nl80211: fix beacon head validation

If the beacon head attribute (NL80211_ATTR_BEACON_HEAD)
is too short to even contain the frame control field,
we access uninitialized data beyond the buffer. Fix this
by checking the minimal required size first. We used to
do this until S1G support was added, where the fixed
data portion has a different size.

Reported-and-tested-by: [email protected]
Suggested-by: Eric Dumazet <[email protected]>
Fixes: 1d47f11 ("nl80211: correctly validate S1G beacon head")
Signed-off-by: Johannes Berg <[email protected]>
Link: https://lore.kernel.org/r/20210408154518.d9b06d39b4ee.Iff908997b2a4067e8d456b3cb96cab9771d252b8@changeid
Signed-off-by: Johannes Berg <[email protected]>

Author: jmberg-intel

net/wireless/nl80211.c

@@ -229,9 +229,13 @@ static int validate_beacon_head(const struct nlattr *attr,
 	unsigned int len = nla_len(attr);
 	const struct element *elem;
 	const struct ieee80211_mgmt *mgmt = (void *)data;
-	bool s1g_bcn = ieee80211_is_s1g_beacon(mgmt->frame_control);
 	unsigned int fixedlen, hdrlen;
+	bool s1g_bcn;
 
+	if (len < offsetofend(typeof(*mgmt), frame_control))
+		goto err;
+
+	s1g_bcn = ieee80211_is_s1g_beacon(mgmt->frame_control);
 	if (s1g_bcn) {
 		fixedlen = offsetof(struct ieee80211_ext,
 				    u.s1g_beacon.variable);