[PATCH 4.2.y-ckt 237/273] flow_dissector: Fix unaligned access in __skb_flow_dissector when used by eth_get_headlen

[Kamal Mostafa]

4.2.8-ckt5 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: Alexander Duyck <aduyck at mirantis.com>

[ Upstream commit 461547f3158978c180d74484d58e82be9b8e7357, since
  we lack the flow dissector flags in this release we guard the
  flow label access using a test on 'skb' being NULL ]

This patch fixes an issue with unaligned accesses when using
eth_get_headlen on a page that was DMA aligned instead of being IP aligned.
The fact is when trying to check the length we don't need to be looking at
the flow label so we can reorder the checks to first check if we are
supposed to gather the flow label and then make the call to actually get
it.

v2:  Updated path so that either STOP_AT_FLOW_LABEL or KEY_FLOW_LABEL can
     cause us to check for the flow label.

Reported-by: Sowmini Varadhan <sowmini.varadhan at oracle.com>
Signed-off-by: Alexander Duyck <aduyck at mirantis.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 net/core/flow_dissector.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 2a834c6..bb98f8b 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -192,7 +192,6 @@ ip:
 	case htons(ETH_P_IPV6): {
 		const struct ipv6hdr *iph;
 		struct ipv6hdr _iph;
-		__be32 flow_label;
 
 ipv6:
 		iph = __skb_header_pointer(skb, nhoff, sizeof(_iph), data, hlen, &_iph);
@@ -214,8 +213,9 @@ ipv6:
 			key_control->addr_type = FLOW_DISSECTOR_KEY_IPV6_ADDRS;
 		}
 
-		flow_label = ip6_flowlabel(iph);
-		if (flow_label) {
+		if (skb && ip6_flowlabel(iph)) {
+			__be32 flow_label = ip6_flowlabel(iph);
+
 			if (skb_flow_dissector_uses_key(flow_dissector,
 				FLOW_DISSECTOR_KEY_FLOW_LABEL)) {
 				key_tags = skb_flow_dissector_target(flow_dissector,
-- 
2.7.0