[PATCH 4.9 125/264] random: zero buffer after reading entropy from userspace

[Greg Kroah-Hartman]

From: "Jason A. Donenfeld" <Jason@xxxxxxxxx>

commit 7b5164fb1279bf0251371848e40bae646b59b3a8 upstream.

This buffer may contain entropic data that shouldn't stick around longer
than needed, so zero out the temporary buffer at the end of write_pool().

Reviewed-by: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>
Reviewed-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/char/random.c |   20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -552,6 +552,7 @@ static void crng_reseed(void)
 	int entropy_count;
 	unsigned long next_gen;
 	u8 key[CHACHA20_KEY_SIZE];
+	bool finalize_init = false;
 
 	/*
 	 * First we make sure we have POOL_MIN_BITS of entropy in the pool,
@@ -579,12 +580,14 @@ static void crng_reseed(void)
 		++next_gen;
 	WRITE_ONCE(base_crng.generation, next_gen);
 	WRITE_ONCE(base_crng.birth, jiffies);
-	spin_unlock_irqrestore(&base_crng.lock, flags);
-	memzero_explicit(key, sizeof(key));
-
 	if (crng_init < 2) {
 		invalidate_batched_entropy();
 		crng_init = 2;
+		finalize_init = true;
+	}
+	spin_unlock_irqrestore(&base_crng.lock, flags);
+	memzero_explicit(key, sizeof(key));
+	if (finalize_init) {
 		process_random_ready_list();
 		wake_up_interruptible(&crng_init_wait);
 		kill_fasync(&fasync, SIGIO, POLL_IN);
@@ -1386,19 +1389,24 @@ static unsigned int random_poll(struct f
 static int write_pool(const char __user *ubuf, size_t count)
 {
 	size_t len;
+	int ret = 0;
 	u8 block[BLAKE2S_BLOCK_SIZE];
 
 	while (count) {
 		len = min(count, sizeof(block));
-		if (copy_from_user(block, ubuf, len))
-			return -EFAULT;
+		if (copy_from_user(block, ubuf, len)) {
+			ret = -EFAULT;
+			goto out;
+		}
 		count -= len;
 		ubuf += len;
 		mix_pool_bytes(block, len);
 		cond_resched();
 	}
 
-	return 0;
+out:
+	memzero_explicit(block, sizeof(block));
+	return ret;
 }
 
 static ssize_t random_write(struct file *file, const char __user *buffer,