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Message-ID: <202005211415.5A1ECC638@keescook>
Date: Thu, 21 May 2020 14:33:22 -0700
From: Kees Cook <keescook@...omium.org>
To: Kristen Carlson Accardi <kristen@...ux.intel.com>
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
"H. Peter Anvin" <hpa@...or.com>, Jessica Yu <jeyu@...nel.org>,
arjan@...ux.intel.com, linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com, rick.p.edgecombe@...el.com,
Ard Biesheuvel <ardb@...nel.org>, Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH v2 9/9] module: Reorder functions
On Thu, May 21, 2020 at 09:56:40AM -0700, Kristen Carlson Accardi wrote:
> 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@...ux.intel.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> Acked-by: Ard Biesheuvel <ardb@...nel.org>
> Tested-by: Ard Biesheuvel <ardb@...nel.org>
> Reviewed-by: Tony Luck <tony.luck@...el.com>
> Tested-by: Tony Luck <tony.luck@...el.com>
> ---
> arch/x86/Kconfig | 1 +
> arch/x86/Makefile | 3 ++
> init/Kconfig | 11 +++++++
> kernel/module.c | 81 +++++++++++++++++++++++++++++++++++++++++++++++
> 4 files changed, 96 insertions(+)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 50e83ea57d70..d0bdd5c8c432 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2187,6 +2187,7 @@ config FG_KASLR
> bool "Function Granular Kernel Address Space Layout Randomization"
> depends on $(cc-option, -ffunction-sections)
> depends on RANDOMIZE_BASE && X86_64
> + select MODULE_FG_KASLR
> help
> This option improves the randomness of the kernel text
> over basic Kernel Address Space Layout Randomization (KASLR)
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index b65ec63c7db7..8c830c37c74c 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -51,6 +51,9 @@ ifdef CONFIG_X86_NEED_RELOCS
> LDFLAGS_vmlinux := --emit-relocs --discard-none
> endif
>
> +ifdef CONFIG_MODULE_FG_KASLR
> + KBUILD_CFLAGS_MODULE += -ffunction-sections
> +endif
With CONFIG_FG_KASLR=y, will -ffunction-sections appear twice on the
compiler command line?
> #
> # Prevent GCC from generating any FP code by mistake.
> #
> diff --git a/init/Kconfig b/init/Kconfig
> index 74a5ac65644f..b19920413bcc 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -2227,6 +2227,17 @@ config UNUSED_KSYMS_WHITELIST
> one per line. The path can be absolute, or relative to the kernel
> source tree.
>
> +config MODULE_FG_KASLR
Oh, er, yes. I'd suggested moving 'config FG_KASLR' into arch/Kconfig,
but I think init/Kconfig is more correct.
> + depends on $(cc-option, -ffunction-sections)
Oh! And I am reminded suddenly about CONFIG_FG_KASLR needing to interact
correctly with CONFIG_LD_DEAD_CODE_DATA_ELIMINATION in that we do NOT
want the sections to be collapsed at link time:
#ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
#define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
(I think I had fixed this in some earlier version?)
I think you want this (untested):
diff --git a/Makefile b/Makefile
index 04f5662ae61a..a0d9acd3b900 100644
--- a/Makefile
+++ b/Makefile
@@ -853,8 +853,11 @@ ifdef CONFIG_DEBUG_SECTION_MISMATCH
KBUILD_CFLAGS += $(call cc-option, -fno-inline-functions-called-once)
endif
+ifneq ($(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION)$(CONFIG_FG_KASLR),)
+KBUILD_CFLAGS_KERNEL += -ffunction-sections
+endif
ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
-KBUILD_CFLAGS_KERNEL += -ffunction-sections -fdata-sections
+KBUILD_CFLAGS_KERNEL += -fdata-sections
LDFLAGS_vmlinux += --gc-sections
endif
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 71e387a5fe90..5f5c692751dd 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -93,20 +93,31 @@
* sections to be brought in with rodata.
*/
#ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
-#define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..LPBX*
#define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*
#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]*
#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]*
#define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]*
#else
-#define TEXT_MAIN .text
#define DATA_MAIN .data
#define SDATA_MAIN .sdata
#define RODATA_MAIN .rodata
#define BSS_MAIN .bss
#define SBSS_MAIN .sbss
#endif
+/*
+ * Both LD_DEAD_CODE_DATA_ELIMINATION and CONFIG_FG_KASLR options enable
+ * -ffunction-sections, which produces separately named .text sections. In
+ * the case of CONFIG_FG_KASLR, they need to stay distinct so they can be
+ * separately randomized. Without CONFIG_FG_KASLR, the separate .text
+ * sections can be collected back into a common section, which makes the
+ * resulting image slightly smaller.
+ */
+#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) && !defined(CONFIG_FG_KASLR)
+#define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
+#else
+#define TEXT_MAIN .text
+#endif
/*
* Align to a 32 byte boundary equal to the
> + 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/module.c b/kernel/module.c
> index 646f1e2330d2..e3cd619c60c2 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -53,6 +53,7 @@
> #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"
>
> @@ -2370,6 +2371,83 @@ 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
> @@ -2460,6 +2538,9 @@ 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
>
Everything else looks good.
--
Kees Cook
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