cfg80211: prevent speculation on cfg80211_classify8021d() return

It's possible that the caller of cfg80211_classify8021d() uses the
value to index an array, like mac80211 in ieee80211_downgrade_queue().
Prevent speculation on the return value.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

CVE-2017-5753

(backported from commit 1fc9b725)
[juergh: Adjusted context.]
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/wireless/util.c

@@ -13,6 +13,7 @@
 #include <net/dsfield.h>
 #include <linux/if_vlan.h>
 #include <linux/mpls.h>
+#include <linux/nospec.h>
 #include "core.h"
 #include "rdev-ops.h"
 
@@ -749,20 +750,25 @@ unsigned int cfg80211_classify8021d(struct sk_buff *skb,
 {
 	unsigned int dscp;
 	unsigned char vlan_priority;
+	unsigned int ret;
 
 	/* skb->priority values from 256->263 are magic values to
 	 * directly indicate a specific 802.1d priority.  This is used
 	 * to allow 802.1d priority to be passed directly in from VLAN
 	 * tags, etc.
 	 */
-	if (skb->priority >= 256 && skb->priority <= 263)
-		return skb->priority - 256;
+	if (skb->priority >= 256 && skb->priority <= 263) {
+		ret = skb->priority - 256;
+		goto out;
+	}
 
 	if (skb_vlan_tag_present(skb)) {
 		vlan_priority = (skb_vlan_tag_get(skb) & VLAN_PRIO_MASK)
 			>> VLAN_PRIO_SHIFT;
-		if (vlan_priority > 0)
-			return vlan_priority;
+		if (vlan_priority > 0) {
+			ret = vlan_priority;
+			goto out;
+		}
 	}
 
 	switch (skb->protocol) {
@@ -781,8 +787,9 @@ unsigned int cfg80211_classify8021d(struct sk_buff *skb,
 		if (!mpls)
 			return 0;
 
-		return (ntohl(mpls->entry) & MPLS_LS_TC_MASK)
+		ret = (ntohl(mpls->entry) & MPLS_LS_TC_MASK)
 			>> MPLS_LS_TC_SHIFT;
+		goto out;
 	}
 	case htons(ETH_P_80221):
 		/* 802.21 is always network control traffic */
@@ -795,18 +802,24 @@ unsigned int cfg80211_classify8021d(struct sk_buff *skb,
 		unsigned int i, tmp_dscp = dscp >> 2;
 
 		for (i = 0; i < qos_map->num_des; i++) {
-			if (tmp_dscp == qos_map->dscp_exception[i].dscp)
-				return qos_map->dscp_exception[i].up;
+			if (tmp_dscp == qos_map->dscp_exception[i].dscp) {
+				ret = qos_map->dscp_exception[i].up;
+				goto out;
+			}
 		}
 
 		for (i = 0; i < 8; i++) {
 			if (tmp_dscp >= qos_map->up[i].low &&
-			    tmp_dscp <= qos_map->up[i].high)
-				return i;
+			    tmp_dscp <= qos_map->up[i].high) {
+				ret = i;
+				goto out;
+			}
 		}
 	}
 
-	return dscp >> 5;
+	ret = dscp >> 5;
+out:
+	return array_index_nospec(ret, IEEE80211_NUM_TIDS);
 }
 EXPORT_SYMBOL(cfg80211_classify8021d);