[PATCH 5.4 79/85] net/core: check length before updating Ethertype in skb_mpls_{push,pop}

[Greg Kroah-Hartman]

From: Guillaume Nault <gnault@xxxxxxxxxx>

commit 4296adc3e32f5d544a95061160fe7e127be1b9ff upstream.

Openvswitch allows to drop a packet's Ethernet header, therefore
skb_mpls_push() and skb_mpls_pop() might be called with ethernet=true
and mac_len=0. In that case the pointer passed to skb_mod_eth_type()
doesn't point to an Ethernet header and the new Ethertype is written at
unexpected locations.

Fix this by verifying that mac_len is big enough to contain an Ethernet
header.

Fixes: fa4e0f8855fc ("net/sched: fix corrupted L2 header with MPLS 'push' and 'pop' actions")
Signed-off-by: Guillaume Nault <gnault@xxxxxxxxxx>
Acked-by: Davide Caratti <dcaratti@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 net/core/skbuff.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5515,7 +5515,7 @@ int skb_mpls_push(struct sk_buff *skb, _
 	lse->label_stack_entry = mpls_lse;
 	skb_postpush_rcsum(skb, lse, MPLS_HLEN);
 
-	if (ethernet)
+	if (ethernet && mac_len >= ETH_HLEN)
 		skb_mod_eth_type(skb, eth_hdr(skb), mpls_proto);
 	skb->protocol = mpls_proto;
 
@@ -5555,7 +5555,7 @@ int skb_mpls_pop(struct sk_buff *skb, __
 	skb_reset_mac_header(skb);
 	skb_set_network_header(skb, mac_len);
 
-	if (ethernet) {
+	if (ethernet && mac_len >= ETH_HLEN) {
 		struct ethhdr *hdr;
 
 		/* use mpls_hdr() to get ethertype to account for VLANs. */