hwrng: core - Don't use a stack buffer in add_early_randomness()

commit 6d4952d9 upstream.

hw_random carefully avoids using a stack buffer except in
add_early_randomness().  This causes a crash in virtio_rng if
CONFIG_VMAP_STACK=y.
Reported-by: Matt Mullins <mmullins@mmlx.us>
Tested-by: Matt Mullins <mmullins@mmlx.us>
Fixes: d3cc7996 ("hwrng: fetch randomness only after device init")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/char/hw_random/core.c

@@ -65,12 +65,12 @@ static size_t rng_buffer_size(void)
 
 static void add_early_randomness(struct hwrng *rng)
 {
-	unsigned char bytes[16];
 	int bytes_read;
+	size_t size = min_t(size_t, 16, rng_buffer_size());
 
-	bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
+	bytes_read = rng_get_data(rng, rng_buffer, size, 1);
 	if (bytes_read > 0)
-		add_device_randomness(bytes, bytes_read);
+		add_device_randomness(rng_buffer, bytes_read);
 }
 
 static inline int hwrng_init(struct hwrng *rng)