net: skbuff: Remove errornous length validation in skb_vlan_pop()

BugLink: https://bugs.launchpad.net/bugs/1878232

commit 636c2628 upstream.

In 93515d53
  "net: move vlan pop/push functions into common code"
skb_vlan_pop was moved from its private location in openvswitch to
skbuff common code.

In case skb has non hw-accel vlan tag, the original 'pop_vlan()' assured
that skb->len is sufficient (if skb->len < VLAN_ETH_HLEN then pop was
considered a no-op).

This validation was moved as is into the new common 'skb_vlan_pop'.

Alas, in its original location (openvswitch), there was a guarantee that
'data' points to the mac_header, therefore the 'skb->len < VLAN_ETH_HLEN'
condition made sense.
However there's no such guarantee in the generic 'skb_vlan_pop'.

For short packets received in rx path going through 'skb_vlan_pop',
this causes 'skb_vlan_pop' to fail pop-ing a valid vlan hdr (in the non
hw-accel case) or to fail moving next tag into hw-accel tag.

Remove the 'skb->len < VLAN_ETH_HLEN' condition entirely:
It is superfluous since inner '__skb_vlan_pop' already verifies there
are VLAN_ETH_HLEN writable bytes at the mac_header.

Note this presents a slight change to skb_vlan_pop() users:
In case total length is smaller than VLAN_ETH_HLEN, skb_vlan_pop() now
returns an error, as opposed to previous "no-op" behavior.
Existing callers (e.g. tc act vlan, ovs) usually drop the packet if
'skb_vlan_pop' fails.

Fixes: 93515d53 ("net: move vlan pop/push functions into common code")
Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Cc: Pravin Shelar <pshelar@ovn.org>
Reviewed-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ian May <ian.may@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/core/skbuff.c

@@ -4562,9 +4562,8 @@ int skb_vlan_pop(struct sk_buff *skb)
 	if (likely(skb_vlan_tag_present(skb))) {
 		skb->vlan_tci = 0;
 	} else {
-		if (unlikely((skb->protocol != htons(ETH_P_8021Q) &&
-			      skb->protocol != htons(ETH_P_8021AD)) ||
-			     skb->len < VLAN_ETH_HLEN))
+		if (unlikely(skb->protocol != htons(ETH_P_8021Q) &&
+			     skb->protocol != htons(ETH_P_8021AD)))
 			return 0;
 
 		err = __skb_vlan_pop(skb, &vlan_tci);
@@ -4572,9 +4571,8 @@ int skb_vlan_pop(struct sk_buff *skb)
 			return err;
 	}
 	/* move next vlan tag to hw accel tag */
-	if (likely((skb->protocol != htons(ETH_P_8021Q) &&
-		    skb->protocol != htons(ETH_P_8021AD)) ||
-		   skb->len < VLAN_ETH_HLEN))
+	if (likely(skb->protocol != htons(ETH_P_8021Q) &&
+		   skb->protocol != htons(ETH_P_8021AD)))
 		return 0;
 
 	vlan_proto = skb->protocol;