Re: [RFC PATCH 04/11] x86/boot/KASLR: Introduce PRNG for faster shuffling

Andy Lutomirski
2020-02-06T01:11:32Z

> On Feb 5, 2020, at 2:39 PM, Kristen Carlson Accardi <kristen@linux.intel.com> wrote:
>
> ﻿From: Kees Cook <keescook@chromium.org>
>
> This *might* improve shuffling speed at boot. Possibly only marginally.
> This has not yet been tested, and would need to have some performance
> tests run to determine if it helps before merging.

Ugh, don’t do this. Use a real DRBG. Someone is going to break the construction in your patch just to prove they can.

ChaCha20 is a good bet.

>
> (notes from Kristen) - initial performance tests suggest that any
> improvement is indeed marginal. However, this code is useful
> for using a known seed.
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
> ---
> arch/x86/boot/compressed/kaslr.c | 4 +--
> arch/x86/include/asm/kaslr.h | 3 +-
> arch/x86/lib/kaslr.c | 50 +++++++++++++++++++++++++++++++-
> arch/x86/mm/init.c | 2 +-
> arch/x86/mm/kaslr.c | 2 +-
> 5 files changed, 55 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index d7408af55738..ae4dce76a9bd 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -598,7 +598,7 @@ static unsigned long slots_fetch_random(void)
> if (slot_max == 0)
> return 0;
>
> - slot = kaslr_get_random_long("Physical") % slot_max;
> + slot = kaslr_get_random_seed("Physical") % slot_max;
>
> for (i = 0; i < slot_area_index; i++) {
> if (slot >= slot_areas[i].num) {
> @@ -880,7 +880,7 @@ static unsigned long find_random_virt_addr(unsigned long minimum,
> slots = (KERNEL_IMAGE_SIZE - minimum - image_size) /
> CONFIG_PHYSICAL_ALIGN + 1;
>
> - random_addr = kaslr_get_random_long("Virtual") % slots;
> + random_addr = kaslr_get_random_seed("Virtual") % slots;
>
> return random_addr * CONFIG_PHYSICAL_ALIGN + minimum;
> }
> diff --git a/arch/x86/include/asm/kaslr.h b/arch/x86/include/asm/kaslr.h
> index db7ba2feb947..47d5b25e5b11 100644
> --- a/arch/x86/include/asm/kaslr.h
> +++ b/arch/x86/include/asm/kaslr.h
> @@ -2,7 +2,8 @@
> #ifndef _ASM_KASLR_H_
> #define _ASM_KASLR_H_
>
> -unsigned long kaslr_get_random_long(const char *purpose);
> +unsigned long kaslr_get_random_seed(const char *purpose);
> +unsigned long kaslr_get_prandom_long(void);
>
> #ifdef CONFIG_RANDOMIZE_MEMORY
> void kernel_randomize_memory(void);
> diff --git a/arch/x86/lib/kaslr.c b/arch/x86/lib/kaslr.c
> index 2b3eb8c948a3..41b5610855a3 100644
> --- a/arch/x86/lib/kaslr.c
> +++ b/arch/x86/lib/kaslr.c
> @@ -46,7 +46,7 @@ static inline u16 i8254(void)
> return timer;
> }
>
> -unsigned long kaslr_get_random_long(const char *purpose)
> +unsigned long kaslr_get_random_seed(const char *purpose)
> {
> #ifdef CONFIG_X86_64
> const unsigned long mix_const = 0x5d6008cbf3848dd3UL;
> @@ -96,3 +96,51 @@ unsigned long kaslr_get_random_long(const char *purpose)
>
> return random;
> }
> +
> +/*
> + * 64bit variant of Bob Jenkins' public domain PRNG
> + * 256 bits of internal state
> + */
> +struct prng_state {
> + u64 a, b, c, d;
> +};
> +
> +static struct prng_state state;
> +static bool initialized;
> +
> +#define rot(x, k) (((x)<<(k))|((x)>>(64-(k))))
> +static u64 prng_u64(struct prng_state *x)
> +{
> + u64 e;
> +
> + e = x->a - rot(x->b, 7);
> + x->a = x->b ^ rot(x->c, 13);
> + x->b = x->c + rot(x->d, 37);
> + x->c = x->d + e;
> + x->d = e + x->a;
> +
> + return x->d;
> +}
> +
> +static void prng_init(struct prng_state *state)
> +{
> + int i;
> +
> + state->a = kaslr_get_random_seed(NULL);
> + state->b = kaslr_get_random_seed(NULL);
> + state->c = kaslr_get_random_seed(NULL);
> + state->d = kaslr_get_random_seed(NULL);
> +
> + for (i = 0; i < 30; ++i)
> + (void)prng_u64(state);
> +
> + initialized = true;
> +}
> +
> +unsigned long kaslr_get_prandom_long(void)
> +{
> + if (!initialized)
> + prng_init(&state);
> +
> + return prng_u64(&state);
> +}
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index e7bb483557c9..c974dbab2293 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -722,7 +722,7 @@ void __init poking_init(void)
> */
> poking_addr = TASK_UNMAPPED_BASE;
> if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
> - poking_addr += (kaslr_get_random_long("Poking") & PAGE_MASK) %
> + poking_addr += (kaslr_get_random_seed("Poking") & PAGE_MASK) %
> (TASK_SIZE - TASK_UNMAPPED_BASE - 3 * PAGE_SIZE);
>
> if (((poking_addr + PAGE_SIZE) & ~PMD_MASK) == 0)
> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> index dc6182eecefa..b30bd1db7543 100644
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
> @@ -123,7 +123,7 @@ void __init kernel_randomize_memory(void)
> for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
> remain_entropy -= get_padding(&kaslr_regions[i]);
>
> - prandom_seed_state(&rand_state, kaslr_get_random_long("Memory"));
> + prandom_seed_state(&rand_state, kaslr_get_random_seed("Memory"));
>
> for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) {
> unsigned long entropy;
> --
> 2.24.1
>