Re: [PATCH v3] random: fix crash on multiple early calls to add_bootloader_randomness()

[Ivan T. Ivanov]

On 11-05 07:04, Dominik Brodowski wrote:
> Date: Fri, 5 Nov 2021 07:04:36 +0100
> From: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
> To: tytso@xxxxxxx
> Cc: "Ivan T. Ivanov" <iivanov@xxxxxxx>, Ard Biesheuvel <ardb@xxxxxxxxxx>,
>  linux-efi@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
> Subject: [PATCH v3] random: fix crash on multiple early calls to
>  add_bootloader_randomness()
> Message-ID: <YYTJdLuuFAShnblb@xxxxxxxxxxxxxxxxxxxxxxxxxx>
Tags: all dt linux me watch
> 

Hi,

> If add_bootloader_randomness() or add_hwgenerator_randomness() is
> called for the first time during early boot, crng_init equals 0. Then,
> crng_fast_load() gets called -- which is safe to do even if the input
> pool is not yet properly set up.
> 
> If the added entropy suffices to increase crng_init to 1, future calls
> to add_bootloader_randomness() or add_hwgenerator_randomness() used to
> progress to credit_entropy_bits(). However, if the input pool is not yet
> properly set up, the cmpxchg call within that function can lead to an
> infinite recursion. This is not only a hypothetical problem, as qemu
> on arm64 may provide bootloader entropy via EFI and via devicetree.
> 
> As crng_global_init_time is set to != 0 once the input pool is properly
> set up, check (also) for this condition to determine which branch to take.
> 
> Calls to crng_fast_load() do not modify the input pool; therefore, the
> entropy_count for the input pool must not be modified at that early
> stage.
> 
> Reported-by: Ivan T. Ivanov <iivanov@xxxxxxx>
> Fixes: 18b915ac6b0a ("efi/random: Treat EFI_RNG_PROTOCOL output as bootloader randomness")
> Tested-by: Ivan T. Ivanov <iivanov@xxxxxxx>
> Signed-off-by: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>

What is the plan for this fix?

Regards,
Ivan

> ---
> v2->v3: onle one unlikely (Ard Biesheuvel)
> v1->v2: fix commit message; unmerge Reported-and-tested-by-tag (Ard Biesheuvel)
> 
>  drivers/char/random.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 605969ed0f96..18fe804c1bf8 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1763,8 +1763,8 @@ static void __init init_std_data(struct entropy_store *r)
>  }
>  
>  /*
> - * Note that setup_arch() may call add_device_randomness()
> - * long before we get here. This allows seeding of the pools
> + * add_device_randomness() or add_bootloader_randomness() may be
> + * called long before we get here. This allows seeding of the pools
>   * with some platform dependent data very early in the boot
>   * process. But it limits our options here. We must use
>   * statically allocated structures that already have all
> @@ -2274,7 +2274,12 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
>  {
>  	struct entropy_store *poolp = &input_pool;
>  
> -	if (unlikely(crng_init == 0)) {
> +	/* We cannot do much with the input pool until it is set up in
> +	 * rand_initalize(); therefore just mix into the crng state.
> +	 * As this does not affect the input pool, we cannot credit
> +	 * entropy for this.
> +	 */
> +	if (unlikely(crng_init == 0 || crng_global_init_time == 0)) {
>  		crng_fast_load(buffer, count);
>  		return;
>  	}