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From:   Kees Cook <keescook@chromium.org>
Date:   Tue, 29 May 2018 07:49:14 -0700
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Subject: Re: [PATCH] x86, random: Fix get_random_bytes() warning in x86 start_kernel
To:     Prarit Bhargava <prarit@redhat.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        "Theodore Ts'o" <tytso@mit.edu>, Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Rik van Riel <riel@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Philippe Ombredanne <pombredanne@nexb.com>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Kate Stewart <kstewart@linuxfoundation.org>
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On Tue, May 29, 2018 at 5:38 AM, Prarit Bhargava <prarit@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 <prarit@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: x86@kernel.org
> Cc: "Theodore Ts'o" <tytso@mit.edu>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Philippe Ombredanne <pombredanne@nexb.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Prarit Bhargava <prarit@redhat.com>
> Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
> Cc: Kate Stewart <kstewart@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