mac80211: fix fragmentation code, particularly for encryption

The "new" fragmentation code (since my rewrite almost 5 years ago)
erroneously sets skb->len rather than using skb_trim() to adjust
the length of the first fragment after copying out all the others.
This leaves the skb tail pointer pointing to after where the data
originally ended, and thus causes the encryption MIC to be written
at that point, rather than where it belongs: immediately after the
data.

The impact of this is that if software encryption is done, then
 a) encryption doesn't work for the first fragment, the connection
    becomes unusable as the first fragment will never be properly
    verified at the receiver, the MIC is practically guaranteed to
    be wrong
 b) we leak up to 8 bytes of plaintext (!) of the packet out into
    the air

This is only mitigated by the fact that many devices are capable
of doing encryption in hardware, in which case this can't happen
as the tail pointer is irrelevant in that case. Additionally,
fragmentation is not used very frequently and would normally have
to be configured manually.

Fix this by using skb_trim() properly.

Cc: stable@vger.kernel.org
Fixes: 2de8e0d999b8 ("mac80211: rewrite fragmentation")
Reported-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
ieee80211_tx_h_check_assoc(struct ieee80211_tx_data *tx)
{

	struct ieee80211_hdr *hdr = (struct ieee80211_hdr *)tx->skb->data;
	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(tx->skb);
	bool assoc = false;

	if (unlikely(info->flags & IEEE80211_TX_CTL_INJECTED))
		return TX_CONTINUE;

	if (unlikely(test_bit(SCAN_SW_SCANNING, &tx->local->scanning)) &&
	    test_bit(SDATA_STATE_OFFCHANNEL, &tx->sdata->state) &&
	    !ieee80211_is_probe_req(hdr->frame_control) &&
	    !ieee80211_is_nullfunc(hdr->frame_control))
		/*
		 * When software scanning only nullfunc frames (to notify
		 * the sleep state to the AP) and probe requests (for the
		 * active scan) are allowed, all other frames should not be
		 * sent and we should not get here, but if we do
		 * nonetheless, drop them to avoid sending them
		 * off-channel. See the link below and
		 * ieee80211_start_scan() for more.
		 *
		 * http://article.gmane.org/gmane.linux.kernel.wireless.general/30089
		 */
		return TX_DROP;

	if (tx->sdata->vif.type == NL80211_IFTYPE_WDS)
		return TX_CONTINUE;

	if (tx->sdata->vif.type == NL80211_IFTYPE_MESH_POINT)
		return TX_CONTINUE;

	if (tx->flags & IEEE80211_TX_PS_BUFFERED)
		return TX_CONTINUE;

	if (tx->sta)
		assoc = test_sta_flag(tx->sta, WLAN_STA_ASSOC);

	if (likely(tx->flags & IEEE80211_TX_UNICAST)) {
		if (unlikely(!assoc &&
			     ieee80211_is_data(hdr->frame_control))) {
#ifdef CONFIG_MAC80211_VERBOSE_DEBUG
			sdata_info(tx->sdata,
				   "dropped data frame to not associated station %pM\n",
				   hdr->addr1);
#endif
			I802_DEBUG_INC(tx->local->tx_handlers_drop_not_assoc);
			return TX_DROP;
		}
	} else if (unlikely(tx->sdata->vif.type == NL80211_IFTYPE_AP &&
			    ieee80211_is_data(hdr->frame_control) &&
			    !atomic_read(&tx->sdata->u.ap.num_mcast_sta))) {
		/*
		 * No associated STAs - no need to send multicast
		 * frames.
		 */
		return TX_DROP;
	}

	return TX_CONTINUE;
}

ieee80211_tx_h_check_assoc(struct ieee80211_tx_data *tx)
{

	struct ieee80211_hdr *hdr = (struct ieee80211_hdr *)tx->skb->data;
	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(tx->skb);
	bool assoc = false;

	if (unlikely(info->flags & IEEE80211_TX_CTL_INJECTED))
		return TX_CONTINUE;

	if (unlikely(test_bit(SCAN_SW_SCANNING, &tx->local->scanning)) &&
	    test_bit(SDATA_STATE_OFFCHANNEL, &tx->sdata->state) &&
	    !ieee80211_is_probe_req(hdr->frame_control) &&
	    !ieee80211_is_nullfunc(hdr->frame_control))
		/*
		 * When software scanning only nullfunc frames (to notify
		 * the sleep state to the AP) and probe requests (for the
		 * active scan) are allowed, all other frames should not be
		 * sent and we should not get here, but if we do
		 * nonetheless, drop them to avoid sending them
		 * off-channel. See the link below and
		 * ieee80211_start_scan() for more.
		 *
		 * http://article.gmane.org/gmane.linux.kernel.wireless.general/30089
		 */
		return TX_DROP;

	if (tx->sdata->vif.type == NL80211_IFTYPE_WDS)
		return TX_CONTINUE;

	if (tx->sdata->vif.type == NL80211_IFTYPE_MESH_POINT)
		return TX_CONTINUE;

	if (tx->flags & IEEE80211_TX_PS_BUFFERED)
		return TX_CONTINUE;

	if (tx->sta)
		assoc = test_sta_flag(tx->sta, WLAN_STA_ASSOC);

	if (likely(tx->flags & IEEE80211_TX_UNICAST)) {
		if (unlikely(!assoc &&
			     ieee80211_is_data(hdr->frame_control))) {
#ifdef CONFIG_MAC80211_VERBOSE_DEBUG
			sdata_info(tx->sdata,
				   "dropped data frame to not associated station %pM\n",
				   hdr->addr1);
#endif
			I802_DEBUG_INC(tx->local->tx_handlers_drop_not_assoc);
			return TX_DROP;
		}
	} else if (unlikely(tx->sdata->vif.type == NL80211_IFTYPE_AP &&
			    ieee80211_is_data(hdr->frame_control) &&
			    !atomic_read(&tx->sdata->u.ap.num_mcast_sta))) {
		/*
		 * No associated STAs - no need to send multicast
		 * frames.
		 */
		return TX_DROP;
	}

	return TX_CONTINUE;
}
