[PATCH net v6 2/3] tun: clear stale skb->protocol of NAPI frags skbs

[Eric Dumazet <edumazet@kernel.org>]

With IFF_NAPI_FRAGS, tun_napi_alloc_frags() gets its skb from
napi_get_frags(). If GRO merged the previous frame with
GRO_MERGED_FREE, that skb was recycled by napi_reuse_skb(), which does
not reset skb->protocol set by napi_frags_skb() for the previous frame.

tun_get_user() calls tun_vnet_hdr_tnl_to_skb() before eth_type_trans(),
so the virtio header checks see the protocol of the previous frame:
GSO packets without VIRTIO_NET_HDR_F_NEEDS_CSUM are dissected with the
wrong protocol. A following change checks the outer protocol of UDP
tunnel GSO packets, which would then reject a valid frame when the two
ethertypes differ.

This needs GRO to hold packets across writes, and IFF_NAPI_FRAGS needs
CAP_NET_ADMIN.

Clear skb->protocol in tun_napi_alloc_frags(), like skbs allocated
by tun_alloc_skb() or build_skb(). eth_type_trans() sets it later.

Fixes: 90e33d459407 ("tun: enable napi_gro_frags() for TUN/TAP driver")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/netdev/179386753589.488856.11455181721004759454@kernel.org/
Suggested-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
---
 drivers/net/tun.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 3b292bce4438..d1fcb25ab026 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1500,6 +1500,12 @@ static struct sk_buff *tun_napi_alloc_frags(struct tun_file *tfile,
 	if (!skb)
 		return ERR_PTR(-ENOMEM);
 
+	/* An skb recycled by napi_reuse_skb() keeps the protocol of a
+	 * previous frame. eth_type_trans() sets it after the virtio
+	 * header checks, which must not see a stale one.
+	 */
+	skb->protocol = 0;
+
 	linear = iov_iter_single_seg_count(it);
 	err = __skb_grow(skb, linear);
 	if (err)
-- 
2.53.0