mac80211: Skip tailroom reservation for full HW-crypto devices with race fix

Based on inputs from Johannes Berg <johannes@sipsolutions.net>
from http://article.gmane.org/gmane.linux.kernel.wireless.general/68193
and http://article.gmane.org/gmane.linux.kernel.wireless.general/71702

In xmit path, devices that do full hardware crypto (including
MMIC and ICV) need no tailroom. For such devices, tailroom
reservation can be skipped if all the keys are programmed into
the hardware (i.e software crypto is not used for any of the
keys) and none of the keys wants software to generate Michael
MIC and IV.

v2: Added check for IV along with MMIC.
Reported-by: Fabio Rossi <rossi.f@inwind.it>
Tested-by: Fabio Rossi <rossi.f@inwind.it>
Signed-off-by: Mohammed Shafi Shajakhan <mshajakhan@atheros.com>
Cc: Mohammed Shafi Shajakhan <mshajakhan@atheros.com>

v3: Fixing races to avoid WARNING: at net/mac80211/wpa.c:397
    ccmp_encrypt_skb+0xc4/0x1f0
Reported-by: Andreas Hartmann <andihartmann@01019freenet.de>
Tested-by: Andreas Hartmann <andihartmann@01019freenet.de>

v4: Added links with message ID
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/ieee80211_i.h

@@ -544,6 +544,9 @@ struct ieee80211_sub_if_data {
 	/* keys */
 	struct list_head key_list;
 
+	/* count for keys needing tailroom space allocation */
+	int crypto_tx_tailroom_needed_cnt;
+
 	struct net_device *dev;
 	struct ieee80211_local *local;
 

net/mac80211/key.c

@@ -61,6 +61,36 @@ static struct ieee80211_sta *get_sta_for_key(struct ieee80211_key *key)
 	return NULL;
 }
 
+static void increment_tailroom_need_count(struct ieee80211_sub_if_data *sdata)
+{
+	/*
+	 * When this count is zero, SKB resizing for allocating tailroom
+	 * for IV or MMIC is skipped. But, this check has created two race
+	 * cases in xmit path while transiting from zero count to one:
+	 *
+	 * 1. SKB resize was skipped because no key was added but just before
+	 * the xmit key is added and SW encryption kicks off.
+	 *
+	 * 2. SKB resize was skipped because all the keys were hw planted but
+	 * just before xmit one of the key is deleted and SW encryption kicks
+	 * off.
+	 *
+	 * In both the above case SW encryption will find not enough space for
+	 * tailroom and exits with WARN_ON. (See WARN_ONs at wpa.c)
+	 *
+	 * Solution has been explained at
+	 * http://mid.gmane.org/1308590980.4322.19.camel@jlt3.sipsolutions.net
+	 */
+
+	if (!sdata->crypto_tx_tailroom_needed_cnt++) {
+		/*
+		 * Flush all XMIT packets currently using HW encryption or no
+		 * encryption at all if the count transition is from 0 -> 1.
+		 */
+		synchronize_net();
+	}
+}
+
 static int ieee80211_key_enable_hw_accel(struct ieee80211_key *key)
 {
 	struct ieee80211_sub_if_data *sdata;
@@ -101,6 +131,11 @@ static int ieee80211_key_enable_hw_accel(struct ieee80211_key *key)
 
 	if (!ret) {
 		key->flags |= KEY_FLAG_UPLOADED_TO_HARDWARE;
+
+		if (!((key->conf.flags & IEEE80211_KEY_FLAG_GENERATE_MMIC) ||
+		      (key->conf.flags & IEEE80211_KEY_FLAG_GENERATE_IV)))
+			sdata->crypto_tx_tailroom_needed_cnt--;
+
 		return 0;
 	}
 
@@ -142,6 +177,10 @@ static void ieee80211_key_disable_hw_accel(struct ieee80211_key *key)
 	sta = get_sta_for_key(key);
 	sdata = key->sdata;
 
