mac80211: prevent mixed key and fragment cache attacks

Simultaneously prevent mixed key attacks (CVE-2020-24587) and fragment
cache attacks (CVE-2020-24586). This is accomplished by assigning a
unique color to every key (per interface) and using this to track which
key was used to decrypt a fragment. When reassembling frames, it is
now checked whether all fragments were decrypted using the same key.

To assure that fragment cache attacks are also prevented, the ID that is
assigned to keys is unique even over (re)associations and (re)connects.
This means fragments separated by a (re)association or (re)connect will
not be reassembled. Because mac80211 now also prevents the reassembly of
mixed encrypted and plaintext fragments, all cache attacks are prevented.

Cc: [e-mail]
Signed-off-by: Mathy Vanhoef <[e-mail]>
Link: https://lore.kernel.org/r/20210511200110.3f8290e59823.I622a67769ed39257327a362cfc09c812320eb979@changeid
Signed-off-by: Johannes Berg <[e-mail]>

net/mac80211/ieee80211_i.h

···
 	u8 rx_queue;
 	bool check_sequential_pn; /* needed for CCMP/GCMP */
 	u8 last_pn[6]; /* PN of the last fragment if CCMP was used */
+	unsigned int key_color;
 };
 
 

net/mac80211/key.c

···
 		       struct ieee80211_sub_if_data *sdata,
 		       struct sta_info *sta)
 {
+	static atomic_t key_color = ATOMIC_INIT(0);
 	struct ieee80211_key *old_key;
 	int idx = key->conf.keyidx;
 	bool pairwise = key->conf.flags & IEEE80211_KEY_FLAG_PAIRWISE;
···
 	key->local = sdata->local;
 	key->sdata = sdata;
 	key->sta = sta;
+
+	/*
+	 * Assign a unique ID to every key so we can easily prevent mixed
+	 * key and fragment cache attacks.
+	 */
+	key->color = atomic_inc_return(&key_color);
 
 	increment_tailroom_need_count(sdata);
 

net/mac80211/key.h

···
 	} debugfs;
 #endif
 
+	unsigned int color;
+
 	/*
 	 * key config, must be last because it contains key
 	 * material as variable length member

net/mac80211/rx.c

···
 			 * next fragment has a sequential PN value.
 			 */
 			entry->check_sequential_pn = true;
+			entry->key_color = rx->key->color;
 			memcpy(entry->last_pn,
 			       rx->key->u.ccmp.rx_pn[queue],
 			       IEEE80211_CCMP_PN_LEN);
···
 
 		if (!requires_sequential_pn(rx, fc))
 			return RX_DROP_UNUSABLE;
+
+		/* Prevent mixed key and fragment cache attacks */
+		if (entry->key_color != rx->key->color)
+			return RX_DROP_UNUSABLE;
+
 		memcpy(pn, entry->last_pn, IEEE80211_CCMP_PN_LEN);
 		for (i = IEEE80211_CCMP_PN_LEN - 1; i >= 0; i--) {
 			pn[i]++;