tuntap: correctly linearize skb when zerocopy is used

Userspace may produce vectors greater than MAX_SKB_FRAGS. When we try to
linearize parts of the skb to let the rest of iov to be fit in
the frags, we need count copylen into linear when calling tun_alloc_skb()
instead of partly counting it into data_len. Since this breaks
zerocopy_sg_from_iovec() since its inner counter assumes nr_frags should
be zero at beginning. This cause nr_frags to be increased wrongly without
setting the correct frags.

This bug were introduced from 0690899b4d4501b3505be069b9a687e68ccbe15b
(tun: experimental zero copy tx support)

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 3 deletions

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 7eab5fcd064f..5cdcf92eb310 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1042,7 +1042,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 {
         struct tun_pi pi = { 0, cpu_to_be16(ETH_P_IP) };
         struct sk_buff *skb;
-        size_t len = total_len, align = NET_SKB_PAD;
+        size_t len = total_len, align = NET_SKB_PAD, linear;
         struct virtio_net_hdr gso = { 0 };
         int offset = 0;
         int copylen;
@@ -1106,10 +1106,13 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
                         copylen = gso.hdr_len;
                 if (!copylen)
                         copylen = GOODCOPY_LEN;
-        } else
+                linear = copylen;
+        } else {
                 copylen = len;
+                linear = gso.hdr_len;
+        }
 
-        skb = tun_alloc_skb(tfile, align, copylen, gso.hdr_len, noblock);
+        skb = tun_alloc_skb(tfile, align, copylen, linear, noblock);
         if (IS_ERR(skb)) {
                 if (PTR_ERR(skb) != -EAGAIN)
                         tun->dev->stats.rx_dropped++;