linux-kernel - Re: [PATCH] random: fix crash on multiple early calls to add_bootloader

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Date:   Sun, 31 Oct 2021 13:33:34 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Dominik Brodowski <linux@...inikbrodowski.net>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>,
        "Ivan T. Ivanov" <iivanov@...e.de>,
        Bhupesh Sharma <bhsharma@...hat.com>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] random: fix crash on multiple early calls to add_bootloader_randomness()

On Sun, 31 Oct 2021 at 07:31, Dominik Brodowski
<linux@...inikbrodowski.net> wrote:
>
> 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 x86 may provide bootloader entropy via EFI and via devicetree.
>

arm64 not x86

> 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-and-tested-by: Ivan T. Ivanov <iivanov@...e.de>

Nit: fancy tags like this are more difficult to grep for

Better to use separate Reported-by and Tested-by tags

> Fixes: 18b915ac6b0a ("efi/random: Treat EFI_RNG_PROTOCOL output as bootloader randomness")
> Signed-off-by: Dominik Brodowski <linux@...inikbrodowski.net>
>

Please don't drop the diffstat. Are you using git format-patch?


> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 605969ed0f96..4211ff3092f9 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) || unlikely(crng_global_init_time == 0)) {

Can we just drop the unlikely()s here?

>                 crng_fast_load(buffer, count);
>                 return;
>         }