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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: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Link: https://github.com/NixOS/nixpkgs/pull/165355
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
This commit is contained in:
Jason A. Donenfeld 2022-03-22 21:43:12 -06:00
parent af704c856e
commit d97c68d178
3 changed files with 15 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
Documentation/admin-guide/kernel-parameters.txt
View File

@ -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

3
drivers/char/Kconfig
View File

@ -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

8
drivers/char/random.c
View File

@ -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);