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

[Andy Lutomirski]

> On Feb 5, 2020, at 2:39 PM, Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx> wrote:
> 
> ïFrom: Kees Cook <keescook@xxxxxxxxxxxx>
> 
> 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, donât 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@xxxxxxxxxxxx>
> Signed-off-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
> ---
> 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
>