cfg80211: check vendor command doit pointer before use

BugLink: https://bugs.launchpad.net/bugs/1892822

[ Upstream commit 4052d3d2 ]

In the case where a vendor command does not implement doit, and has no
flags set, doit would not be validated and a NULL pointer dereference
would occur, for example when invoking the vendor command via iw.

I encountered this while developing new vendor commands.  Perhaps in
practice it is advisable to always implement doit along with dumpit,
but it seems reasonable to me to always check doit anyway, not just
when NEED_WDEV.
Signed-off-by: Julian Squires <julian@cipht.net>
Link: https://lore.kernel.org/r/20200706211353.2366470-1-julian@cipht.netSigned-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Ian May <ian.may@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

net/wireless/nl80211.c

@@ -10181,13 +10181,13 @@ static int nl80211_vendor_cmd(struct sk_buff *skb, struct genl_info *info)
 				if (!wdev->netdev && !wdev->p2p_started)
 					return -ENETDOWN;
 			}
-
-			if (!vcmd->doit)
-				return -EOPNOTSUPP;
 		} else {
 			wdev = NULL;
 		}
 
+		if (!vcmd->doit)
+			return -EOPNOTSUPP;
+
 		if (info->attrs[NL80211_ATTR_VENDOR_DATA]) {
 			data = nla_data(info->attrs[NL80211_ATTR_VENDOR_DATA]);
 			len = nla_len(info->attrs[NL80211_ATTR_VENDOR_DATA]);