tun: fix issues of iovec iterators using in tun_put_user()

This patch fixes two issues after using iovec iterators:
- vlan_offset should be initialized to zero, otherwise unexpected offset
  will be used in skb_copy_datagram_iter()
- advance iovec iterator when vnet_hdr_sz is greater than sizeof(gso), this
  is the case when mergeable rx buffer were enabled for a virt guest.

Fixes e0b46d0ee9c240c7430a47e9b0365674d4a04522 ("tun: Use iovec iterators")
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 1 deletions

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 2ff769bf3f35..e3fa65a4d96c 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1235,7 +1235,7 @@ static ssize_t tun_put_user(struct tun_struct *tun,
 {
         struct tun_pi pi = { 0, skb->protocol };
         ssize_t total;
-        int vlan_offset;
+        int vlan_offset = 0;
         int vlan_hlen = 0;
         int vnet_hdr_sz = 0;
 
@@ -1304,6 +1304,8 @@ static ssize_t tun_put_user(struct tun_struct *tun,
 
                 if (copy_to_iter(&gso, sizeof(gso), iter) != sizeof(gso))
                         return -EFAULT;
+
+                iov_iter_advance(iter, vnet_hdr_sz - sizeof(gso));
         }
 
         if (vlan_hlen) {