WNM: Fix WNM-Sleep Mode Request bounds checking

ieee802_11_rx_wnmsleep_req() might be called for a short frame that has
no more payload after the Public Action field, i.e., with len == 0. The
bounds checking for the payload length was done only for the information
elements while the one octet Dialog Token field was read
unconditionally. This could result in reading one octet beyond the end
of the received frame data.

Depending on driver interface specific mechanism used for fetching the
frame, this could result in reading one octet beyond the end of a
stack/hash buffer or reading an uninitialized octet from within a
buffer. The actual value that was read as the Dialog Token field is not
used since the function returns immediately after having read this value
when there is no information elements following the field.

This issue was initially added in commit d32d94dbf47a ("WNM: Add
WNM-Sleep Mode implementation for AP") (with CONFIG_IEEE80211V=y build
option) and it remained in place during number of cleanup and fix
changes in this area and renaming of the build parameter to
CONFIG_WNM=y. The impacted function was not included in any default
build without one of the these optional build options being explicitly
enabled. CONFIG_WNM=y is still documented as "experimental and not
complete implementation" in hostapd/defconfig. In addition, commit
114f2830d2c2 ("WNM: Ignore WNM-Sleep Mode Request in wnm_sleep_mode=0
case") made this function exit before the impact read if WNM-Sleep Mode
support was not explicitly enabled in runtime configuration
(wnm_sleep_mode=1 in hostapd.conf).

Fix this by explicitly checking the frame has enough payload before
reading the Dialog Token field.

Bug: 111893132
Change-Id: I4b61e22c39d1a5683923eff34e43bb0c509913d4
Merged-In: I4b61e22c39d1a5683923eff34e43bb0c509913d4
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
(cherry picked from commit 7a543744db8ece2376b019040b5668ede68ebd8b)
diff --git a/src/ap/wnm_ap.c b/src/ap/wnm_ap.c
index 41d50ce..02daa9b 100644
--- a/src/ap/wnm_ap.c
+++ b/src/ap/wnm_ap.c
@@ -202,6 +202,13 @@
 	u8 *tfsreq_ie_end = NULL;
 	u16 tfsreq_ie_len = 0;
+	if (len < 1) {
+		wpa_printf(MSG_DEBUG,
+			   "WNM: Ignore too short WNM-Sleep Mode Request from "
+			   MACSTR, MAC2STR(addr));
+		return;
+	}
 	dialog_token = *pos++;
 	while (pos + 1 < frm + len) {
 		u8 ie_len = pos[1];