[PATCH v3] 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 primary_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 primary_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. Plus it's a trylock so it has the same considerations
as calling crng_fast_load(), which we're already using.

Cc: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Suggested-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
v3 uses a trylock instead of a spinlock, just like all the other locks
taken in hard irq. (Incidentally, we're now talking about moving this
into the deferred stage, so that at can be a spinlock, but at least with
what we have here, this really must be a trylock.)

 drivers/char/random.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index d31b0b3afe2e..f3179c67010b 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -850,6 +850,10 @@ 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;
+			if (spin_trylock(&input_pool.lock)) {
+				_mix_pool_bytes(&fast_pool->pool, sizeof(fast_pool->pool));
+				spin_unlock(&input_pool.lock);
+			}
 		}
 		return;
 	}
-- 
2.35.0