LinuxLists.cc - [PATCH] random: treat bootloader trust toggle the same way as cpu trust toggle

2022-03-23 11:03:52

Subject: [PATCH] random: treat bootloader trust toggle the same way as cpu trust toggle

2022-03-24 12:45:50

Subject: [PATCH v2] 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 <[email protected]>
Cc: Graham Christensen <[email protected]>
Reviewed-by: Ard Biesheuvel <[email protected]>
Reviewed-by: Dominik Brodowski <[email protected]>
Link: https://github.com/NixOS/nixpkgs/pull/165355
Signed-off-by: Jason A. Donenfeld <[email protected]>
---
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

2022-03-24 21:06:34

by Ard Biesheuvel

[permalink] [raw]

Subject: Re: [PATCH] random: treat bootloader trust toggle the same way as cpu trust toggle

On Wed, 23 Mar 2022 at 05:11, Jason A. Donenfeld <[email protected]> wrote:
>
> 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: Dominik Brodowski <[email protected]>
> Cc: Theodore Ts'o <[email protected]>
> Cc: Graham Christensen <[email protected]>
> Link: https://github.com/NixOS/nixpkgs/pull/165355
> Signed-off-by: Jason A. Donenfeld <[email protected]>

Reviewed-by: Ard Biesheuvel <[email protected]>

> ---
> drivers/char/random.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 7b7f5e6596c2..c8974e5f57e1 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -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);
> --
> 2.35.1
>