[v9,12/15] module: Reorder functions

Commit Message

Alexander Lobakin Dec. 23, 2021, 12:22 a.m. UTC

From: Kristen Carlson Accardi <[e-mail]>

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 <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Acked-by: Ard Biesheuvel <[e-mail]>
Tested-by: Ard Biesheuvel <[e-mail]>
Reviewed-by: Tony Luck <[e-mail]>
Tested-by: Tony Luck <[e-mail]>
Acked-by: Jessica Yu <[e-mail]>
Tested-by: Jessica Yu <[e-mail]>
Reported-by: kernel test robot <[e-mail]> # swap.cocci
Co-developed-by: Alexander Lobakin <[e-mail]>
Signed-off-by: Alexander Lobakin <[e-mail]>
---
 Makefile                |  6 +++-
 include/linux/linkage.h |  1 +
 init/Kconfig            | 14 ++++++++
 kernel/module.c         | 73 +++++++++++++++++++++++++++++++++++++++--
 4 files changed, 91 insertions(+), 3 deletions(-)

Patch

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)