net, bonding: Disallow vlan+srcmac with XDP

The new vlan+srcmac xmit policy is not implementable with XDP since
in many cases the 802.1Q payload is not present in the packet. This
can be for example due to hardware offload or in the case of veth
due to use of skbuffs internally.

This also fixes the NULL deref with the vlan+srcmac xmit policy
reported by Jonathan Toppins by additionally checking the skb
pointer.

Fixes: a815bde56b15 ("net, bonding: Refactor bond_xmit_hash for use with xdp_buff")
Reported-by: Jonathan Toppins <[e-mail]>
Signed-off-by: Jussi Maki <[e-mail]>
Reviewed-by: Jonathan Toppins <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/bonding/bond_main.c

···
 	switch (BOND_MODE(bond)) {
 	case BOND_MODE_ROUNDROBIN:
 	case BOND_MODE_ACTIVEBACKUP:
+		return true;
 	case BOND_MODE_8023AD:
 	case BOND_MODE_XOR:
-		return true;
+		/* vlan+srcmac is not supported with XDP as in most cases the 802.1q
+		 * payload is not in the packet due to hardware offload.
+		 */
+		if (bond->params.xmit_policy != BOND_XMIT_POLICY_VLAN_SRCMAC)
+			return true;
+		fallthrough;
 	default:
 		return false;
 	}
···
 
 static u32 bond_vlan_srcmac_hash(struct sk_buff *skb, const void *data, int mhoff, int hlen)
 {
-	struct ethhdr *mac_hdr;
 	u32 srcmac_vendor = 0, srcmac_dev = 0;
-	u16 vlan;
+	struct ethhdr *mac_hdr;
+	u16 vlan = 0;
 	int i;
 
 	data = bond_pull_data(skb, data, hlen, mhoff + sizeof(struct ethhdr));
···
 	for (i = 3; i < ETH_ALEN; i++)
 		srcmac_dev = (srcmac_dev << 8) | mac_hdr->h_source[i];
 
-	if (!skb_vlan_tag_present(skb))
-		return srcmac_vendor ^ srcmac_dev;
-
-	vlan = skb_vlan_tag_get(skb);
+	if (skb && skb_vlan_tag_present(skb))
+		vlan = skb_vlan_tag_get(skb);
 
 	return vlan ^ srcmac_vendor ^ srcmac_dev;
 }