* [PATCH v2] powerpc/xmon: add read-only mode
@ 2019-04-08 3:08 Christopher M. Riedl
2019-04-08 5:34 ` Oliver
2019-04-08 6:37 ` Andrew Donnellan
0 siblings, 2 replies; 8+ messages in thread
From: Christopher M. Riedl @ 2019-04-08 3:08 UTC (permalink / raw)
To: linuxppc-dev; +Cc: Christopher M. Riedl, andrew.donnellan
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 <cmr@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"
+ 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.
+
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)
#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";
+
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
--
2.21.0
^ permalink raw reply related [flat|nested] 8+ messages in thread
* Re: [PATCH v2] powerpc/xmon: add read-only mode
2019-04-08 3:08 [PATCH v2] powerpc/xmon: add read-only mode Christopher M. Riedl
@ 2019-04-08 5:34 ` Oliver
2019-04-10 2:35 ` Christopher M Riedl
2019-04-08 6:37 ` Andrew Donnellan
1 sibling, 1 reply; 8+ messages in thread
From: Oliver @ 2019-04-08 5:34 UTC (permalink / raw)
To: Christopher M. Riedl; +Cc: linuxppc-dev, Andrew Donnellan
On Mon, Apr 8, 2019 at 1:06 PM Christopher M. Riedl <cmr@informatik.wtf> 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 <cmr@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
Do you mean make it dependent on XMON?
> 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"
> + 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.
Maybe I am a dumb, but I found this *extremely* confusing.
Conventionally Kconfig options will control what code is and is not
included in the kernel (see XMON_DISASSEMBLY) rather than changing the
default behaviour of code. It's not wrong to do so and I'm going to
assume that you were following the pattern of XMON_DEFAULT, but I
think you need to be a little more clear about what option actually
does. Renaming it to XMON_DEFAULT_RO_MODE and re-wording the
description to indicate it's a only a mode change would help a lot.
Sorry if this comes across as pointless bikeshedding since it's the
opposite of what Christophe said in the last patch, but this was a bit
of a head scratcher.
> 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)
> #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";
> +
> 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
> --
> 2.21.0
>
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH v2] powerpc/xmon: add read-only mode
2019-04-08 3:08 [PATCH v2] powerpc/xmon: add read-only mode Christopher M. Riedl
2019-04-08 5:34 ` Oliver
@ 2019-04-08 6:37 ` Andrew Donnellan
2019-04-10 2:40 ` Christopher M Riedl
1 sibling, 1 reply; 8+ messages in thread
From: Andrew Donnellan @ 2019-04-08 6:37 UTC (permalink / raw)
To: Christopher M. Riedl, linuxppc-dev; +Cc: Oliver O'Halloran
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 <cmr@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.donnellan@au1.ibm.com IBM Australia Limited
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH v2] powerpc/xmon: add read-only mode
2019-04-08 5:34 ` Oliver
@ 2019-04-10 2:35 ` Christopher M Riedl
2019-04-10 6:51 ` Oliver
2019-04-11 12:37 ` Michael Ellerman
0 siblings, 2 replies; 8+ messages in thread
From: Christopher M Riedl @ 2019-04-10 2:35 UTC (permalink / raw)
To: Oliver; +Cc: linuxppc-dev, Andrew Donnellan
> On April 8, 2019 at 1:34 AM Oliver <oohall@gmail.com> wrote:
>
>
> On Mon, Apr 8, 2019 at 1:06 PM Christopher M. Riedl <cmr@informatik.wtf> 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 <cmr@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
> Do you mean make it dependent on XMON?
>
Yeah that's really not clear at all -- XMON_RW is set based on the value of
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"
> > + 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.
>
> Maybe I am a dumb, but I found this *extremely* confusing.
> Conventionally Kconfig options will control what code is and is not
> included in the kernel (see XMON_DISASSEMBLY) rather than changing the
> default behaviour of code. It's not wrong to do so and I'm going to
> assume that you were following the pattern of XMON_DEFAULT, but I
> think you need to be a little more clear about what option actually
> does. Renaming it to XMON_DEFAULT_RO_MODE and re-wording the
> description to indicate it's a only a mode change would help a lot.
>
> Sorry if this comes across as pointless bikeshedding since it's the
> opposite of what Christophe said in the last patch, but this was a bit
> of a head scratcher.
>
If anyone is dumb here it's me for making this confusing :)
I chatted with Michael Ellerman about this, so let me try to explain this more clearly.
