[PATCH v6 kspp-next 19/22] module: Reorder functions

Alexander Lobakin
2021-08-31T14:42:56Z

From: Kristen Carlson Accardi <kristen@linux.intel.com>

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>
Acked-by: Jessica Yu <jeyu@kernel.org>
Tested-by: Jessica Yu <jeyu@kernel.org>
Reported-by: kernel test robot <lkp@intel.com> # swap.cocci
[ alobakin: make it work with ClangCFI ]
Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
---
Makefile | 4 ++
init/Kconfig | 12 ++++++
kernel/kallsyms.c | 4 +-
kernel/livepatch/core.c | 2 +-
kernel/module.c | 91 ++++++++++++++++++++++++++++++++++++++++-
5 files changed, 108 insertions(+), 5 deletions(-)

diff --git a/Makefile b/Makefile
index 283876e170f7..2bde66addb89 100644
--- a/Makefile
+++ b/Makefile
@@ -921,6 +921,10 @@ endif
# ClangLTO implies -ffunction-sections -fdata-sections, no need
# to specify them manually and trigger a pointless full rebuild
ifndef CONFIG_LTO_CLANG
+ifdef CONFIG_MODULE_FG_KASLR
+KBUILD_CFLAGS_MODULE += -ffunction-sections
+endif
+
ifneq ($(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION)$(CONFIG_FG_KASLR),)
KBUILD_CFLAGS_KERNEL += -ffunction-sections
endif
diff --git a/init/Kconfig b/init/Kconfig
index e72633f4f8a9..e8158c256ee9 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -2330,6 +2330,18 @@ config UNUSED_KSYMS_WHITELIST
one per line. The path can be absolute, or relative to the kernel
source tree.

+config MODULE_FG_KASLR
+ bool "Module Function Granular Layout Randomization"
+ default FG_KASLR
+ depends on BROKEN
+ 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 5ffdcc2fb88e..6906cc726149 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -841,7 +841,7 @@ 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.
*/
-#ifdef 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)
{
@@ -985,7 +985,7 @@ static int kallsyms_random_open(struct inode *inode, struct file *file)
#define kallsyms_open kallsyms_random_open
#else
#define kallsyms_open __kallsyms_open
-#endif /* !CONFIG_FG_KASLR */
+#endif /* !CONFIG_FG_KASLR && !CONFIG_MODULE_FG_KASLR */

#ifdef CONFIG_KGDB_KDB
const char *kdb_walk_kallsyms(loff_t *pos)
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 852bbfa9da7b..15c4cd25592d 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -177,7 +177,7 @@ static int klp_find_object_symbol(const char *objname, const char *name,
* force the algorithm to require that only unique symbols are
* allowed to be patched.
*/
- if (IS_ENABLED(CONFIG_FG_KASLR))
+ if (IS_ENABLED(CONFIG_FG_KASLR) || IS_ENABLED(CONFIG_MODULE_FG_KASLR))
sympos = 0;

/*
diff --git a/kernel/module.c b/kernel/module.c
index ed13917ea5f3..08747e5f2442 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -57,6 +57,7 @@
#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"

@@ -1527,7 +1528,7 @@ static void free_sect_attrs(struct module_sect_attrs *sect_attrs)

for (section = 0; section < sect_attrs->nsections; section++)
kfree(sect_attrs->attrs[section].battr.attr.name);
- kfree(sect_attrs);
+ kvfree(sect_attrs);
}

static void add_sect_attrs(struct module *mod, const struct load_info *info)
@@ -1544,7 +1545,7 @@ static void add_sect_attrs(struct module *mod, const struct load_info *info)
size[0] = ALIGN(struct_size(sect_attrs, attrs, nloaded),
sizeof(sect_attrs->grp.bin_attrs[0]));
size[1] = (nloaded + 1) * sizeof(sect_attrs->grp.bin_attrs[0]);
- sect_attrs = kzalloc(size[0] + size[1], GFP_KERNEL);
+ sect_attrs = kvzalloc(size[0] + size[1], GFP_KERNEL);
if (sect_attrs == NULL)
return;

@@ -2416,6 +2417,89 @@ static bool module_init_layout_section(const char *sname)
return module_init_section(sname);
}

+/*
+ * 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)
+{
+ u32 i, j;
+
+ for (i = size - 1; i > 0; i--) {
+ /*
+ * pick a random index from 0 to i
+ */
+ j = get_random_u32() % (i + 1);
+
+ swap(list[i], list[j]);
+ }
+}
+
+/*
+ * 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 max_sections = info->hdr->e_shnum;
+ int num_text_sections = 0;
+ Elf_Shdr **text_list;
+ int i, size;
+
+ text_list = kvmalloc_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) ||
+ (shdr->sh_flags & ARCH_SHF_SMALL) ||
+ module_init_layout_section(sname))
+ continue;
+
+ /*
+ * With CONFIG_CFI_CLANG, .text with __cfi_check() must come
+ * before any other text sections, and be aligned to PAGE_SIZE.
+ * Don't include it in the shuffle list.
+ */
+ if (IS_ENABLED(CONFIG_CFI_CLANG) && !strcmp(sname, ".text"))
+ continue;
+
+ if (!num_text_sections)
+ size = shdr->sh_entsize;
+
+ text_list[num_text_sections] = shdr;
+ num_text_sections++;
+ }
+
+ if (!num_text_sections)
+ goto exit;
+
+ 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 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);
+ }
+
+exit:
+ kvfree(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
@@ -2510,6 +2594,9 @@ 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)
--
2.31.1