random: always fill buffer in get_random_bytes_wait

commit 25e3fca492 upstream.

In the unfortunate event that a developer fails to check the return
value of get_random_bytes_wait, or simply wants to make a "best effort"
attempt, for whatever that's worth, it's much better to still fill the
buffer with _something_ rather than catastrophically failing in the case
of an interruption. This is both a defense in depth measure against
inevitable programming bugs, as well as a means of making the API a bit
more useful.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/random.h

@@ -63,10 +63,8 @@ static inline unsigned long get_random_long(void)
 static inline int get_random_bytes_wait(void *buf, int nbytes)
 {
 	int ret = wait_for_random_bytes();
-	if (unlikely(ret))
-		return ret;
 	get_random_bytes(buf, nbytes);
-	return 0;
+	return ret;
 }
 
 #define declare_get_random_var_wait(var) \