On Mon, Apr 15, 2019 at 10:17 AM Christopher M. Riedl
<c...@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 <c...@informatik.wtf>
Reviewed-by: Oliver O'Halloran <ooh...@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
>
