mac80211: Filter duplicate IE ids

mac80211 is lenient with respect to reception of corrupted beacons.
Even if the frame is corrupted as a whole, the available IE elements
are still passed back and accepted, sometimes replacing legitimate
data.  It is unknown to what extent this "feature" is made use of,
but it is clear that in some cases, this is detrimental.  One such
case is reported in http://crosbug.com/26832 where an AP corrupts
its beacons but not its probe responses.

One approach would be to completely reject frames with invaid data
(for example, if the last tag extends beyond the end of the enclosing
PDU).  The enclosed approach is much more conservative: we simply
prevent later IEs from overwriting the state from previous ones.
This approach hopes that there might be some salient data in the
IE stream before the corruption, and seeks to at least prevent that
data from being overwritten.  This approach will fix the case above.

Further, we flag element structures that contain data we think might
be corrupted, so that as we fill the mac80211 BSS structure, we try
not to replace data from an un-corrupted probe response with that
of a corrupted beacon, for example.

Short of any statistics gathering in the various forms of AP breakage,
it's not possible to ascertain the side effects of more stringent
discarding of data.

Signed-off-by: Paul Stewart <pstew@chromium.org>
Cc: Sam Leffler <sleffler@chromium.org>
Cc: Eliad Peller <eliad@wizery.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

1 files changed, 35 insertions, 2 deletions

diff --git a/net/mac80211/util.c b/net/mac80211/util.c
index 0a5ad95ac8b..32f7a3b3d43 100644
--- a/net/mac80211/util.c
+++ b/net/mac80211/util.c
@@ -572,24 +572,40 @@ u32 ieee802_11_parse_elems_crc(u8 *start, size_t len,
 	size_t left = len;
 	u8 *pos = start;
 	bool calc_crc = filter != 0;
+	DECLARE_BITMAP(seen_elems, 256);
 
+	bitmap_zero(seen_elems, 256);
 	memset(elems, 0, sizeof(*elems));
 	elems->ie_start = start;
 	elems->total_len = len;
 
 	while (left >= 2) {
 		u8 id, elen;
+		bool elem_parse_failed;
 
 		id = *pos++;
 		elen = *pos++;
 		left -= 2;
 
-		if (elen > left)
+		if (elen > left) {
+			elems->parse_error = true;
 			break;
+		}
+
+		if (id != WLAN_EID_VENDOR_SPECIFIC &&
+		    id != WLAN_EID_QUIET &&
+		    test_bit(id, seen_elems)) {
+			elems->parse_error = true;
+			left -= elen;
+			pos += elen;
+			continue;
+		}
 
 		if (calc_crc && id < 64 && (filter & (1ULL << id)))
 			crc = crc32_be(crc, pos - 2, elen + 2);
 
+		elem_parse_failed = false;
+
 		switch (id) {
 		case WLAN_EID_SSID:
 			elems->ssid = pos;
@@ -615,7 +631,8 @@ u32 ieee802_11_parse_elems_crc(u8 *start, size_t len,
 			if (elen >= sizeof(struct ieee80211_tim_ie)) {
 				elems->tim = (void *)pos;
 				elems->tim_len = elen;
-			}
+			} else
+				elem_parse_failed = true;
 			break;
 		case WLAN_EID_IBSS_PARAMS:
 			elems->ibss_params = pos;
@@ -664,10 +681,14 @@ u32 ieee802_11_parse_elems_crc(u8 *start, size_t len,
 		case WLAN_EID_HT_CAPABILITY:
 			if (elen >= sizeof(struct ieee80211_ht_cap))
 				elems->ht_cap_elem = (void *)pos;
+			else
+				elem_parse_failed = true;
 			break;
 		case WLAN_EID_HT_INFORMATION:
 			if (elen >= sizeof(struct ieee80211_ht_info))
 				elems->ht_info_elem = (void *)pos;
+			else
+				elem_parse_failed = true;
 			break;
 		case WLAN_EID_MESH_ID:
 			elems->mesh_id = pos;
@@ -676,6 +697,8 @@ u32 ieee802_11_parse_elems_crc(u8 *start, size_t len,
 		case WLAN_EID_MESH_CONFIG:
 			if (elen >= sizeof(struct ieee80211_meshconf_ie))
 				elems->mesh_config = (void *)pos;
+			else
+				elem_parse_failed = true;
 			break;
 		case WLAN_EID_PEER_MGMT:
 			elems->peering = pos;
@@ -696,6 +719,8 @@ u32 ieee802_11_parse_elems_crc(u8 *start, size_t len,
 		case WLAN_EID_RANN:
 			if (elen >= sizeof(struct ieee80211_rann_ie))
 				elems->rann = (void *)pos;
+			else
+				elem_parse_failed = true;
 			break;
 		case WLAN_EID_CHANNEL_SWITCH:
 			elems->ch_switch_elem = pos;
@@ -724,10 +749,18 @@ u32 ieee802_11_parse_elems_crc(u8 *start, size_t len,
 			break;
 		}
 
+		if (elem_parse_failed)
+			elems->parse_error = true;
+		else
+			set_bit(id, seen_elems);
+
 		left -= elen;
 		pos += elen;
 	}
 
+	if (left != 0)
+		elems->parse_error = true;
+
 	return crc;
 }