On 8/4/19 1:08 pm, Christopher M. Riedl wrote:
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 <c...@informatik.wtf>
---
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.
+ 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.
+
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.
#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.
+
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.donnel...@au1.ibm.com IBM Australia Limited
