Code: static int ieee80211_fragment(struct ieee80211_tx_data *tx,
			      struct sk_buff *skb, int hdrlen,
			      int frag_threshold)
{
	struct ieee80211_local *local = tx->local;
	struct ieee80211_tx_info *info;
	struct sk_buff *tmp;
	int per_fragm = frag_threshold - hdrlen - FCS_LEN;
	int pos = hdrlen + per_fragm;
	int rem = skb->len - hdrlen - per_fragm;

	if (WARN_ON(rem < 0))
		return -EINVAL;

	/* first fragment was already added to queue by caller */

	while (rem) {
		int fraglen = per_fragm;

		if (fraglen > rem)
			fraglen = rem;
		rem -= fraglen;
		tmp = dev_alloc_skb(local->tx_headroom +
				    frag_threshold +
				    tx->sdata->encrypt_headroom +
				    IEEE80211_ENCRYPT_TAILROOM);
		if (!tmp)
			return -ENOMEM;

		__skb_queue_tail(&tx->skbs, tmp);

		skb_reserve(tmp,
			    local->tx_headroom + tx->sdata->encrypt_headroom);

		/* copy control information */
		memcpy(tmp->cb, skb->cb, sizeof(tmp->cb));

		info = IEEE80211_SKB_CB(tmp);
		info->flags &= ~(IEEE80211_TX_CTL_CLEAR_PS_FILT |
				 IEEE80211_TX_CTL_FIRST_FRAGMENT);

		if (rem)
			info->flags |= IEEE80211_TX_CTL_MORE_FRAMES;

		skb_copy_queue_mapping(tmp, skb);
		tmp->priority = skb->priority;
		tmp->dev = skb->dev;

		/* copy header and data */
		memcpy(skb_put(tmp, hdrlen), skb->data, hdrlen);
		memcpy(skb_put(tmp, fraglen), skb->data + pos, fraglen);

		pos += fraglen;
 	}
 
 	/* adjust first fragment's length */
	skb->len = hdrlen + per_fragm;
 	return 0;
 }

Vulnerability Type: +Info 
CWE ID: CWE-200
Summary: The ieee80211_fragment function in net/mac80211/tx.c in the Linux kernel before 3.13.5 does not properly maintain a certain tail pointer, which allows remote attackers to obtain sensitive cleartext information by reading packets.
Commit Message: 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>