Re: [PATCH v1 10/10] lkdtm: Fix execute_[user]_location()

[Christophe Leroy]

Le 13/10/2021 à 09:16, Kees Cook a écrit :
> On Mon, Oct 11, 2021 at 05:25:37PM +0200, Christophe Leroy wrote:
> > 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 <christophe.leroy@xxxxxxxxxx>
> > ---
> >   drivers/misc/lkdtm/perms.c | 45 +++++++++++++++++++++++++++++++-------
> >   1 file changed, 37 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> > index da16564e1ecd..1d03cd44c21d 100644
> > --- a/drivers/misc/lkdtm/perms.c
> > +++ b/drivers/misc/lkdtm/perms.c
> > @@ -44,19 +44,42 @@ static noinline void do_overwritten(void)
> >   	return;
> >   }
> >   
> > +static void *setup_function_descriptor(funct_descr_t *fdesc, void *dst)
> > +{
> > +	int err;
> > +
> > +	if (!__is_defined(HAVE_DEREFERENCE_FUNCTION_DESCRIPTOR))
> > +		return dst;
> > +
> > +	err = copy_from_kernel_nofault(fdesc, do_nothing, sizeof(*fdesc));
> > +	if (err < 0)
> > +		return ERR_PTR(err);
> > +
> > +	fdesc->addr = (unsigned long)dst;
> > +	barrier();
> > +
> > +	return fdesc;
> > +}
> > +
> >   static noinline void execute_location(void *dst, bool write)
> >   {
> > -	void (*func)(void) = dst;
> > +	void (*func)(void);
> > +	funct_descr_t fdesc;
> > +	void *do_nothing_text = dereference_symbol_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);
> > +	func = setup_function_descriptor(&fdesc, dst);
> > +	if (IS_ERR(func))
> > +		return;
>
> I think this error case should complain with some details. :) Maybe:
>
> 	pr_err("FAIL: could not build function descriptor for %px\n", dst);

Ok, I was going to add that in v2, but after one more thought I realise 
that there's no need to use copy_from_kernel_nofault() here, we are 
copying a static object into our stack, so a memcpy() will be good enough.

> > +
> > +	pr_info("attempting bad execution at %px\n", dst);
>
> And then leave this pr_info() as it was, before the
> setup_function_descriptor() call.
>
> >   	func();
> >   	pr_err("FAIL: func returned\n");
> >   }
> > @@ -66,16 +89,22 @@ static void execute_user_location(void *dst)
> >   	int copied;
> >   
> >   	/* Intentionally crossing kernel/user memory boundary. */
> > -	void (*func)(void) = dst;
> > +	void (*func)(void);
> > +	funct_descr_t fdesc;
> > +	void *do_nothing_text = dereference_symbol_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);
> > +	func = setup_function_descriptor(&fdesc, dst);
> > +	if (IS_ERR(func))
> > +		return;
> > +
> > +	pr_info("attempting bad execution at %px\n", dst);
>
> Same here.
>
> >   	func();
> >   	pr_err("FAIL: func returned\n");
> >   }
> > --
> > 2.31.1