Re: [RFC PATCH 10/11] module: Reorder functions

[Jessica Yu]

+++ Kees Cook [06/02/20 04:41 -0800]:
> On Wed, Feb 05, 2020 at 02:39:49PM -0800, Kristen Carlson Accardi wrote:
> > If a module has functions split out into separate text sections
> > (i.e. compiled with the -ffunction-sections flag), reorder the
> > functions to provide some code diversification to modules.
> >
> > Signed-off-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
>
> Probably a good idea to add Jessica to CC in next version:
> 	Jessica Yu <jeyu@xxxxxxxxxx>

Thanks :)

> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>
> -Kees
>
> > ---
> >  kernel/module.c | 85 +++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 85 insertions(+)
> >
> > diff --git a/kernel/module.c b/kernel/module.c
> > index b56f3224b161..231563e95e61 100644
> > --- a/kernel/module.c
> > +++ b/kernel/module.c
> > @@ -53,6 +53,8 @@
> >  #include <linux/bsearch.h>
> >  #include <linux/dynamic_debug.h>
> >  #include <linux/audit.h>
> > +#include <linux/random.h>
> > +#include <asm/setup.h>
> >  #include <uapi/linux/module.h>
> >  #include "module-internal.h"
> >
> > @@ -3245,6 +3247,87 @@ static int find_module_sections(struct module *mod, struct load_info *info)
> >  	return 0;
> >  }
> >
> > +/*
> > + * shuffle_text_list()
> > + * Use a Fisher Yates algorithm to shuffle a list of text sections.
> > + */
> > +static void shuffle_text_list(Elf_Shdr **list, int size)
> > +{
> > +	int i;
> > +	unsigned int j;
> > +	Elf_Shdr *temp;
> > +
> > +	for (i = size - 1; i > 0; i--) {
> > +		/*
> > +		 * TBD - seed. We need to be able to use a known
> > +		 * seed so that we can non-randomly randomize for
> > +		 * debugging.
> > +		 */
> > +
> > +		// pick a random index from 0 to i
> > +		get_random_bytes(&j, sizeof(j));
> > +		j = j % (i + 1);
> > +
> > +		temp = list[i];
> > +		list[i] = list[j];
> > +		list[j] = temp;
> > +	}
> > +}
> > +
> > +/*
> > + * randomize_text()
> > + * Look through the core section looking for executable code sections.
> > + * Store sections in an array and then shuffle the sections
> > + * to reorder the functions.
> > + */
> > +static void randomize_text(struct module *mod, struct load_info *info)
> > +{
> > +	int i;
> > +	int num_text_sections = 0;
> > +	Elf_Shdr **text_list;
> > +	int size = 0;
> > +	int max_sections = info->hdr->e_shnum;
> > +	unsigned int sec = find_sec(info, ".text");
> > +
> > +	if (!IS_ENABLED(CONFIG_FG_KASLR) || !kaslr_enabled())
> > +		return;

Maybe put this conditional before the call to randomize_text() in
move_module()? Just for code readability reasons (i.e., we only call
randomize_text() when CONFIG_FG_KASLR || kaslr_enabled()). But this is
just a matter of preference.

> > +
> > +	if (sec == 0)
> > +		return;
> > +
> > +	text_list = kmalloc_array(max_sections, sizeof(*text_list), GFP_KERNEL);
> > +	if (text_list == NULL)
> > +		return;
> > +
> > +	for (i = 0; i < max_sections; i++) {
> > +		Elf_Shdr *shdr = &info->sechdrs[i];
> > +		const char *sname = info->secstrings + shdr->sh_name;
> > +
> > +		if (!(shdr->sh_flags & SHF_ALLOC) ||
> > +		    !(shdr->sh_flags & SHF_EXECINSTR) ||
> > +		    strstarts(sname, ".init"))
> > +			continue;
> > +
> > +		text_list[num_text_sections] = shdr;
> > +		num_text_sections++;
> > +	}
> > +
> > +	shuffle_text_list(text_list, num_text_sections);
> > +
> > +	for (i = 0; i < num_text_sections; i++) {
> > +		Elf_Shdr *shdr = text_list[i];
> > +		unsigned int infosec;
> > +		const char *sname;
> > +
> > +		sname = info->secstrings + shdr->sh_name;

sname doesn't appear to be used after this (?)

> > +		infosec	= shdr->sh_info;
> > +
> > +		shdr->sh_entsize = get_offset(mod, &size, shdr, infosec);

get_offset() expects a section index as the last argument, but sh_info
is 0 for text sections (SHT_PROGBITS). It's really only used in
arch_mod_section_prepend() though, which is only defined for parisc.
We could perhaps save the section index in sh_info in the previous for
loop (hacky, I have to double check if this is actually safe, but it
seems sh_info only has signficance for SHT_REL* and SHT_SYMTAB
sections).

> > +	}
> > +
> > +	kfree(text_list);
> > +}
> > +
> >  static int move_module(struct module *mod, struct load_info *info)
> >  {
> >  	int i;
> > @@ -3282,6 +3365,8 @@ static int move_module(struct module *mod, struct load_info *info)
> >  	} else
> >  		mod->init_layout.base = NULL;
> >
> > +	randomize_text(mod, info);

Hm, I wonder if we should not incorporate the randomize_text() logic into
layout_sections() instead of move_module(), since logically speaking
layout_sections() determines all the section offsets and stores them
in sh_entsize, whereas move_module() does all the allocations and
copying to final destination. Just a thought.


Thanks!

Jessica