mac80211: assure all fragments are encrypted

Do not mix plaintext and encrypted fragments in protected Wi-Fi
networks. This fixes CVE-2020-26147.

Previously, an attacker was able to first forward a legitimate encrypted
fragment towards a victim, followed by a plaintext fragment. The
encrypted and plaintext fragment would then be reassembled. For further
details see Section 6.3 and Appendix D in the paper "Fragment and Forge:
Breaking Wi-Fi Through Frame Aggregation and Fragmentation".

Because of this change there are now two equivalent conditions in the
code to determine if a received fragment requires sequential PNs, so we
also move this test to a separate function to make the code easier to
maintain.

Cc: [e-mail]
Signed-off-by: Mathy Vanhoef <[e-mail]>
Link: https://lore.kernel.org/r/20210511200110.30c4394bb835.I5acfdb552cc1d20c339c262315950b3eac491397@changeid
Signed-off-by: Johannes Berg <[e-mail]>

net/mac80211/rx.c

···
 	return NULL;
 }
 
+static bool requires_sequential_pn(struct ieee80211_rx_data *rx, __le16 fc)
+{
+	return rx->key &&
+		(rx->key->conf.cipher == WLAN_CIPHER_SUITE_CCMP ||
+		 rx->key->conf.cipher == WLAN_CIPHER_SUITE_CCMP_256 ||
+		 rx->key->conf.cipher == WLAN_CIPHER_SUITE_GCMP ||
+		 rx->key->conf.cipher == WLAN_CIPHER_SUITE_GCMP_256) &&
+		ieee80211_has_protected(fc);
+}
+
 static ieee80211_rx_result debug_noinline
 ieee80211_rx_h_defragment(struct ieee80211_rx_data *rx)
 {
···
 		/* This is the first fragment of a new frame. */
 		entry = ieee80211_reassemble_add(rx->sdata, frag, seq,
 						 rx->seqno_idx, &(rx->skb));
-		if (rx->key &&
-		    (rx->key->conf.cipher == WLAN_CIPHER_SUITE_CCMP ||
-		     rx->key->conf.cipher == WLAN_CIPHER_SUITE_CCMP_256 ||
-		     rx->key->conf.cipher == WLAN_CIPHER_SUITE_GCMP ||
-		     rx->key->conf.cipher == WLAN_CIPHER_SUITE_GCMP_256) &&
-		    ieee80211_has_protected(fc)) {
+		if (requires_sequential_pn(rx, fc)) {
 			int queue = rx->security_idx;
 
 			/* Store CCMP/GCMP PN so that we can verify that the
···
 		u8 pn[IEEE80211_CCMP_PN_LEN], *rpn;
 		int queue;
 
-		if (!rx->key ||
-		    (rx->key->conf.cipher != WLAN_CIPHER_SUITE_CCMP &&
-		     rx->key->conf.cipher != WLAN_CIPHER_SUITE_CCMP_256 &&
-		     rx->key->conf.cipher != WLAN_CIPHER_SUITE_GCMP &&
-		     rx->key->conf.cipher != WLAN_CIPHER_SUITE_GCMP_256))
+		if (!requires_sequential_pn(rx, fc))
 			return RX_DROP_UNUSABLE;
 		memcpy(pn, entry->last_pn, IEEE80211_CCMP_PN_LEN);
 		for (i = IEEE80211_CCMP_PN_LEN - 1; i >= 0; i--) {