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

[Christophe Leroy]

Le 13/10/2021 à 09:09, 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().
>
> I don't understand this. Why does the next patch not fix this?

Well, probably it would, but it looked incorrect in my mind.

lkdtm_rodata_do_nothing() is a function which has its own function 
descriptor, it is not a copy of do_nothing().

If we use execute_location() as modified by next patch, then we will 
execute it using the function descriptor of do_nothing(). Allthough it 
most likely works (at least on powerpc as it uses the same TOC) it looks 
odd to me to do so.

Am I missing something ?

Christophe


> -Kees
>
> > 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");
> >   }
> >   
> >   void lkdtm_EXEC_USERSPACE(void)
> > --
> > 2.31.1