[PATCH net 4/5] net: openvswitch: ensure a linear inner Ethernet header on TEB pop

The issue:
pop_mpls() restores MAC_PROTO_ETHERNET when the popped ethertype is
ETH_P_TEB, but skb_mpls_pop() only guarantees that the MPLS shim is in
the linear area.  When the inner Ethernet header sits in a page
fragment, the datapath now claims an Ethernet header it cannot access
linearly.

Output to an OVS internal vport then calls eth_type_trans(), which
pulls ETH_HLEN from an empty linear area and triggers the BUG in
__skb_pull():

  kernel BUG at ./include/linux/skbuff.h:2830!
  RIP: 0010:eth_type_trans+0x335/0x500
  internal_dev_recv
  ovs_vport_send
  do_output
  do_execute_actions
  ovs_execute_actions
  ovs_dp_process_packet
  ovs_vport_receive
  internal_dev_xmit

Conditions to recreate the bug:
On a system with 4 KiB pages and CONFIG_OPENVSWITCH and CONFIG_PACKET
enabled, use CAP_NET_ADMIN to create an OVS datapath and bring its local
internal port up with MTU 9000.  Install a flow matching an Ethernet/MPLS
packet with one bottom-of-stack label and actions pop_eth,
pop_mpls(0x6558), output to the local port.  With CAP_NET_RAW, send a
4128-byte AF_PACKET/SOCK_RAW frame containing outer Ethernet, the MPLS
label, inner Ethernet and 4096 payload bytes.  The two pops consume the
linear headers and expose the inner Ethernet header in page fragments.

The Fix:
Pull the inner Ethernet header into the linear area before restoring
MAC_PROTO_ETHERNET.

Fixes: f66b53fdbb22 ("openvswitch: New MPLS actions for layer 2 tunnelling")
Link: https://lore.kernel.org/netdev/e85fa16e-df3c-43dd-bb3a-9edca07200ea@kernel.org/ (local)
Cc: stable@vger.kernel.org
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 net/openvswitch/actions.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
index 27152bca8a8c..03b31e33e880 100644
--- a/net/openvswitch/actions.c
+++ b/net/openvswitch/actions.c

@@ -153,8 +153,11 @@ static int pop_mpls(struct sk_buff *skb, struct sw_flow_key *key,
 	if (err)
 		return err;
 
-	if (ethertype == htons(ETH_P_TEB))
+	if (ethertype == htons(ETH_P_TEB)) {
+		if (!pskb_may_pull(skb, ETH_HLEN))
+			return -ENOMEM;
 		key->mac_proto = MAC_PROTO_ETHERNET;
+	}
 
 	invalidate_flow_key(key);
 	return 0;

-- 
2.43.0