[PATCH net] tun: correct header offsets in napi frags mode

[Willem de Bruijn]

From: Willem de Bruijn <will...@google.com>

Tun in IFF_NAPI_FRAGS mode calls napi_gro_frags. Unlike netif_rx and
netif_gro_receive, this expects skb->data to point to the mac layer.

But skb_probe_transport_header, __skb_get_hash_symmetric, and
xdp_do_generic in tun_get_user need skb->data to point to the network
header. Flow dissection also needs skb->protocol set, so
eth_type_trans has to be called.

Temporarily pull ETH_HLEN to make control flow the same for frags and
not frags. Then push the header just before calling napi_gro_frags.

Fixes: 90e33d459407 ("tun: enable napi_gro_frags() for TUN/TAP driver")
Signed-off-by: Willem de Bruijn <will...@google.com>
---
 drivers/net/tun.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 44889eba1dbc..b984733c6c31 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1871,8 +1871,11 @@ static ssize_t tun_get_user(struct tun_struct *tun, 
struct tun_file *tfile,
                skb->dev = tun->dev;
                break;
        case IFF_TAP:
-               if (!frags)
-                       skb->protocol = eth_type_trans(skb, tun->dev);
+               if (frags && !pskb_may_pull(skb, ETH_HLEN)) {
+                       err = -ENOMEM;
+                       goto drop;
+               }
+               skb->protocol = eth_type_trans(skb, tun->dev);
                break;
        }
 
@@ -1929,9 +1932,12 @@ static ssize_t tun_get_user(struct tun_struct *tun, 
struct tun_file *tfile,
        }
 
        if (frags) {
+               u32 headlen;
+
                /* Exercise flow dissector code path. */
-               u32 headlen = eth_get_headlen(tun->dev, skb->data,
-                                             skb_headlen(skb));
+               skb_push(skb, ETH_HLEN);
+               headlen = eth_get_headlen(tun->dev, skb->data,
+                                         skb_headlen(skb));
 
                if (unlikely(headlen > skb_headlen(skb))) {
                        this_cpu_inc(tun->pcpu_stats->rx_dropped);
-- 
2.27.0.rc0.183.gde8f92d652-goog