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Date: Wed, 15 Apr 2020 14:04:51 -0700
From: Kristen Carlson Accardi <kristen@...ux.intel.com>
To: keescook@...omium.org,
tglx@...utronix.de,
mingo@...hat.com,
bp@...en8.de,
hpa@...or.com,
Jessica Yu <jeyu@...nel.org>
Cc: arjan@...ux.intel.com,
x86@...nel.org,
linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com,
rick.p.edgecomb@...el.com
Subject: [PATCH 9/9] module: Reorder functions
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@...ux.intel.com>
Reviewed-by: Kees Cook <keescook@...omium.org>
---
kernel/module.c | 82 +++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 82 insertions(+)
diff --git a/kernel/module.c b/kernel/module.c
index 646f1e2330d2..e432ec5f6df4 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"
@@ -2370,6 +2372,83 @@ static long get_offset(struct module *mod, unsigned int *size,
return ret;
}
+/*
+ * 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--) {
+ /*
+ * 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 (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];
+
+ /*
+ * get_offset has a section index for it's last
+ * argument, that is only used by arch_mod_section_prepend(),
+ * which is only defined by parisc. Since this this type
+ * of randomization isn't supported on parisc, we can
+ * safely pass in zero as the last argument, as it is
+ * ignored.
+ */
+ shdr->sh_entsize = get_offset(mod, &size, shdr, 0);
+ }
+
+ kfree(text_list);
+}
+
/* Lay out the SHF_ALLOC sections in a way not dissimilar to how ld
might -- code, read-only data, read-write data, small data. Tally
sizes, and place the offsets into sh_entsize fields: high bit means it
@@ -2460,6 +2539,9 @@ static void layout_sections(struct module *mod, struct load_info *info)
break;
}
}
+
+ if (IS_ENABLED(CONFIG_FG_KASLR) && kaslr_enabled())
+ randomize_text(mod, info);
}
static void set_license(struct module *mod, const char *license)
--
2.20.1
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