random: do not ignore early device randomness

The add_device_randomness() function would ignore incoming bytes if the
crng wasn't ready.  This additionally makes sure to make an early enough
call to add_latent_entropy() to influence the initial stack canary,
which is especially important on non-x86 systems where it stays the same
through the life of the boot.

Link: http://lkml.kernel.org/r/20170626233038.GA48751@beast
Signed-off-by: Kees Cook <[email protected]>
Cc: "Theodore Ts'o" <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jessica Yu <[email protected]>
Cc: Steven Rostedt (VMware) <[email protected]>
Cc: Viresh Kumar <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Prarit Bhargava <[email protected]>
Cc: Lokesh Vutla <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: AKASHI Takahiro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

drivers/char/random.c

@@ -987,6 +987,11 @@ void add_device_randomness(const void *buf, unsigned int size)
 	unsigned long time = random_get_entropy() ^ jiffies;
 	unsigned long flags;
 
+	if (!crng_ready()) {
+		crng_fast_load(buf, size);
+		return;
+	}
+
 	trace_add_device_randomness(size, _RET_IP_);
 	spin_lock_irqsave(&input_pool.lock, flags);
 	_mix_pool_bytes(&input_pool, buf, size);

init/main.c

@@ -518,6 +518,7 @@ asmlinkage __visible void __init start_kernel(void)
 	/*
 	 * Set up the initial canary ASAP:
 	 */
+	add_latent_entropy();
 	boot_init_stack_canary();
 
 	cgroup_init_early();