Subject: Re: [PATCH v3 11/12] lkdtm: Fix execute_[user]_location()

From: Kees Cook <[email-masked]>
Date: Wed, 19 Jan 2022 14:00:26 -0800

On Wed, Jan 19, 2022 at 08:28:54PM +0100, Christophe Leroy wrote:
> Hi Kees,
> 
> 
> Le 17/12/2021 à 12:49, Christophe Leroy a écrit :
> > Hi Kees,
> > 
> > Le 17/10/2021 à 14:38, Christophe Leroy a écrit :
> > > execute_location() and execute_user_location() intent
> > > to copy do_nothing() text and execute it at a new location.
> > > However, at the time being it doesn't copy do_nothing() function
> > > but do_nothing() function descriptor which still points to the
> > > original text. So at the end it still executes do_nothing() at
> > > its original location allthough using a copied function descriptor.
> > > 
> > > So, fix that by really copying do_nothing() text and build a new
> > > function descriptor by copying do_nothing() function descriptor and
> > > updating the target address with the new location.
> > > 
> > > Also fix the displayed addresses by dereferencing do_nothing()
> > > function descriptor.
> > > 
> > > Signed-off-by: Christophe Leroy <[email-masked]>
> > 
> > Do you have any comment to this patch and to patch 12 ?
> > 
> > If not, is it ok to get your acked-by ?
> 
> Any feedback please, even if it's to say no feedback ?

Hi! Thanks for the ping; I haven't had time yet to look at this, but
with -rc1 coming, I should be able to task-switch back to LKDTM for the
dev cycle and I can give some feedback.

-Kees

> 
> Many thanks,
> Christophe
> 
> > 
> > Thanks
> > Christophe
> > 
> > > ---
> > >   drivers/misc/lkdtm/perms.c | 37 ++++++++++++++++++++++++++++---------
> > >   1 file changed, 28 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> > > index 035fcca441f0..1cf24c4a79e9 100644
> > > --- a/drivers/misc/lkdtm/perms.c
> > > +++ b/drivers/misc/lkdtm/perms.c
> > > @@ -44,19 +44,34 @@ static noinline void do_overwritten(void)
> > >       return;
> > >   }
> > > +static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
> > > +{
> > > +    if (!have_function_descriptors())
> > > +        return dst;
> > > +
> > > +    memcpy(fdesc, do_nothing, sizeof(*fdesc));
> > > +    fdesc->addr = (unsigned long)dst;
> > > +    barrier();
> > > +
> > > +    return fdesc;
> > > +}
> > > +
> > >   static noinline void execute_location(void *dst, bool write)
> > >   {
> > > -    void (*func)(void) = dst;
> > > +    void (*func)(void);
> > > +    func_desc_t fdesc;
> > > +    void *do_nothing_text = dereference_function_descriptor(do_nothing);
> > > -    pr_info("attempting ok execution at %px\n", do_nothing);
> > > +    pr_info("attempting ok execution at %px\n", do_nothing_text);
> > >       do_nothing();
> > >       if (write == CODE_WRITE) {
> > > -        memcpy(dst, do_nothing, EXEC_SIZE);
> > > +        memcpy(dst, do_nothing_text, EXEC_SIZE);
> > >           flush_icache_range((unsigned long)dst,
> > >                      (unsigned long)dst + EXEC_SIZE);
> > >       }
> > > -    pr_info("attempting bad execution at %px\n", func);
> > > +    pr_info("attempting bad execution at %px\n", dst);
> > > +    func = setup_function_descriptor(&fdesc, dst);
> > >       func();
> > >       pr_err("FAIL: func returned\n");
> > >   }
> > > @@ -66,16 +81,19 @@ static void execute_user_location(void *dst)
> > >       int copied;
> > >       /* Intentionally crossing kernel/user memory boundary. */
> > > -    void (*func)(void) = dst;
> > > +    void (*func)(void);
> > > +    func_desc_t fdesc;
> > > +    void *do_nothing_text = dereference_function_descriptor(do_nothing);
> > > -    pr_info("attempting ok execution at %px\n", do_nothing);
> > > +    pr_info("attempting ok execution at %px\n", do_nothing_text);
> > >       do_nothing();
> > > -    copied = access_process_vm(current, (unsigned long)dst, do_nothing,
> > > +    copied = access_process_vm(current, (unsigned long)dst,
> > > do_nothing_text,
> > >                      EXEC_SIZE, FOLL_WRITE);
> > >       if (copied < EXEC_SIZE)
> > >           return;
> > > -    pr_info("attempting bad execution at %px\n", func);
> > > +    pr_info("attempting bad execution at %px\n", dst);
> > > +    func = setup_function_descriptor(&fdesc, dst);
> > >       func();
> > >       pr_err("FAIL: func returned\n");
> > >   }
> > > @@ -153,7 +171,8 @@ void lkdtm_EXEC_VMALLOC(void)
> > >   void lkdtm_EXEC_RODATA(void)
> > >   {
> > > -    execute_location(lkdtm_rodata_do_nothing, CODE_AS_IS);
> > > +    execute_location(dereference_function_descriptor(lkdtm_rodata_do_nothing),
> > > 
> > > +             CODE_AS_IS);
> > >   }
> > >   void lkdtm_EXEC_USERSPACE(void)
> > > 

-- 
Kees Cook