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

[Kees Cook]

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>

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;
> +
> +	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;
> +		infosec	= shdr->sh_info;
> +
> +		shdr->sh_entsize = get_offset(mod, &size, shdr, infosec);
> +	}
> +
> +	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);
> +
>  	/* Transfer each section which specifies SHF_ALLOC */
>  	pr_debug("final section addresses:\n");
>  	for (i = 0; i < info->hdr->e_shnum; i++) {
> -- 
> 2.24.1
> 

-- 
Kees Cook