[PATCH v9 12/15] module: Reorder functions

Alexander Lobakin
2021-12-23T00:26:38Z

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.

alobakin:
Make it work with ClangCFI -- in such builds, .text section must
always come first and be page-aligned. Exclude it from the shuffle
list and leave as it is.
Make this feature depend on `-z unique-symbol` as well, due to the
very same reasons.
Traditionally, use common shuffle_array() from <linux/random.h>.

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
Co-developed-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
---
Makefile | 6 +++-
include/linux/linkage.h | 1 +
init/Kconfig | 14 ++++++++
kernel/module.c | 73 +++++++++++++++++++++++++++++++++++++++--
4 files changed, 91 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index 3346269341d4..74d270c77d96 100644
--- a/Makefile
+++ b/Makefile
@@ -900,7 +900,7 @@ export SECSUBST_AFLAGS
endif

# Same for modules. LD DCE doesn't work for them, thus not checking for it
-ifneq ($(CONFIG_LTO_CLANG),)
+ifneq ($(CONFIG_MODULE_FG_KASLR)$(CONFIG_LTO_CLANG),)
KBUILD_AFLAGS_MODULE += -Wa,--sectname-subst
KBUILD_CFLAGS_MODULE += -Wa,--sectname-subst
endif
@@ -909,6 +909,10 @@ endif # CONFIG_HAVE_ASM_FUNCTION_SECTIONS
# ClangLTO implies `-ffunction-sections -fdata-sections`, no need
# to specify them manually and trigger a pointless full rebuild
ifndef CONFIG_LTO_CLANG
+ifdef CONFIG_MODULE_FG_KASLR
+KBUILD_CFLAGS_MODULE += -ffunction-sections
+endif
+
ifneq ($(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION)$(CONFIG_FG_KASLR),)
KBUILD_CFLAGS_KERNEL += -ffunction-sections
endif
diff --git a/include/linux/linkage.h b/include/linux/linkage.h
index f3c96fb6a534..deb26069278a 100644
--- a/include/linux/linkage.h
+++ b/include/linux/linkage.h
@@ -80,6 +80,7 @@
#if defined(CONFIG_HAVE_ASM_FUNCTION_SECTIONS) && \
((defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) && !defined(MODULE)) || \
(defined(CONFIG_FG_KASLR) && !defined(MODULE)) || \
+ (defined(CONFIG_MODULE_FG_KASLR) && defined(MODULE)) || \
(defined(CONFIG_LTO_CLANG)))

#define SYM_PUSH_SECTION(name) \
diff --git a/init/Kconfig b/init/Kconfig
index 381b063b4925..38c82e21efd7 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -2376,6 +2376,20 @@ 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"
+ depends on $(cc-option,-ffunction-sections)
+ depends on LD_HAS_Z_UNIQUE_SYMBOL || !LIVEPATCH
+ 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/module.c b/kernel/module.c
index 84a9141a5e15..802e1098eaf4 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 +2417,71 @@ static bool module_init_layout_section(const char *sname)
return module_init_section(sname);
}

+/*
+ * 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_array(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 +2576,9 @@ 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)
--
2.33.1