There are two things I am trying to address with XMON_RW:
1) provide a default access mode for xmon based on system "security"
2) replace XMON in the decision to write-protect kernel text at compile-time
I think a single Kconfig for both of those things is sensible as ultimately the
point is to allow xmon to operate in read-only mode on "secure" systems -- without
violating any integrity/security guarantees (such as write-protected kernel text).
Christophe suggested looking at STRICT_KERNEL_RWX and I think that option makes the
most sense to base XMON_RW on since the description for STRICT_KERNEL_RWX states:
> If this is set, kernel text and rodata memory will be made read-only,
> and non-text memory will be made non-executable. This provides
> protection against certain security exploits (e.g. executing the heap
> or modifying text)
>
> These features are considered standard security practice these days.
> You should say Y here in almost all cases.
Considering this, does XMON_DEFAULT_RO_MODE really make things more clear?
With that said, I will remove the 'xmon=rw' cmdline option as it really doesn't work
since kernel text is write-protected at compile time.
'xmon=ro' remains for those who would optionally like to prevent shooting themselves
in the foot/feet while using xmon.
Hopefully this makes a bit more sense now?
>
> > 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)
> > #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";
> > +
> > 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
> > --
> > 2.21.0
> >
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH v2] powerpc/xmon: add read-only mode
2019-04-08 6:37 ` Andrew Donnellan
@ 2019-04-10 2:40 ` Christopher M Riedl
0 siblings, 0 replies; 8+ messages in thread
From: Christopher M Riedl @ 2019-04-10 2:40 UTC (permalink / raw)
To: Andrew Donnellan, linuxppc-dev; +Cc: Oliver O'Halloran
> On April 8, 2019 at 2:37 AM Andrew Donnellan <andrew.donnellan@au1.ibm.com> wrote:
>
>
> 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 <cmr@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.
>
Agreed, if the option isn't renamed again I will fix this in the next version :)
>
> > + 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.
>
> > +
> > 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.
>
> > #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.
>
> > +
> > 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
>
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH v2] powerpc/xmon: add read-only mode
2019-04-10 2:35 ` Christopher M Riedl
@ 2019-04-10 6:51 ` Oliver
2019-04-11 12:37 ` Michael Ellerman
1 sibling, 0 replies; 8+ messages in thread
From: Oliver @ 2019-04-10 6:51 UTC (permalink / raw)
To: Christopher M Riedl; +Cc: linuxppc-dev, Andrew Donnellan
On Wed, Apr 10, 2019 at 12:35 PM Christopher M Riedl <cmr@informatik.wtf> wrote:
>
>
> > On April 8, 2019 at 1:34 AM Oliver <oohall@gmail.com> wrote:
> >
> >
> > On Mon, Apr 8, 2019 at 1:06 PM Christopher M. Riedl <cmr@informatik.wtf> 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 <cmr@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
> > Do you mean make it dependent on XMON?
> >
>
> Yeah that's really not clear at all -- XMON_RW is set based on the value of
> 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"
> > > + 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.
> >
> > Maybe I am a dumb, but I found this *extremely* confusing.
> > Conventionally Kconfig options will control what code is and is not
> > included in the kernel (see XMON_DISASSEMBLY) rather than changing the
> > default behaviour of code. It's not wrong to do so and I'm going to
> > assume that you were following the pattern of XMON_DEFAULT, but I
> > think you need to be a little more clear about what option actually
> > does. Renaming it to XMON_DEFAULT_RO_MODE and re-wording the
> > description to indicate it's a only a mode change would help a lot.
> >
> > Sorry if this comes across as pointless bikeshedding since it's the
> > opposite of what Christophe said in the last patch, but this was a bit
> > of a head scratcher.
> >
>
> If anyone is dumb here it's me for making this confusing :)
> I chatted with Michael Ellerman about this, so let me try to explain this more clearly.
>
> There are two things I am trying to address with XMON_RW:
> 1) provide a default access mode for xmon based on system "security"
> 2) replace XMON in the decision to write-protect kernel text at compile-time
>
> I think a single Kconfig for both of those things is sensible as ultimately the
> point is to allow xmon to operate in read-only mode on "secure" systems -- without
> violating any integrity/security guarantees (such as write-protected kernel text).
