Re: [PATCH v2] lkdtm: fix memory copy size for WRITE_KERN

[Candle Sun]

On Thu, Jan 28, 2021 at 1:40 AM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> On Wed, Jan 27, 2021 at 3:05 AM Candle Sun <candlesea@xxxxxxxxx> wrote:
> >
> > From: Candle Sun <candle.sun@xxxxxxxxxx>
> >
> > Though do_overwritten() follows do_nothing() in source code, the final
> > memory address order is determined by compiler. We can't always assume
>
> ^ "by the compiler."
>
> > 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*
>
> Hopefully nothing else gets placed in between the two, otherwise we're
> going to overwrite that, too.
>
> > big and WRITE_KERN test on ARM32 arch will fail.
> >
> > Get absolute value of the address substraction for memcpy() size.
>
> ^ typo: subtraction
>
> >
> > Signed-off-by: Candle Sun <candle.sun@xxxxxxxxxx>
> > ---
> > Changes in v2:
> > - Use abs() in place of address comparison.
> > ---
> >  drivers/misc/lkdtm/perms.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> > index 2dede2ef658f..fbb7f4554054 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. */
>
> ^ This comment is now obsolete and should be removed together with
> this patch.  That will also fix the typo in it.
>
> > -static void do_overwritten(void)
> > +static noinline void do_overwritten(void)
> >  {
> >         pr_info("do_overwritten wasn't overwritten!\n");
> >         return;
> > @@ -113,7 +113,7 @@ void lkdtm_WRITE_KERN(void)
> >         size_t size;
> >         volatile unsigned char *ptr;
> >
> > -       size = (unsigned long)do_overwritten - (unsigned long)do_nothing;
> > +       size = (size_t)abs((uintptr_t)do_overwritten - (uintptr_t)do_nothing);
> >         ptr = (unsigned char *)do_overwritten;
> >
> >         pr_info("attempting bad %zu byte write at %px\n", size, ptr);
> > --
> > 2.17.0
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers

Thanks Nick. I will clean the typo and the comment line in the next patch.

Regards,
Candle