wifi: mac80211: Avoid address calculations via out of bounds array indexing

req->n_channels must be set before req->channels[] can be used.

This patch fixes one of the issues encountered in [1].

[   83.964255] UBSAN: array-index-out-of-bounds in net/mac80211/scan.c:364:4
[   83.964258] index 0 is out of range for type 'struct ieee80211_channel *[]'
[...]
[   83.964264] Call Trace:
[   83.964267]  <TASK>
[   83.964269]  dump_stack_lvl+0x3f/0xc0
[   83.964274]  __ubsan_handle_out_of_bounds+0xec/0x110
[   83.964278]  ieee80211_prep_hw_scan+0x2db/0x4b0
[   83.964281]  __ieee80211_start_scan+0x601/0x990
[   83.964291]  nl80211_trigger_scan+0x874/0x980
[   83.964295]  genl_family_rcv_msg_doit+0xe8/0x160
[   83.964298]  genl_rcv_msg+0x240/0x270
[...]

[1] https://bugzilla.kernel.org/show_bug.cgi?id=218810Co-authored-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <kees@kernel.org>
Signed-off-by: Kenton Groombridge <concord@gentoo.org>
Link: https://msgid.link/20240605152218.236061-1-concord@gentoo.orgSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/scan.c

@@ -358,7 +358,8 @@ static bool ieee80211_prep_hw_scan(struct ieee80211_sub_if_data *sdata)
 	struct cfg80211_scan_request *req;
 	struct cfg80211_chan_def chandef;
 	u8 bands_used = 0;
-	int i, ielen, n_chans;
+	int i, ielen;
+	u32 *n_chans;
 	u32 flags = 0;
 
 	req = rcu_dereference_protected(local->scan_req,
@@ -368,34 +369,34 @@ static bool ieee80211_prep_hw_scan(struct ieee80211_sub_if_data *sdata)
 		return false;
 
 	if (ieee80211_hw_check(&local->hw, SINGLE_SCAN_ON_ALL_BANDS)) {
+		local->hw_scan_req->req.n_channels = req->n_channels;
+
 		for (i = 0; i < req->n_channels; i++) {
 			local->hw_scan_req->req.channels[i] = req->channels[i];
 			bands_used |= BIT(req->channels[i]->band);
 		}
-
-		n_chans = req->n_channels;
 	} else {
 		do {
 			if (local->hw_scan_band == NUM_NL80211_BANDS)
 				return false;
 
-			n_chans = 0;
+			n_chans = &local->hw_scan_req->req.n_channels;
+			*n_chans = 0;
 
 			for (i = 0; i < req->n_channels; i++) {
 				if (req->channels[i]->band !=
 				    local->hw_scan_band)
 					continue;
-				local->hw_scan_req->req.channels[n_chans] =
+				local->hw_scan_req->req.channels[(*n_chans)++] =
 							req->channels[i];
-				n_chans++;
+
 				bands_used |= BIT(req->channels[i]->band);
 			}
 
 			local->hw_scan_band++;
-		} while (!n_chans);
+		} while (!*n_chans);
 	}
 
-	local->hw_scan_req->req.n_channels = n_chans;
 	ieee80211_prepare_scan_chandef(&chandef);
 
 	if (req->flags & NL80211_SCAN_FLAG_MIN_PREQ_CONTENT)