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@xxxxxxxxxxxxxxx> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> Acked-by: Ard Biesheuvel <ardb@xxxxxxxxxx> Tested-by: Ard Biesheuvel <ardb@xxxxxxxxxx> Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx> Tested-by: Tony Luck <tony.luck@xxxxxxxxx> --- 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 e29c032e4d66..87706a08a2ca 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 df2b20e1b7f2..da65593bffc7 100644 --- a/kernel/kallsyms.c +++ b/kernel/kallsyms.c @@ -761,7 +761,7 @@ int get_all_symbol_name(void *data, const char *name, struct module *mod, return 0; } -#if defined(CONFIG_FG_KASLR) +#if defined(CONFIG_FG_KASLR) || defined(CONFIG_MODULE_FG_KASLR) /* * When fine grained kaslr is enabled, we need to * print out the symbols sorted by name rather than by diff --git a/kernel/module.c b/kernel/module.c index e8a198588f26..ff1e82b54127 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" @@ -2388,6 +2389,83 @@ 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 @@ -2478,6 +2556,9 @@ 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