[PATCH v2 6/9] random: absorb fast pool into input pool after fast load

["Jason A. Donenfeld" <Jason@zx2c4.com>]

During crng_init == 0, we never credit entropy in add_interrupt_
randomness(), but instead dump it directly into the base_crng. That's
fine, except for the fact that we then wind up throwing away that
entropy later when we switch to extracting from the input pool and
overwriting the base_crng key. The two other early init sites --
add_hwgenerator_randomness()'s use crng_fast_load() and add_device_
randomness()'s use of crng_slow_load() -- always additionally give their
inputs to the input pool. But not add_interrupt_randomness().

This commit fixes that shortcoming by calling mix_pool_bytes() after
crng_fast_load() in add_interrupt_randomness(). That's partially
verboten on PREEMPT_RT, where it implies taking spinlock_t from an IRQ
handler. But this also only happens during early boot and then never
again after that. Should this turn into a larger problem later for some
esoteric reason, we can always use `if (IS_ENABLED(PREEMPT_RT))` or
something similar to that. But for now, this should do.

Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Suggested-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/char/random.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 785a4545c9d7..20374bce9a07 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -864,6 +864,13 @@ void add_interrupt_randomness(int irq)
 		    crng_fast_load((u8 *)fast_pool->pool, sizeof(fast_pool->pool)) > 0) {
 			fast_pool->count = 0;
 			fast_pool->last = now;
+
+			/* Technically this call means that we're using a spinlock_t
+			 * in the IRQ handler, which isn't terrific for PREEMPT_RT.
+			 * However, this only happens during very early boot, and then
+			 * never again, so we live with it.
+			 */
+			mix_pool_bytes(&fast_pool->pool, sizeof(fast_pool->pool));
 		}
 		return;
 	}
-- 
2.35.0