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Subject[PATCH v4 10/10] module: Reorder functions
Date
Introduce a new config option to allow modules to be re-ordered
by function. This option can be enabled independently of the
kernel text KASLR or FG_KASLR settings so that it can be used
by architectures that do not support either of these features.
This option will be selected by default if CONFIG_FG_KASLR is
selected.

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>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Tested-by: Tony Luck <tony.luck@intel.com>
---
arch/x86/Makefile | 5 +++
init/Kconfig | 12 +++++++
kernel/kallsyms.c | 2 +-
kernel/module.c | 81 +++++++++++++++++++++++++++++++++++++++++++++++
4 files changed, 99 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 00e378de8bc0..0f2dbc46eb5c 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -51,6 +51,11 @@ ifdef CONFIG_X86_NEED_RELOCS
LDFLAGS_vmlinux := --emit-relocs --discard-none
endif

+ifndef CONFIG_FG_KASLR
+ ifdef CONFIG_MODULE_FG_KASLR
+ KBUILD_CFLAGS_MODULE += -ffunction-sections
+ endif
+endif
#
# Prevent GCC from generating any FP code by mistake.
#
diff --git a/init/Kconfig b/init/Kconfig
index 82f042a1062f..b4741838da40 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1994,6 +1994,7 @@ config FG_KASLR
bool "Function Granular Kernel Address Space Layout Randomization"
depends on $(cc-option, -ffunction-sections)
depends on ARCH_HAS_FG_KASLR
+ select MODULE_FG_KASLR
default n
help
This option improves the randomness of the kernel text
@@ -2278,6 +2279,17 @@ config UNUSED_KSYMS_WHITELIST
one per line. The path can be absolute, or relative to the kernel
source tree.

+config MODULE_FG_KASLR
+ depends on $(cc-option, -ffunction-sections)
+ bool "Module Function Granular Layout Randomization"
+ help
+ This option randomizes the module text section by reordering the text
+ section by function at module load time. In order to use this
+ feature, the module must have been compiled with the
+ -ffunction-sections compiler flag.
+
+ If unsure, say N.
+
endif # MODULES

config MODULES_TREE_LOOKUP
diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 45d147f7f10e..e3f7d0fd3270 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -692,7 +692,7 @@ static int __kallsyms_open(struct inode *inode, struct file *file)
* When function granular kaslr is enabled, we need to print out the symbols
* at random so we don't reveal the new layout.
*/
-#if defined(CONFIG_FG_KASLR)
+#if defined(CONFIG_FG_KASLR) || defined(CONFIG_MODULE_FG_KASLR)
static int update_random_pos(struct kallsyms_shuffled_iter *s_iter,
loff_t pos, loff_t *new_pos)
{
diff --git a/kernel/module.c b/kernel/module.c
index aa183c9ac0a2..0f4f4e567a42 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -56,6 +56,7 @@
#include <linux/bsearch.h>
#include <linux/dynamic_debug.h>
#include <linux/audit.h>
+#include <linux/random.h>
#include <uapi/linux/module.h>
#include "module-internal.h"

@@ -2391,6 +2392,83 @@ 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)
+ 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
@@ -2481,6 +2559,9 @@ static void layout_sections(struct module *mod, struct load_info *info)
break;
}
}
+
+ if (IS_ENABLED(CONFIG_MODULE_FG_KASLR))
+ randomize_text(mod, info);
}

static void set_license(struct module *mod, const char *license)
--
2.20.1
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