linux-kernel - [PATCH 4.4 307/312] net: skbuff: Remove errornous length validation in skb_vlan

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Date:   Fri,  8 May 2020 14:34:58 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Shmulik Ladkani <shmulik.ladkani@...il.com>,
        Pravin Shelar <pshelar@....org>,
        "David S. Miller" <davem@...emloft.net>
Subject: [PATCH 4.4 307/312] net: skbuff: Remove errornous length validation in skb_vlan_pop()

From: Shmulik Ladkani <shmulik.ladkani@...ellosystems.com>

commit 636c2628086e40c86dac7ddc84a1c4b4fcccc6e3 upstream.

In 93515d53b1
  "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: 93515d53b1 ("net: move vlan pop/push functions into common code")
Signed-off-by: Shmulik Ladkani <shmulik.ladkani@...il.com>
Cc: Pravin Shelar <pshelar@....org>
Reviewed-by: Pravin B Shelar <pshelar@....org>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 net/core/skbuff.c |   10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4451,9 +4451,8 @@ 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);
@@ -4461,9 +4460,8 @@ 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;