On Tue, May 29, 2018 at 5:38 AM, Prarit Bhargava <pra...@redhat.com> wrote:
> After 43838a23a05f ("random: fix crng_ready() test") early boot calls
> to get_random_bytes() will warn on each cpu on x86 because the crng
> is not initialized. For example,
>
> random: get_random_bytes called from start_kernel+0x8e/0x587 with crng_init=0
>
> x86 only uses get_random_bytes() for better randomization of the stack
> canary value so the warning is of no consequence.
>
> Export crng_ready() for x86 and test if the crng is initialized before
> calling get_random_bytes().
NAK. This leaves the stack canary with very little entropy. This needs
to pull from whatever pool is available, not skip it.
-Kees
>
> Signed-off-by: Prarit Bhargava <pra...@redhat.com>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: Ingo Molnar <mi...@redhat.com>
> Cc: "H. Peter Anvin" <h...@zytor.com>
> Cc: x...@kernel.org
> Cc: "Theodore Ts'o" <ty...@mit.edu>
> Cc: Arnd Bergmann <a...@arndb.de>
> Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org>
> Cc: Rik van Riel <r...@redhat.com>
> Cc: Andrew Morton <a...@linux-foundation.org>
> Cc: Philippe Ombredanne <pombreda...@nexb.com>
> Cc: Kees Cook <keesc...@chromium.org>
> Cc: Prarit Bhargava <pra...@redhat.com>
> Cc: "Jason A. Donenfeld" <ja...@zx2c4.com>
> Cc: Kate Stewart <kstew...@linuxfoundation.org>
> ---
> arch/x86/include/asm/stackprotector.h | 3 ++-
> drivers/char/random.c | 5 ++++-
> include/linux/random.h | 1 +
> 3 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/stackprotector.h
> b/arch/x86/include/asm/stackprotector.h
> index 371b3a4af000..4e2223aa34fc 100644
> --- a/arch/x86/include/asm/stackprotector.h
> +++ b/arch/x86/include/asm/stackprotector.h
> @@ -72,7 +72,8 @@ static __always_inline void boot_init_stack_canary(void)
> * there it already has some randomness on most systems. Later
> * on during the bootup the random pool has true entropy too.
> */
> - get_random_bytes(&canary, sizeof(canary));
> + if (crng_ready())
> + get_random_bytes(&canary, sizeof(canary));
> tsc = rdtsc();
> canary += tsc + (tsc << 32UL);
> canary &= CANARY_MASK;
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index cd888d4ee605..003091d104bf 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -428,7 +428,10 @@ struct crng_state primary_crng = {
> * its value (from 0->1->2).
> */
> static int crng_init = 0;
> -#define crng_ready() (likely(crng_init > 1))
> +int crng_ready(void)
> +{
> + return likely(crng_init > 1);
> +}
> static int crng_init_cnt = 0;
> static unsigned long crng_global_init_time = 0;
> #define CRNG_INIT_CNT_THRESH (2*CHACHA20_KEY_SIZE)
> diff --git a/include/linux/random.h b/include/linux/random.h
> index 2ddf13b4281e..45616513abd9 100644
> --- a/include/linux/random.h
> +++ b/include/linux/random.h
> @@ -196,4 +196,5 @@ static inline u32 next_pseudo_random32(u32 seed)
> return seed * 1664525 + 1013904223;
> }
>
> +extern int crng_ready(void);
> #endif /* _LINUX_RANDOM_H */
> --
> 2.14.3
>
--
Kees Cook
Pixel Security
