p, "ro", 2) == 0) {
+		xmon_init(1);
+		xmon_on = 1;
+		xmon_ro = 1;
 	} else if (strncmp(p, "off", 3) == 0)
 		xmon_on = 0;
 	else
-- 
2.21.0