Re: [PATCH] lkdtm: fix memory copy size for WRITE_KERN

[Candle Sun <candlesea@gmail.com>]

On Tue, Jan 26, 2021 at 5:16 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Mon, Jan 25, 2021 at 12:56 AM Candle Sun <candlesea@gmail.com> wrote:
> >
> > From: Candle Sun <candle.sun@unisoc.com>
> >
> > Though do_overwritten() follows do_nothing() in source code, the final
> > memory address order is determined by compiler. We can't always assume
> > address of do_overwritten() is bigger than do_nothing(). At least the
> > Clang we are using places do_overwritten() before do_nothing() in the
> > object. This causes the copy size in lkdtm_WRITE_KERN() is *really*
> > big and WRITE_KERN test on ARM32 arch will fail.
> >
> > Compare the address order before doing the subtraction.
> >
> > Signed-off-by: Candle Sun <candle.sun@unisoc.com>
> > ---
> >  drivers/misc/lkdtm/perms.c | 19 +++++++++----------
> >  1 file changed, 9 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> > index 2dede2ef658f..fbfbdf89d668 100644
> > --- a/drivers/misc/lkdtm/perms.c
> > +++ b/drivers/misc/lkdtm/perms.c
> > @@ -31,13 +31,13 @@ static unsigned long ro_after_init __ro_after_init = 0x55AA5500;
> >   * This just returns to the caller. It is designed to be copied into
> >   * non-executable memory regions.
> >   */
> > -static void do_nothing(void)
> > +static noinline void do_nothing(void)
> >  {
> >         return;
> >  }
> >
> >  /* Must immediately follow do_nothing for size calculuations to work out. */
> > -static void do_overwritten(void)
> > +static noinline void do_overwritten(void)
> >  {
> >         pr_info("do_overwritten wasn't overwritten!\n");
> >         return;
> > @@ -110,15 +110,14 @@ void lkdtm_WRITE_RO_AFTER_INIT(void)
> >
> >  void lkdtm_WRITE_KERN(void)
> >  {
> > -       size_t size;
> > -       volatile unsigned char *ptr;
> > +       unsigned long value_dow = (unsigned long)do_overwritten;
> > +       unsigned long value_do =  (unsigned long)do_nothing;
> > +       size_t size = (size_t)(value_dow > value_do ?
> > +                       value_dow - value_do : value_do - value_dow);
> >
> > -       size = (unsigned long)do_overwritten - (unsigned long)do_nothing;
>
> Thanks for the patch! Might it be simpler to do:
>
> size = abs((uintptr_t)do_overwritten - (uintptr_t)do_nothing));
>
> Then I don't think much of this function needs to be changed.
>

Thanks Nick.

abs() is better, I will check the patch.

Regards,
Candle


> > -       ptr = (unsigned char *)do_overwritten;
> > -
> > -       pr_info("attempting bad %zu byte write at %px\n", size, ptr);
> > -       memcpy((void *)ptr, (unsigned char *)do_nothing, size);
> > -       flush_icache_range((unsigned long)ptr, (unsigned long)(ptr + size));
> > +       pr_info("attempting bad %zu byte write at %px\n", size, do_overwritten);
> > +       memcpy((void *)value_dow, (void *)value_do, size);
> > +       flush_icache_range(value_dow, value_dow + (unsigned long)size);
> >         pr_err("FAIL: survived bad write\n");
> >
> >         do_overwritten();
> > --
> > 2.17.0
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers