Re: [PATCH v2] x86, kaslr: propagate base load address calculation

Kees Cook
2015-02-13T17:49:50Z

On Fri, Feb 13, 2015 at 7:04 AM, Jiri Kosina <jkosina@suse.cz> wrote:
> Commit e2b32e678 ("x86, kaslr: randomize module base load address") makes
> the base address for module to be unconditionally randomized in case when
> CONFIG_RANDOMIZE_BASE is defined and "nokaslr" option isn't present on the
> commandline.
>
> This is not consistent with how choose_kernel_location() decides whether
> it will randomize kernel load base.
>
> Namely, CONFIG_HIBERNATION disables kASLR (unless "kaslr" option is
> explicitly specified on kernel commandline), which makes the state space
> larger than what module loader is looking at. IOW CONFIG_HIBERNATION &&
> CONFIG_RANDOMIZE_BASE is a valid config option, kASLR wouldn't be applied
> by default in that case, but module loader is not aware of that.
>
> Instead of fixing the logic in module.c, this patch takes more generic
> aproach. It introduces a new bootparam setup data_type SETUP_KASLR and
> uses that to pass the information whether kaslr has been applied during
> kernel decompression, and sets a global 'kaslr_enabled' variable
> accordingly, so that any kernel code (module loading, livepatching, ...)
> can make decisions based on its value.
>
> x86 module loader is converted to make use of this flag.
>
> Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Thanks for working on this! If others are happy with the setup_data
approach, I think this is fine. My only concern is confusion over
seeing SETUP_KASLR that was added by a boot loader.

Another way to handle it might be to do some kind of relocs-like
poking of a value into the decompressed kernel?

> ---
>
> v1 -> v2:
>
> Originally I just calculated the fact on the fly from difference between
> __START_KERNEL and &text, but Kees correctly pointed out that this doesn't
> properly catch the case when the offset is randomized to zero. I don't see
> a better option how to propagate the information from
> choose_kernel_location() to the decompressed kernel than introducing new
> bootparam setup type. Comments welcome.
>
> arch/x86/boot/compressed/aslr.c | 34 +++++++++++++++++++++++++++++++++-
> arch/x86/boot/compressed/misc.c | 3 ++-
> arch/x86/boot/compressed/misc.h | 6 ++++--
> arch/x86/include/asm/page_types.h | 3 +++
> arch/x86/include/uapi/asm/bootparam.h | 1 +
> arch/x86/kernel/module.c | 11 ++---------
> arch/x86/kernel/setup.c | 10 ++++++++++
> 7 files changed, 55 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/aslr.c b/arch/x86/boot/compressed/aslr.c
> index bb13763..d9d1da9 100644
> --- a/arch/x86/boot/compressed/aslr.c
> +++ b/arch/x86/boot/compressed/aslr.c
> @@ -14,6 +14,13 @@
> static const char build_str[] = UTS_RELEASE " (" LINUX_COMPILE_BY "@"
> LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION;
>
> +struct kaslr_setup_data {

Should this be "static"?

