[PATCH] random: Move rand_initialize() earlier

[Kees Cook]

Right now rand_initialize() is run as an early_initcall(), but it only
depends on timekeeping_init() (for mixing ktime_get_real() into the
pools). However, the call to boot_init_stack_canary() for stack canary
initialization runs earlier, which triggers a warning at boot:

random: get_random_bytes called from start_kernel+0x357/0x548 with crng_init=0

Instead, this moves rand_initialize() to after timekeeping_init(), and moves
canary initialization here as well.

Note that this warning may still remain for machines that do not have
UEFI RNG support (which initializes the RNG pools during setup_arch()),
or for x86 machines without RDRAND (or booting without "random.trust=on"
or CONFIG_RANDOM_TRUST_CPU=y).

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
Alternatively, ktime_get_real() could get mixed into the pools after
timekeeping_init(), and rand_initialize() could be run MUCH early,
like after setup_arch()...
---
 drivers/char/random.c  |  5 ++---
 include/linux/random.h |  1 +
 init/main.c            | 21 ++++++++++++++-------
 3 files changed, 17 insertions(+), 10 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index c75b6cdf0053..deff1aa4d000 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1784,7 +1784,7 @@ EXPORT_SYMBOL(get_random_bytes_arch);
  * data into the pool to prepare it for use. The pool is not cleared
  * as that can only decrease the entropy in the pool.
  */
-static void init_std_data(struct entropy_store *r)
+static void __init init_std_data(struct entropy_store *r)
 {
 	int i;
 	ktime_t now = ktime_get_real();
@@ -1811,7 +1811,7 @@ static void init_std_data(struct entropy_store *r)
  * take care not to overwrite the precious per platform data
  * we were given.
  */
-static int rand_initialize(void)
+int __init rand_initialize(void)
 {
 	init_std_data(&input_pool);
 	init_std_data(&blocking_pool);
@@ -1823,7 +1823,6 @@ static int rand_initialize(void)
 	}
 	return 0;
 }
-early_initcall(rand_initialize);
 
 #ifdef CONFIG_BLOCK
 void rand_initialize_disk(struct gendisk *disk)
diff --git a/include/linux/random.h b/include/linux/random.h
index 445a0ea4ff49..13aeaf5a4bd4 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -36,6 +36,7 @@ extern void add_interrupt_randomness(int irq, int irq_flags) __latent_entropy;
 
 extern void get_random_bytes(void *buf, int nbytes);
 extern int wait_for_random_bytes(void);
+extern int __init rand_initialize(void);
 extern bool rng_is_initialized(void);
 extern int add_random_ready_callback(struct random_ready_callback *rdy);
 extern void del_random_ready_callback(struct random_ready_callback *rdy);
diff --git a/init/main.c b/init/main.c
index 18f8f0140fa0..e2b073bf846f 100644
--- a/init/main.c
+++ b/init/main.c
@@ -550,13 +550,6 @@ asmlinkage __visible void __init start_kernel(void)
 	page_address_init();
 	pr_notice("%s", linux_banner);
 	setup_arch(&command_line);
-	/*
-	 * Set up the the initial canary and entropy after arch
-	 * and after adding latent and command line entropy.
-	 */
-	add_latent_entropy();
-	add_device_randomness(command_line, strlen(command_line));
-	boot_init_stack_canary();
 	mm_init_cpumask(&init_mm);
 	setup_command_line(command_line);
 	setup_nr_cpu_ids();
@@ -641,6 +634,20 @@ asmlinkage __visible void __init start_kernel(void)
 	hrtimers_init();
 	softirq_init();
 	timekeeping_init();
+
+	/*
+	 * For best initial stack canary entropy, prepare it after:
+	 * - setup_arch() for any UEFI RNG entropy and boot cmdline access
+	 * - timekeeping_init() for ktime entropy used in rand_initialize()
+	 * - rand_initialize() to get any arch-specific entropy like RDRAND
+	 * - add_latent_entropy() to get any latent entropy
+	 * - adding command line entropy
+	 */
+	rand_initialize();
+	add_latent_entropy();
+	add_device_randomness(command_line, strlen(command_line));
+	boot_init_stack_canary();
+
 	time_init();
 	printk_safe_init();
 	perf_event_init();
-- 
2.17.1


-- 
Kees Cook
Pixel Security