On Wed, Apr 10, 2019 at 12:35 PM Christopher M Riedl <c...@informatik.wtf>
wrote:
>
>
> > On April 8, 2019 at 1:34 AM Oliver <ooh...@gmail.com> wrote:
> >
> >
> > On Mon, Apr 8, 2019 at 1:06 PM 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_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 <c...@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
> > >