+	if (!((key->conf.flags & IEEE80211_KEY_FLAG_GENERATE_MMIC) ||
+	      (key->conf.flags & IEEE80211_KEY_FLAG_GENERATE_IV)))
+		increment_tailroom_need_count(sdata);
+
 	if (sdata->vif.type == NL80211_IFTYPE_AP_VLAN)
 		sdata = container_of(sdata->bss,
 				     struct ieee80211_sub_if_data,
@@ -394,8 +433,10 @@ static void __ieee80211_key_destroy(struct ieee80211_key *key)
 		ieee80211_aes_key_free(key->u.ccmp.tfm);
 	if (key->conf.cipher == WLAN_CIPHER_SUITE_AES_CMAC)
 		ieee80211_aes_cmac_key_free(key->u.aes_cmac.tfm);
-	if (key->local)
+	if (key->local) {
 		ieee80211_debugfs_key_remove(key);
+		key->sdata->crypto_tx_tailroom_needed_cnt--;
+	}
 
 	kfree(key);
 }
@@ -452,6 +493,8 @@ int ieee80211_key_link(struct ieee80211_key *key,
 	else
 		old_key = key_mtx_dereference(sdata->local, sdata->keys[idx]);
 
+	increment_tailroom_need_count(sdata);
+
 	__ieee80211_key_replace(sdata, sta, pairwise, old_key, key);
 	__ieee80211_key_destroy(old_key);
 
@@ -498,8 +541,12 @@ void ieee80211_enable_keys(struct ieee80211_sub_if_data *sdata)
 
 	mutex_lock(&sdata->local->key_mtx);
 
-	list_for_each_entry(key, &sdata->key_list, list)
+	sdata->crypto_tx_tailroom_needed_cnt = 0;
+
+	list_for_each_entry(key, &sdata->key_list, list) {
+		increment_tailroom_need_count(sdata);
 		ieee80211_key_enable_hw_accel(key);
+	}
 
 	mutex_unlock(&sdata->local->key_mtx);
 }

net/mac80211/tx.c

@@ -1474,18 +1474,14 @@ static bool ieee80211_tx(struct ieee80211_sub_if_data *sdata,
 
 /* device xmit handlers */
 
-static int ieee80211_skb_resize(struct ieee80211_local *local,
+static int ieee80211_skb_resize(struct ieee80211_sub_if_data *sdata,
 				struct sk_buff *skb,
 				int head_need, bool may_encrypt)
 {
+	struct ieee80211_local *local = sdata->local;
 	int tail_need = 0;
 
-	/*
-	 * This could be optimised, devices that do full hardware
-	 * crypto (including TKIP MMIC) need no tailroom... But we
-	 * have no drivers for such devices currently.
-	 */
-	if (may_encrypt) {
+	if (may_encrypt && sdata->crypto_tx_tailroom_needed_cnt) {
 		tail_need = IEEE80211_ENCRYPT_TAILROOM;
 		tail_need -= skb_tailroom(skb);
 		tail_need = max_t(int, tail_need, 0);
@@ -1578,7 +1574,7 @@ static void ieee80211_xmit(struct ieee80211_sub_if_data *sdata,
 	headroom -= skb_headroom(skb);
 	headroom = max_t(int, 0, headroom);
 
-	if (ieee80211_skb_resize(local, skb, headroom, may_encrypt)) {
+	if (ieee80211_skb_resize(sdata, skb, headroom, may_encrypt)) {
 		dev_kfree_skb(skb);
 		rcu_read_unlock();
 		return;
@@ -1945,7 +1941,7 @@ netdev_tx_t ieee80211_subif_start_xmit(struct sk_buff *skb,
 		head_need += IEEE80211_ENCRYPT_HEADROOM;
 		head_need += local->tx_headroom;
 		head_need = max_t(int, 0, head_need);
-		if (ieee80211_skb_resize(local, skb, head_need, true))
+		if (ieee80211_skb_resize(sdata, skb, head_need, true))
 			goto fail;
 	}