[PATCH v2] powerpc/xmon: add read-only mode
Andrew Donnellan
andrew.donnellan at au1.ibm.com
Mon Apr 8 16:37:02 AEST 2019
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 at 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 at au1.ibm.com IBM Australia Limited
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