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

commit d97c68d178fbf8aaaf21b69b446f2dfb13909316 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index bb25aae..fd3d14d 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4307,6 +4307,12 @@
                        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

diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index 7408118..55f4837 100644 (file)
--- 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 382ff01..fd7b234 100644 (file)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -945,11 +945,17 @@ static bool drain_entropy(void *buf, size_t nbytes)
  **********************************************************************/
 
 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
@@ -1157,7 +1163,7 @@ 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);