>
> Christophe suggested looking at STRICT_KERNEL_RWX and I think that option makes the
> most sense to base XMON_RW on since the description for STRICT_KERNEL_RWX states:
>
> > If this is set, kernel text and rodata memory will be made read-only,
> > and non-text memory will be made non-executable. This provides
> > protection against certain security exploits (e.g. executing the heap
> > or modifying text)
> >
> > These features are considered standard security practice these days.
> > You should say Y here in almost all cases.
>
> Considering this, does XMON_DEFAULT_RO_MODE really make things more clear?
Not really.
> With that said, I will remove the 'xmon=rw' cmdline option as it really doesn't work
> since kernel text is write-protected at compile time.
I think you're overestimating the importance of being able to write to
the kernel text. The only features that require a writeable text are
using the mem commands to modify the kernel text (a bad idea) and
software breakpoints.
Funnily enough, enabling STRICT_RWX make software breakpoints work
again since it enables a smarter implementation of patch_instruction()
that uses a temporary RW mapping to do the patch. Considering that
kprobes and dynamic ftrace are also implemented with
patch_instruction() too we could just default to the newer
implementation and force PAGE_KERNEL_TEXT to be PAGE_KERNEL_ROX all
the time.
> 'xmon=ro' remains for those who would optionally like to prevent shooting themselves
> in the foot/feet while using xmon.
>
> Hopefully this makes a bit more sense now?
>
> > > 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)
> > > #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";
> > > +
> > > 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
> > > --
> > > 2.21.0
> > >
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH v2] powerpc/xmon: add read-only mode
2019-04-10 2:35 ` Christopher M Riedl
2019-04-10 6:51 ` Oliver
@ 2019-04-11 12:37 ` Michael Ellerman
2019-04-12 1:42 ` Christopher M Riedl
1 sibling, 1 reply; 8+ messages in thread
From: Michael Ellerman @ 2019-04-11 12:37 UTC (permalink / raw)
To: Christopher M Riedl, Oliver; +Cc: linuxppc-dev, Andrew Donnellan
Christopher M Riedl <cmr@informatik.wtf> writes:
>> On April 8, 2019 at 1:34 AM Oliver <oohall@gmail.com> wrote:
>> On Mon, Apr 8, 2019 at 1:06 PM Christopher M. Riedl <cmr@informatik.wtf> wrote:
...
>> >
>> > 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"
>> > + 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.
>>
>> Maybe I am a dumb, but I found this *extremely* confusing.
>> Conventionally Kconfig options will control what code is and is not
>> included in the kernel (see XMON_DISASSEMBLY) rather than changing the
>> default behaviour of code. It's not wrong to do so and I'm going to
>> assume that you were following the pattern of XMON_DEFAULT, but I
>> think you need to be a little more clear about what option actually
>> does. Renaming it to XMON_DEFAULT_RO_MODE and re-wording the
>> description to indicate it's a only a mode change would help a lot.
>>
>> Sorry if this comes across as pointless bikeshedding since it's the
>> opposite of what Christophe said in the last patch, but this was a bit
>> of a head scratcher.
>
> If anyone is dumb here it's me for making this confusing :)
> I chatted with Michael Ellerman about this, so let me try to explain this more clearly.
Yeah it's my fault :)
> There are two things I am trying to address with XMON_RW:
> 1) provide a default access mode for xmon based on system "security"
I think I've gone off this idea. Tying them together is just enforcing a
linkage that people may not want.
I think XMON_RW should just be an option that stands on its own. It
should probably be default n, to give people a safe default.
> 2) replace XMON in the decision to write-protect kernel text at compile-time
We should do that as a separate patch. That's actually a bug in the
current STRICT_KERNEL_RWX support.
ie. STRICT_KERNEL_RWX should always give you PAGE_KERNEL_ROX, regardless
of XMON or anything else.
> I think a single Kconfig for both of those things is sensible as ultimately the
> point is to allow xmon to operate in read-only mode on "secure" systems -- without
> violating any integrity/security guarantees (such as write-protected kernel text).
>
> Christophe suggested looking at STRICT_KERNEL_RWX and I think that option makes the
> most sense to base XMON_RW on since the description for STRICT_KERNEL_RWX states:
Once we fix the bugs in STRICT_KERNEL_RWX people are going to enable
that by default, so it will essentially be always on in future.
> With that said, I will remove the 'xmon=rw' cmdline option as it really doesn't work
> since kernel text is write-protected at compile time.
