[PATCH net 1/1] wireguard: send: append trailer after expanding head

["Jason A. Donenfeld" <Jason@zx2c4.com>]

With how this is currently written, we add the trailer, zero it out, and
then add the header space on. If that header space requires a
reallocation + copy, the zeros in the trailer aren't copied, because the
skb len hasn't actually been yet expanded to cover that. Instead add the
padding at the end of the process rather than at the beginning.

Fixes: e7096c131e51 ("net: WireGuard secure network tunnel")
Cc: stable@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/net/wireguard/send.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/drivers/net/wireguard/send.c b/drivers/net/wireguard/send.c
index 26e09c30d596..67d01478eb76 100644
--- a/drivers/net/wireguard/send.c
+++ b/drivers/net/wireguard/send.c
@@ -177,16 +177,6 @@ static bool encrypt_packet(struct sk_buff *skb, struct noise_keypair *keypair)
 	trailer_len = padding_len + noise_encrypted_len(0);
 	plaintext_len = skb->len + padding_len;
 
-	/* Expand data section to have room for padding and auth tag. */
-	num_frags = skb_cow_data(skb, trailer_len, &trailer);
-	if (unlikely(num_frags < 0 || num_frags > ARRAY_SIZE(sg)))
-		return false;
-
-	/* Set the padding to zeros, and make sure it and the auth tag are part
-	 * of the skb.
-	 */
-	memset(skb_tail_pointer(trailer), 0, padding_len);
-
 	/* Expand head section to have room for our header and the network
 	 * stack's headers.
 	 */
@@ -198,6 +188,16 @@ static bool encrypt_packet(struct sk_buff *skb, struct noise_keypair *keypair)
 		     skb_checksum_help(skb)))
 		return false;
 
+	/* Expand data section to have room for padding and auth tag. */
+	num_frags = skb_cow_data(skb, trailer_len, &trailer);
+	if (unlikely(num_frags < 0 || num_frags > ARRAY_SIZE(sg)))
+		return false;
+
+	/* Set the padding to zeros, and make sure it and the auth tag are part
+	 * of the skb.
+	 */
+	memset(skb_tail_pointer(trailer), 0, padding_len);
+
 	/* Only after checksumming can we safely add on the padding at the end
 	 * and the header.
 	 */
-- 
2.54.0