[PATCH net] net/openvswitch: check Ethernet header length before key_extract()

["Cen Zhang (Microsoft)" <blbllhy@gmail.com>]

When a packet arrives on an ARPHRD_NONE device (e.g. TUN),
ovs_flow_key_extract() trusts the user-provided skb->protocol field: if
it is ETH_P_TEB, the packet is classified as MAC_PROTO_ETHERNET and
key_extract() is called without ensuring the skb has ETH_HLEN (14) bytes
of linear data. key_extract() unconditionally pulls 2 * ETH_ALEN bytes
for MAC addresses and parse_ethertype() pulls 2 more, either of which
triggers a kernel BUG in __skb_pull() when the linear area is too small.

  kernel BUG at include/linux/skbuff.h:2848!
  RIP: 0010:key_extract+0xa7e/0xd90 net/openvswitch/flow.c:933
  ovs_flow_key_extract+0x419/0xa70
  ovs_vport_receive+0x222/0x390
  netdev_frame_hook+0x3e0/0x630
  tun_get_user+0x2d0c/0x38e0

Fixed by adding check_header(skb, ETH_HLEN) in ovs_flow_key_extract()
before calling key_extract().

Fixes: 217ac77a3c25 ("openvswitch: allow L3 netdev ports")
Reported-by: AutonomousCodeSecurity@microsoft.com
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
---
 net/openvswitch/flow.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/openvswitch/flow.c b/net/openvswitch/flow.c
index 66366982f604..a0f9d788d284 100644
--- a/net/openvswitch/flow.c
+++ b/net/openvswitch/flow.c
@@ -1035,6 +1035,13 @@ int ovs_flow_key_extract(const struct ip_tunnel_info *tun_info,
 		return res;
 	key->mac_proto = res;
 
+	if (res == MAC_PROTO_ETHERNET) {
+		int err = check_header(skb, ETH_HLEN);
+
+		if (unlikely(err))
+			return err;
+	}
+
 #if IS_ENABLED(CONFIG_NET_TC_SKB_EXT)
 	if (tc_skb_ext_tc_enabled()) {
 		tc_ext = skb_ext_find(skb, TC_SKB_EXT);
-- 
2.53.0