[PATCH v2] powerpc/xmon: add read-only mode
Christopher M Riedl
cmr at informatik.wtf
Wed Apr 10 12:35:18 AEST 2019
> On April 8, 2019 at 1:34 AM Oliver <oohall at gmail.com> wrote:
>
>
> On Mon, Apr 8, 2019 at 1:06 PM 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_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
> 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
> >
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