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From 9b25436662d5fb4c66eb527ead53cab15f596ee0 Mon Sep 17 00:00:00 2001
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From: Kees Cook <keescook@chromium.org>
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Date: Mon, 27 Aug 2018 14:51:54 -0700
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Subject: [PATCH] random: make CPU trust a boot parameter
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Instead of forcing a distro or other system builder to choose
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at build time whether the CPU is trusted for CRNG seeding via
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CONFIG_RANDOM_TRUST_CPU, provide a boot-time parameter for end users to
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control the choice. The CONFIG will set the default state instead.
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Signed-off-by: Kees Cook <keescook@chromium.org>
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Signed-off-by: Theodore Ts'o <tytso@mit.edu>
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---
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Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
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drivers/char/Kconfig | 4 ++--
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drivers/char/random.c | 11 ++++++++---
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3 files changed, 16 insertions(+), 5 deletions(-)
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diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
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index 0c8f7889efa1..227c5c6fa4c1 100644
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--- a/Documentation/admin-guide/kernel-parameters.txt
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+++ b/Documentation/admin-guide/kernel-parameters.txt
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@@ -3390,6 +3390,12 @@
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ramdisk_size= [RAM] Sizes of RAM disks in kilobytes
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See Documentation/blockdev/ramdisk.txt.
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+ random.trust_cpu={on,off}
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+ [KNL] Enable or disable trusting the use of the
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+ CPU's random number generator (if available) to
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+ fully seed the kernel's CRNG. Default is controlled
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+ by CONFIG_RANDOM_TRUST_CPU.
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+
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ras=option[,option,...] [KNL] RAS-specific options
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cec_disable [X86]
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diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
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index ce277ee0a28a..40728491f37b 100644
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--- a/drivers/char/Kconfig
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+++ b/drivers/char/Kconfig
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@@ -566,5 +566,5 @@ config RANDOM_TRUST_CPU
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that CPU manufacturer (perhaps with the insistence or mandate
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of a Nation State's intelligence or law enforcement agencies)
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has not installed a hidden back door to compromise the CPU's
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- random number generation facilities.
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-
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+ random number generation facilities. This can also be configured
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+ at boot with "random.trust_cpu=on/off".
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diff --git a/drivers/char/random.c b/drivers/char/random.c
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index bf5f99fc36f1..c75b6cdf0053 100644
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--- a/drivers/char/random.c
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+++ b/drivers/char/random.c
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@@ -779,6 +779,13 @@ static struct crng_state **crng_node_pool __read_mostly;
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static void invalidate_batched_entropy(void);
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+static bool trust_cpu __ro_after_init = IS_ENABLED(CONFIG_RANDOM_TRUST_CPU);
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+static int __init parse_trust_cpu(char *arg)
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+{
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+ return kstrtobool(arg, &trust_cpu);
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+}
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+early_param("random.trust_cpu", parse_trust_cpu);
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+
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static void crng_initialize(struct crng_state *crng)
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{
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int i;
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@@ -799,12 +806,10 @@ static void crng_initialize(struct crng_state *crng)
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}
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crng->state[i] ^= rv;
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}
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-#ifdef CONFIG_RANDOM_TRUST_CPU
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- if (arch_init) {
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+ if (trust_cpu && arch_init) {
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crng_init = 2;
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pr_notice("random: crng done (trusting CPU's manufacturer)\n");
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}
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-#endif
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crng->init_time = jiffies - CRNG_RESEED_INTERVAL - 1;
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}
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--
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2.17.1
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