WNM: Fix WNM-Sleep Mode Request bounds checking

ieee802_11_rx_wnmsleep_req() might have been 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. In the original implementation, this could have
resulted in reading one octet beyond the end of the received frame data.

This case has not been reachable after the commit e0785ebbbd ("Use
more consistent Action frame RX handling in both AP mode paths"), but it
is better to address the specific issue in ieee802_11_rx_wnmsleep_req()
as well for additional protection against accidential removal of the
check and also to have something that can be merged into an older
version (pre-v2.7) if desired. The comments below apply for such older
versions where the case could have been reachable.

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 d32d94dbf4 ("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
114f2830d2 ("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). Commit e0785ebbbd ("Use more
consistent Action frame RX handling in both AP mode paths") made this
code unreachable in practice.

Add an explicit check that the frame has enough payload before reading
the Dialog Token field in ieee802_11_rx_wnmsleep_req().

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>

src/ap/wnm_ap.c

@@ -255,6 +255,13 @@ static void ieee802_11_rx_wnmsleep_req(struct hostapd_data *hapd,
 		return;
 	}
 
+	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];