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    Subject[PATCH 4.19 124/234] random: treat bootloader trust toggle the same way as cpu trust toggle
    Date
    From: "Jason A. Donenfeld" <Jason@zx2c4.com>

    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>
    ---
    Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
    drivers/char/Kconfig | 3 ++-
    drivers/char/random.c | 8 +++++++-
    3 files changed, 15 insertions(+), 2 deletions(-)

    --- a/Documentation/admin-guide/kernel-parameters.txt
    +++ b/Documentation/admin-guide/kernel-parameters.txt
    @@ -3697,6 +3697,12 @@
    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.
    +
    ras=option[,option,...] [KNL] RAS-specific options

    cec_disable [X86]
    --- a/drivers/char/Kconfig
    +++ b/drivers/char/Kconfig
    @@ -576,4 +576,5 @@ 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.
    \ No newline at end of file
    + only mixes the entropy pool. This can also be configured at boot with
    + "random.trust_bootloader=on/off".
    --- a/drivers/char/random.c
    +++ b/drivers/char/random.c
    @@ -940,11 +940,17 @@ static bool drain_entropy(void *buf, siz
    **********************************************************************/

    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
    @@ -1152,7 +1158,7 @@ EXPORT_SYMBOL_GPL(add_hwgenerator_random
    */
    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);

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