Re: [PATCH v2] powerpc/xmon: add read-only mode

On April 8, 2019 at 2:37 AM Andrew Donnellan [off-list ref] wrote:


On 8/4/19 1:08 pm, Christopher M. Riedl wrote:

quoted

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_RW, to control default xmon behavior along
with kernel cmdline options xmon=ro and xmon=rw for explicit control.
Use XMON_RW instead of XMON in the condition to set PAGE_KERNEL_TEXT to
allow xmon in read-only mode alongside write-protected kernel text.
XMON_RW defaults to !STRICT_KERNEL_RWX.

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 <redacted>
---
v1->v2:
	Use bool type for xmon_is_ro flag
	Replace XMON_RO with XMON_RW config option
	Make XMON_RW dependent on STRICT_KERNEL_RWX
	Use XMON_RW to control PAGE_KERNEL_TEXT
	Add printf in xmon read-only mode when dropping/skipping writes
	Disable memzcan (zero-fill memop) in xmon read-only mode

  arch/powerpc/Kconfig.debug                   | 10 +++++
  arch/powerpc/include/asm/book3s/32/pgtable.h |  5 ++-
  arch/powerpc/include/asm/book3s/64/pgtable.h |  5 ++-
  arch/powerpc/include/asm/nohash/pgtable.h    |  5 ++-
  arch/powerpc/xmon/xmon.c                     | 42 ++++++++++++++++++++
  5 files changed, 61 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
index 4e00cb0a5464..0c7f21476018 100644
--- a/arch/powerpc/Kconfig.debug
+++ b/arch/powerpc/Kconfig.debug

@@ -117,6 +117,16 @@ config XMON_DISASSEMBLY
  	  to say Y here, unless you're building for a memory-constrained
  	  system.
  
+config XMON_RW
+	bool "Allow xmon read and write operations"

"Allow xmon write operations" would be clearer. This option has no 
impact on read operations.

Agreed, if the option isn't renamed again I will fix this in the next version :)

quoted

+	depends on XMON
+	default !STRICT_KERNEL_RWX
+	help
+	  Allow xmon to read and write to memory and special-purpose registers.
+          Conversely, prevent xmon write access when set to N. Read and write
+          access can also be explicitly controlled with 'xmon=rw' or 'xmon=ro'
+          (read-only) cmdline options. Default is !STRICT_KERNEL_RWX.

This is an improvement but still doesn't clearly explain the 
relationship between selecting this option and using the cmdline options.

I will reword this in the next version.

quoted

+
  config DEBUGGER
  	bool
  	depends on KGDB || XMON

diff --git a/arch/powerpc/include/asm/book3s/32/pgtable.h b/arch/powerpc/include/asm/book3s/32/pgtable.h
index aa8406b8f7ba..615144ad667d 100644
--- a/arch/powerpc/include/asm/book3s/32/pgtable.h
+++ b/arch/powerpc/include/asm/book3s/32/pgtable.h

@@ -86,8 +86,9 @@ static inline bool pte_user(pte_t pte)
   * set breakpoints anywhere, so don't write protect the kernel text
   * on platforms where such control is possible.
   */
-#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\
-	defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE)
+#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \
+	defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \
+	defined(CONFIG_DYNAMIC_FTRACE)
  #define PAGE_KERNEL_TEXT	PAGE_KERNEL_X
  #else
  #define PAGE_KERNEL_TEXT	PAGE_KERNEL_ROX

diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
index 581f91be9dd4..bc4655122f6b 100644
--- a/arch/powerpc/include/asm/book3s/64/pgtable.h
+++ b/arch/powerpc/include/asm/book3s/64/pgtable.h

@@ -168,8 +168,9 @@
   * set breakpoints anywhere, so don't write protect the kernel text
   * on platforms where such control is possible.
   */
-#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) || \
-	defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE)
+#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \
+	defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \
+	defined(CONFIG_DYNAMIC_FTRACE)

We might need to rethink how the cmdline options work.

What happens if CONFIG_XMON_RW=n, but "xmon=rw" on the command line?

I think I agree with Oliver that we're not being clear whether this 
config option is selecting code to be compiled in, or whether it's just 
setting a default.

Please refer to my reply to Oliver's comments -- 'xmon=rw' is broken and will be removed
in the next version. I think 'xmon=ro' still makes sense to optionally enable xmon's read-
only mode even when writes are allowed on a non-secure system.

quoted

  #define PAGE_KERNEL_TEXT	PAGE_KERNEL_X
  #else
  #define PAGE_KERNEL_TEXT	PAGE_KERNEL_ROX

diff --git a/arch/powerpc/include/asm/nohash/pgtable.h b/arch/powerpc/include/asm/nohash/pgtable.h
index 1ca1c1864b32..c052931bd243 100644
--- a/arch/powerpc/include/asm/nohash/pgtable.h
+++ b/arch/powerpc/include/asm/nohash/pgtable.h

@@ -22,8 +22,9 @@
   * set breakpoints anywhere, so don't write protect the kernel text
   * on platforms where such control is possible.
   */
-#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\
-	defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE)
+#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \
+	defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \
+	defined(CONFIG_DYNAMIC_FTRACE)
  #define PAGE_KERNEL_TEXT	PAGE_KERNEL_X
  #else
  #define PAGE_KERNEL_TEXT	PAGE_KERNEL_ROX

diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index a0f44f992360..224ca0b3506b 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 bool xmon_is_ro = !IS_ENABLED(CONFIG_XMON_RW);
  
  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_is_ro_warn = "xmon read-only mode: skipping write\n";

Yay, error messages! Though something like: "Operation disabled: xmon is 
in read-only mode" would be clearer.

Agreed, changing this in the next version.

quoted

+
  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_is_ro_warn);
+					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_is_ro_warn);
+				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_is_ro_warn);
+				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_is_ro_warn);
+		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_is_ro_warn);
+		return n;
+	}
+
  	if (setjmp(bus_error_jmp) == 0) {
  		catch_memory_errors = 1;
  		sync();

@@ -2884,9 +2910,17 @@ memops(int cmd)
  	scanhex((void *)&mcount);
  	switch( cmd ){
  	case 'm':
+		if (xmon_is_ro) {
+			printf(xmon_is_ro_warn);
+			break;
+		}
  		memmove((void *)mdest, (void *)msrc, mcount);
  		break;
  	case 's':
+		if (xmon_is_ro) {
+			printf(xmon_is_ro_warn);
+			break;
+		}
  		memset((void *)mdest, mval, mcount);
  		break;
  	case 'd':

@@ -3796,6 +3830,14 @@ 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

-- 
Andrew Donnellan              OzLabs, ADL Canberra
andrew.donnellan@au1.ibm.com  IBM Australia Limited