Re: [PATCH v1 09/10] lkdtm: Fix lkdtm_EXEC_RODATA()

[Christophe Leroy]

Le 13/10/2021 à 09:23, Kees Cook a écrit :
> On Mon, Oct 11, 2021 at 05:25:36PM +0200, Christophe Leroy wrote:
> > Behind a location, lkdtm_EXEC_RODATA() executes a real function,
> > not a copy of do_nothing().
> >
> > So do it directly instead of using execute_location().
> >
> > And fix displayed addresses by dereferencing the function descriptors.
> >
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> > ---
> >   drivers/misc/lkdtm/perms.c | 9 ++++++++-
> >   1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> > index 442d60ed25ef..da16564e1ecd 100644
> > --- a/drivers/misc/lkdtm/perms.c
> > +++ b/drivers/misc/lkdtm/perms.c
> > @@ -153,7 +153,14 @@ void lkdtm_EXEC_VMALLOC(void)
> >
> >   void lkdtm_EXEC_RODATA(void)
> >   {
> > -	execute_location(lkdtm_rodata_do_nothing, CODE_AS_IS);
> > +	pr_info("attempting ok execution at %px\n",
> > +		dereference_symbol_descriptor(do_nothing));
> > +	do_nothing();
> > +
> > +	pr_info("attempting bad execution at %px\n",
> > +		dereference_symbol_descriptor(lkdtm_rodata_do_nothing));
> > +	lkdtm_rodata_do_nothing();
> > +	pr_err("FAIL: func returned\n");
> >   }
>
> (In re-reading this more carefully, I see now why kallsyms.h is used
> earlier: _function_ vs _symbol_ descriptor.)
>
> In the next patch:
>
> static noinline void execute_location(void *dst, bool write)
> {
> ...
>         func = setup_function_descriptor(&fdesc, dst);
>         if (IS_ERR(func))
>                 return;
>
>         pr_info("attempting bad execution at %px\n", dst);
>         func();
>         pr_err("FAIL: func returned\n");
> }
>
> What are the conditions for which dereference_symbol_descriptor works
> but dereference _function_descriptor doesn't?

When LKDTM is built as a module I guess ?

Christophe