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

Jessica Yu
2020-02-11T12:39:58Z

+++ 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@linux.intel.com>
>
>Probably a good idea to add Jessica to CC in next version:
> Jessica Yu <jeyu@kernel.org>

Thanks :)

>Reviewed-by: Kees Cook <keescook@chromium.org>
>
>-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