linux-kernel - Re: [RFC] random: UEFI RNG input is bootloader randomness

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Date:   Sat, 28 Sep 2019 10:09:28 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Dominik Brodowski <linux@...inikbrodowski.net>
Cc:     ard.biesheuvel@...aro.org, hsinyi@...omium.org,
        swboyd@...omium.org, robh@...nel.org, tytso@....edu,
        joeyli.kernel@...il.com, linux-kernel@...r.kernel.org,
        linux-efi@...r.kernel.org
Subject: Re: [RFC] random: UEFI RNG input is bootloader randomness

On Sat, Sep 28, 2019 at 12:14:28PM +0200, Dominik Brodowski wrote:
> Depending on RANDOM_TRUST_BOOTLOADER, bootloader-provided randomness
> is credited as entropy. As the UEFI seeding entropy pool is seeded by
> the UEFI firmware/bootloader, add its content as bootloader randomness.
> 
> Note that this UEFI (v2.4 or newer) feature is currently only
> implemented for EFI stub booting on ARM, and further note that
> RANDOM_TRUST_BOOTLOADER must only be enabled if there indeed is
> sufficient trust in the bootloader _and_ its source of randomness.
> 
> Signed-off-by: Dominik Brodowski <linux@...inikbrodowski.net>
> Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Cc: Hsin-Yi Wang <hsinyi@...omium.org>
> Cc: Stephen Boyd <swboyd@...omium.org>
> Cc: Rob Herring <robh@...nel.org>
> Cc: Theodore Ts'o <tytso@....edu>
> Cc: Lee, Chun-Yi <joeyli.kernel@...il.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> 
> ---
> 
> Untested patch, as efi_random_get_seed() is only hooked up on ARM,
> and the firmware on my old x86 laptop only has UEFI v2.31 anyway.
> 
> Thanks,
> 	Dominik
> 
> diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> index 8f1ab04f6743..db0bffce754e 100644
> --- a/drivers/firmware/efi/efi.c
> +++ b/drivers/firmware/efi/efi.c
> @@ -545,7 +545,7 @@ int __init efi_config_parse_tables(void *config_tables, int count, int sz,
>  					      sizeof(*seed) + size);
>  			if (seed != NULL) {
>  				pr_notice("seeding entropy pool\n");
> -				add_device_randomness(seed->bits, seed->size);
> +				add_bootloader_randomness(seed->bits, seed->size);
>  				early_memunmap(seed, sizeof(*seed) + size);
>  			} else {
>  				pr_err("Could not map UEFI random seed!\n");

-- 
Kees Cook