I think 'xmon=rw' still makes sense. Only some of the RW functionality
relies on being able to patch kernel text.
And once you have proccall() you can just call a function to make it
read/write anyway, or use memex to manually frob the page tables.
cheers
^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH v2] powerpc/xmon: add read-only mode
2019-04-11 12:37 ` Michael Ellerman
@ 2019-04-12 1:42 ` Christopher M Riedl
0 siblings, 0 replies; 8+ messages in thread
From: Christopher M Riedl @ 2019-04-12 1:42 UTC (permalink / raw)
To: Michael Ellerman, Oliver; +Cc: linuxppc-dev, Andrew Donnellan
> On April 11, 2019 at 8:37 AM Michael Ellerman <mpe@ellerman.id.au> wrote:
>
>
> Christopher M Riedl <cmr@informatik.wtf> writes:
> >> On April 8, 2019 at 1:34 AM Oliver <oohall@gmail.com> wrote:
> >> On Mon, Apr 8, 2019 at 1:06 PM Christopher M. Riedl <cmr@informatik.wtf> wrote:
> ...
> >> >
> >> > 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"
> >> > + 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.
> >>
> >> Maybe I am a dumb, but I found this *extremely* confusing.
> >> Conventionally Kconfig options will control what code is and is not
> >> included in the kernel (see XMON_DISASSEMBLY) rather than changing the
> >> default behaviour of code. It's not wrong to do so and I'm going to
> >> assume that you were following the pattern of XMON_DEFAULT, but I
> >> think you need to be a little more clear about what option actually
> >> does. Renaming it to XMON_DEFAULT_RO_MODE and re-wording the
> >> description to indicate it's a only a mode change would help a lot.
> >>
> >> Sorry if this comes across as pointless bikeshedding since it's the
> >> opposite of what Christophe said in the last patch, but this was a bit
> >> of a head scratcher.
> >
> > If anyone is dumb here it's me for making this confusing :)
> > I chatted with Michael Ellerman about this, so let me try to explain this more clearly.
>
> Yeah it's my fault :)
>
"Signed-off-by: Christopher M. Riedl" -- I take full responsibility hah.
>
> > There are two things I am trying to address with XMON_RW:
> > 1) provide a default access mode for xmon based on system "security"
>
> I think I've gone off this idea. Tying them together is just enforcing a
> linkage that people may not want.
>
> I think XMON_RW should just be an option that stands on its own. It
> should probably be default n, to give people a safe default.
>
Next version includes this along with making it clear that this option
provides the default mode for XMON.
>
> > 2) replace XMON in the decision to write-protect kernel text at compile-time
>
> We should do that as a separate patch. That's actually a bug in the
> current STRICT_KERNEL_RWX support.
>
> ie. STRICT_KERNEL_RWX should always give you PAGE_KERNEL_ROX, regardless
> of XMON or anything else.
>
> > I think a single Kconfig for both of those things is sensible as ultimately the
> > point is to allow xmon to operate in read-only mode on "secure" systems -- without
> > violating any integrity/security guarantees (such as write-protected kernel text).
> >
> > Christophe suggested looking at STRICT_KERNEL_RWX and I think that option makes the
> > most sense to base XMON_RW on since the description for STRICT_KERNEL_RWX states:
>
> Once we fix the bugs in STRICT_KERNEL_RWX people are going to enable
> that by default, so it will essentially be always on in future.
>
>
> > With that said, I will remove the 'xmon=rw' cmdline option as it really doesn't work
> > since kernel text is write-protected at compile time.
>
> I think 'xmon=rw' still makes sense. Only some of the RW functionality
> relies on being able to patch kernel text.
>
> And once you have proccall() you can just call a function to make it
> read/write anyway, or use memex to manually frob the page tables.
>
> cheers
Great, adding this back in the next version.
^ permalink raw reply [flat|nested] 8+ messages in thread
end of thread, other threads:[~2019-04-12 1:44 UTC | newest]
Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-04-08 3:08 [PATCH v2] powerpc/xmon: add read-only mode Christopher M. Riedl
2019-04-08 5:34 ` Oliver
2019-04-10 2:35 ` Christopher M Riedl
2019-04-10 6:51 ` Oliver
2019-04-11 12:37 ` Michael Ellerman
2019-04-12 1:42 ` Christopher M Riedl
2019-04-08 6:37 ` Andrew Donnellan
2019-04-10 2:40 ` Christopher M Riedl
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