Commit d97c68d1 authored by Jason A. Donenfeld's avatar Jason A. Donenfeld
Browse files

random: treat bootloader trust toggle the same way as cpu trust toggle 



If CONFIG_RANDOM_TRUST_CPU is set, the RNG initializes using RDRAND.
But, the user can disable (or enable) this behavior by setting
`random.trust_cpu=0/1` on the kernel command line. This allows system
builders to do reasonable things while avoiding howls from tinfoil
hatters. (Or vice versa.)

CONFIG_RANDOM_TRUST_BOOTLOADER is basically the same thing, but regards
the seed passed via EFI or device tree, which might come from RDRAND or
a TPM or somewhere else. In order to allow distros to more easily enable
this while avoiding those same howls (or vice versa), this commit adds
the corresponding `random.trust_bootloader=0/1` toggle.

Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Graham Christensen <graham@grahamc.com>
Reviewed-by: default avatarArd Biesheuvel <ardb@kernel.org>
Reviewed-by: default avatarDominik Brodowski <linux@dominikbrodowski.net>
Link: https://github.com/NixOS/nixpkgs/pull/165355


Signed-off-by: default avatarJason A. Donenfeld <Jason@zx2c4.com>
parent af704c85

Documentation/admin-guide/kernel-parameters.txt

+6 −0
Original line number Diff line number Diff line
@@ -4427,6 +4427,12 @@ 
			fully seed the kernel's CRNG. Default is controlled

			by CONFIG_RANDOM_TRUST_CPU.



	random.trust_bootloader={on,off}

			[KNL] Enable or disable trusting the use of a

			seed passed by the bootloader (if available) to

			fully seed the kernel's CRNG. Default is controlled

			by CONFIG_RANDOM_TRUST_BOOTLOADER.



	randomize_kstack_offset=

			[KNL] Enable or disable kernel stack offset

			randomization, which provides roughly 5 bits of

drivers/char/Kconfig

+2 −1
Original line number Diff line number Diff line
@@ -449,6 +449,7 @@ config RANDOM_TRUST_BOOTLOADER 
	device randomness. Say Y here to assume the entropy provided by the

	booloader is trustworthy so it will be added to the kernel's entropy

	pool. Otherwise, say N here so it will be regarded as device input that

	only mixes the entropy pool.

	only mixes the entropy pool. This can also be configured at boot with

	"random.trust_bootloader=on/off".



endmenu

drivers/char/random.c

+7 −1
Original line number Diff line number Diff line
@@ -948,11 +948,17 @@ static bool drain_entropy(void *buf, size_t nbytes, bool force) 
 **********************************************************************/



static bool trust_cpu __ro_after_init = IS_ENABLED(CONFIG_RANDOM_TRUST_CPU);

static bool trust_bootloader __ro_after_init = IS_ENABLED(CONFIG_RANDOM_TRUST_BOOTLOADER);

static int __init parse_trust_cpu(char *arg)

{

	return kstrtobool(arg, &trust_cpu);

}

static int __init parse_trust_bootloader(char *arg)

{

	return kstrtobool(arg, &trust_bootloader);

}

early_param("random.trust_cpu", parse_trust_cpu);

early_param("random.trust_bootloader", parse_trust_bootloader);



/*

 * The first collection of entropy occurs at system boot while interrupts
@@ -1160,7 +1166,7 @@ EXPORT_SYMBOL_GPL(add_hwgenerator_randomness); 
 */

void add_bootloader_randomness(const void *buf, size_t size)

{

	if (IS_ENABLED(CONFIG_RANDOM_TRUST_BOOTLOADER))

	if (trust_bootloader)

		add_hwgenerator_randomness(buf, size, size * 8);

	else

		add_device_randomness(buf, size);