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

[Candle Sun]

On Mon, Jan 25, 2021 at 6:37 PM David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> From: Candle Sun
> > Sent: 25 January 2021 08:56
> >
> > 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
> > 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.
>
> It isn't clear that helps.
> Compile with -ffunction-sections and/or do LTO an there
> is no reason at all why the functions should be together.
>
> Even without that lkdtm_WRITE_KERN() could easily be between them.
>
> You need to get the size of the 'empty function' from the
> symbol table.
>
>         David

Thanks David.

I think using abs() by Nick's advice would be better. But could you
point out which kernel function can get function size?

Regards,
Candle


>
> >
> > Signed-off-by: Candle Sun <candle.sun@xxxxxxxxxx>
> > ---
> >  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;
> > -     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
>
> -
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