random: do not throw away excess input to crng_fast_load

When crng_fast_load() is called by add_hwgenerator_randomness(), we
currently will advance to crng_init==1 once we've acquired 64 bytes, and
then throw away the rest of the buffer. Usually, that is not a problem:
When add_hwgenerator_randomness() gets called via EFI or DT during
setup_arch(), there won't be any IRQ randomness. Therefore, the 64 bytes
passed by EFI exactly matches what is needed to advance to crng_init==1.
Usually, DT seems to pass 64 bytes as well -- with one notable exception
being kexec, which hands over 128 bytes of entropy to the kexec'd kernel.
In that case, we'll advance to crng_init==1 once 64 of those bytes are
consumed by crng_fast_load(), but won't continue onward feeding in bytes
to progress to crng_init==2. This commit fixes the issue by feeding
any leftover bytes into the next phase in add_hwgenerator_randomness().

[linux@dominikbrodowski.net: rewrite commit message]
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -919,12 +919,14 @@ static struct crng_state *select_crng(void)
 
 /*
  * crng_fast_load() can be called by code in the interrupt service
- * path.  So we can't afford to dilly-dally.
+ * path.  So we can't afford to dilly-dally. Returns the number of
+ * bytes processed from cp.
  */
-static int crng_fast_load(const char *cp, size_t len)
+static size_t crng_fast_load(const char *cp, size_t len)
 {
 	unsigned long flags;
 	char *p;
+	size_t ret = 0;
 
 	if (!spin_trylock_irqsave(&primary_crng.lock, flags))
 		return 0;
@@ -935,15 +937,15 @@ static int crng_fast_load(const char *cp, size_t len)
 	p = (unsigned char *) &primary_crng.state[4];
 	while (len > 0 && crng_init_cnt < CRNG_INIT_CNT_THRESH) {
 		p[crng_init_cnt % CHACHA_KEY_SIZE] ^= *cp;
-		cp++; crng_init_cnt++; len--;
+		cp++; crng_init_cnt++; len--; ret++;
 	}
 	spin_unlock_irqrestore(&primary_crng.lock, flags);
 	if (crng_init_cnt >= CRNG_INIT_CNT_THRESH) {
 		invalidate_batched_entropy();
 		crng_init = 1;
 		pr_notice("fast init done\n");
 	}
-	return 1;
+	return ret;
 }
 
 /*
@@ -1294,7 +1296,7 @@ void add_interrupt_randomness(int irq)
 	if (unlikely(crng_init == 0)) {
 		if ((fast_pool->count >= 64) &&
 		    crng_fast_load((char *) fast_pool->pool,
-				   sizeof(fast_pool->pool))) {
+				   sizeof(fast_pool->pool)) > 0) {
 			fast_pool->count = 0;
 			fast_pool->last = now;
 		}
@@ -2295,8 +2297,11 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
 	struct entropy_store *poolp = &input_pool;
 
 	if (unlikely(crng_init == 0)) {
-		crng_fast_load(buffer, count);
-		return;
+		size_t ret = crng_fast_load(buffer, count);
+		count -= ret;
+		buffer += ret;
+		if (!count || crng_init == 0)
+			return;
 	}
 
 	/* Suspend writing if we're above the trickle threshold.