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@xxxxxxx> Cc: Graham Christensen <graham@xxxxxxxxxxx> Reviewed-by: Ard Biesheuvel <ardb@xxxxxxxxxx> Reviewed-by: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx> Link: https://github.com/NixOS/nixpkgs/pull/165355 Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx> --- v2 updates the documentation, using the same verbage as the trust_cpu text. Documentation/admin-guide/kernel-parameters.txt | 6 ++++++ drivers/char/Kconfig | 3 ++- drivers/char/random.c | 8 +++++++- 3 files changed, 15 insertions(+), 2 deletions(-) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index c658e5d2d52c..b9d764a49207 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -4400,6 +4400,12 @@ 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 the + 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 diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig index 740811893c57..55f48375e3fe 100644 --- a/drivers/char/Kconfig +++ b/drivers/char/Kconfig @@ -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 diff --git a/drivers/char/random.c b/drivers/char/random.c index 81dab72b611a..40107f8b9e9e 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -949,11 +949,17 @@ 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 @@ -1161,7 +1167,7 @@ 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); -- 2.35.1
