[PATCH v3] powerpc/xmon: add read-only mode

[Oliver]

On Mon, Apr 15, 2019 at 10:17 AM Christopher M. Riedl
<cmr at 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_DEFAULT_RO_MODE, to set default xmon behavior.
> The kernel cmdline options xmon=ro and xmon=rw override this default.
>
> 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>

Reviewed-by: Oliver O'Halloran <oohall at gmail.com>

> ---
>
> v2->v3:
>         Use XMON_DEFAULT_RO_MODE to set xmon read-only mode
>         Untangle read-only mode from STRICT_KERNEL_RWX and PAGE_KERNEL_ROX
>         Update printed msg string for write ops in read-only mode

Making PAGE_KERNEL_ROX the default in more cases is still a good idea.
Feel free to send a follow up patch if you want to keep pulling on
that thread.

>  arch/powerpc/Kconfig.debug |  8 ++++++++
>  arch/powerpc/xmon/xmon.c   | 42 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 50 insertions(+)
>
> diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
> index 4e00cb0a5464..8de4823dfb86 100644
> --- a/arch/powerpc/Kconfig.debug
> +++ b/arch/powerpc/Kconfig.debug
> @@ -117,6 +117,14 @@ config XMON_DISASSEMBLY
>           to say Y here, unless you're building for a memory-constrained
>           system.
>
> +config XMON_DEFAULT_RO_MODE
> +       bool "Restrict xmon to read-only operations"
> +       depends on XMON
> +       default y
> +       help
> +          Operate xmon in read-only mode. The cmdline options 'xmon=rw' and
> +          'xmon=ro' override this default.
> +
>  config DEBUGGER
>         bool
>         depends on KGDB || XMON
> diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
> index a0f44f992360..ce98c8049eb6 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_DEFAULT_RO_MODE);
>
>  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_ro_msg = "Operation disabled: xmon in read-only mode\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_ro_msg);
> +                                       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_ro_msg);
> +                               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_ro_msg);
> +                               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_ro_msg);
> +               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_ro_msg);
> +               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_ro_msg);
> +                       break;
> +               }
>                 memmove((void *)mdest, (void *)msrc, mcount);
>                 break;
>         case 's':
> +               if (xmon_is_ro) {
> +                       printf(xmon_ro_msg);
> +                       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
>