net: validate untrusted gso packets without csum offload

Syzkaller again found a path to a kernel crash through bad gso input.
By building an excessively large packet to cause an skb field to wrap.

If VIRTIO_NET_HDR_F_NEEDS_CSUM was set this would have been dropped in
skb_partial_csum_set.

GSO packets that do not set checksum offload are suspicious and rare.
Most callers of virtio_net_hdr_to_skb already pass them to
skb_probe_transport_header.

Move that test forward, change it to detect parse failure and drop
packets on failure as those cleary are not one of the legitimate
VIRTIO_NET_HDR_GSO types.

Fixes: bfd5f4a3d605 ("packet: Add GSO/csum offload support.")
Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 10 insertions, 1 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 95d25b010a25..4c1c82a5678c 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2434,7 +2434,7 @@ static inline void skb_probe_transport_header(struct sk_buff *skb,
 
         if (skb_flow_dissect_flow_keys_basic(skb, &keys, NULL, 0, 0, 0, 0))
                 skb_set_transport_header(skb, keys.control.thoff);
-        else
+        else if (offset_hint >= 0)
                 skb_set_transport_header(skb, offset_hint);
 }
 
diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index cb462f9ab7dd..71f2394abbf7 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -57,6 +57,15 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 
                 if (!skb_partial_csum_set(skb, start, off))
                         return -EINVAL;
+        } else {
+                /* gso packets without NEEDS_CSUM do not set transport_offset.
+                 * probe and drop if does not match one of the above types.
+                 */
+                if (gso_type) {
+                        skb_probe_transport_header(skb, -1);
+                        if (!skb_transport_header_was_set(skb))
+                                return -EINVAL;
+                }
         }
 
         if (hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE) {