+++ 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