> + __u64 next;
> + __u32 type;
> + __u32 len;
> + __u8 data[1];
> +} kaslr_setup_data;
> +
> #define I8254_PORT_CONTROL 0x43
> #define I8254_PORT_COUNTER0 0x40
> #define I8254_CMD_READBACK 0xC0
> @@ -295,7 +302,29 @@ static unsigned long find_random_addr(unsigned long minimum,
> return slots_fetch_random();
> }
>
> -unsigned char *choose_kernel_location(unsigned char *input,
> +static void add_kaslr_setup_data(struct boot_params *params, __u8 enabled)
> +{
> + struct setup_data *data;
> +
> + kaslr_setup_data.type = SETUP_KASLR;
> + kaslr_setup_data.len = 1;
> + kaslr_setup_data.next = 0;
> + kaslr_setup_data.data[0] = enabled;
> +
> + data = (struct setup_data *)(unsigned long)params->hdr.setup_data;
> +
> + while (data && data->next)
> + data = (struct setup_data *)(unsigned long)data->next;
> +
> + if (data)
> + data->next = (unsigned long)&kaslr_setup_data;
> + else
> + params->hdr.setup_data = (unsigned long)&kaslr_setup_data;
> +
> +}
> +
> +unsigned char *choose_kernel_location(struct boot_params *params,
> + unsigned char *input,
> unsigned long input_size,
> unsigned char *output,
> unsigned long output_size)
> @@ -306,14 +335,17 @@ unsigned char *choose_kernel_location(unsigned char *input,
> #ifdef CONFIG_HIBERNATION
> if (!cmdline_find_option_bool("kaslr")) {
> debug_putstr("KASLR disabled by default...\n");
> + add_kaslr_setup_data(params, 0);
> goto out;
> }
> #else
> if (cmdline_find_option_bool("nokaslr")) {
> debug_putstr("KASLR disabled by cmdline...\n");
> + add_kaslr_setup_data(params, 0);
> goto out;
> }
> #endif
> + add_kaslr_setup_data(params, 1);
>
> /* Record the various known unsafe memory ranges. */
> mem_avoid_init((unsigned long)input, input_size,
> diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> index dcc1c53..5aecf56 100644
> --- a/arch/x86/boot/compressed/misc.c
> +++ b/arch/x86/boot/compressed/misc.c
> @@ -399,7 +399,8 @@ asmlinkage __visible void *decompress_kernel(void *rmode, memptr heap,
> * the entire decompressed kernel plus relocation table, or the
> * entire decompressed kernel plus .bss and .brk sections.
> */
> - output = choose_kernel_location(input_data, input_len, output,
> + output = choose_kernel_location(real_mode, input_data, input_len,
> + output,
> output_len > run_size ? output_len
> : run_size);
>
> diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
> index 24e3e56..6d67307 100644
> --- a/arch/x86/boot/compressed/misc.h
> +++ b/arch/x86/boot/compressed/misc.h
> @@ -56,7 +56,8 @@ int cmdline_find_option_bool(const char *option);
>
> #if CONFIG_RANDOMIZE_BASE
> /* aslr.c */
> -unsigned char *choose_kernel_location(unsigned char *input,
> +unsigned char *choose_kernel_location(struct boot_params *params,
> + unsigned char *input,
> unsigned long input_size,
> unsigned char *output,
> unsigned long output_size);
> @@ -64,7 +65,8 @@ unsigned char *choose_kernel_location(unsigned char *input,
> bool has_cpuflag(int flag);
> #else
> static inline
> -unsigned char *choose_kernel_location(unsigned char *input,
> +unsigned char *choose_kernel_location(struct boot_params *params,
> + unsigned char *input,
> unsigned long input_size,
> unsigned char *output,
> unsigned long output_size)
> diff --git a/arch/x86/include/asm/page_types.h b/arch/x86/include/asm/page_types.h
> index f97fbe3..3d43ce3 100644
> --- a/arch/x86/include/asm/page_types.h
> +++ b/arch/x86/include/asm/page_types.h
> @@ -3,6 +3,7 @@
>
> #include <linux/const.h>
> #include <linux/types.h>
> +#include <asm/bootparam.h>
>
> /* PAGE_SHIFT determines the page size */
> #define PAGE_SHIFT 12
> @@ -51,6 +52,8 @@ extern int devmem_is_allowed(unsigned long pagenr);
> extern unsigned long max_low_pfn_mapped;
> extern unsigned long max_pfn_mapped;
>
> +extern bool kaslr_enabled;
> +
> static inline phys_addr_t get_max_mapped(void)
> {
> return (phys_addr_t)max_pfn_mapped << PAGE_SHIFT;
> diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h
> index 225b098..44e6dd7 100644
> --- a/arch/x86/include/uapi/asm/bootparam.h
> +++ b/arch/x86/include/uapi/asm/bootparam.h
> @@ -7,6 +7,7 @@
> #define SETUP_DTB 2
> #define SETUP_PCI 3
> #define SETUP_EFI 4
> +#define SETUP_KASLR 5
>
> /* ram_size flags */
> #define RAMDISK_IMAGE_START_MASK 0x07FF
> diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
> index e69f988..c3c59a3 100644
> --- a/arch/x86/kernel/module.c
> +++ b/arch/x86/kernel/module.c
> @@ -32,6 +32,7 @@
>
> #include <asm/page.h>
> #include <asm/pgtable.h>
> +#include <asm/page_types.h>
>
> #if 0
> #define DEBUGP(fmt, ...) \
> @@ -46,21 +47,13 @@ do { \
>
> #ifdef CONFIG_RANDOMIZE_BASE
> static unsigned long module_load_offset;
> -static int randomize_modules = 1;
>
> /* Mutex protects the module_load_offset. */
> static DEFINE_MUTEX(module_kaslr_mutex);
>
> -static int __init parse_nokaslr(char *p)
> -{
> - randomize_modules = 0;
> - return 0;
> -}
> -early_param("nokaslr", parse_nokaslr);
> -
> static unsigned long int get_module_load_offset(void)
> {
> - if (randomize_modules) {
> + if (kaslr_enabled) {
> mutex_lock(&module_kaslr_mutex);
> /*
> * Calculate the module_load_offset the first time this
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index ab4734e..78c91bb 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -121,6 +121,8 @@
> unsigned long max_low_pfn_mapped;
> unsigned long max_pfn_mapped;
>
> +bool __read_mostly kaslr_enabled = false;
> +
> #ifdef CONFIG_DMI
> RESERVE_BRK(dmi_alloc, 65536);
> #endif
> @@ -424,6 +426,11 @@ static void __init reserve_initrd(void)
> }
> #endif /* CONFIG_BLK_DEV_INITRD */
>
> +static void __init parse_kaslr_setup(u64 pa_data, u32 data_len)
> +{
> + kaslr_enabled = (bool)(pa_data + sizeof(struct setup_data));
> +}
> +
> static void __init parse_setup_data(void)
> {
> struct setup_data *data;
> @@ -451,6 +458,9 @@ static void __init parse_setup_data(void)
> case SETUP_EFI:
> parse_efi_setup(pa_data, data_len);
> break;
> + case SETUP_KASLR:
> + parse_kaslr_setup(pa_data, data_len);
> + break;
> default:
> break;
> }
> --
> Jiri Kosina
> SUSE Labs

-Kees

--
Kees Cook
Chrome OS Security