mac80211: Fix ieee80211_amsdu_aggregate frag_tail bug

In ieee80211_amsdu_aggregate() set a pointer frag_tail point to the
end of skb_shinfo(head)->frag_list, and use it to bind other skb in
the end of this function. But when execute ieee80211_amsdu_aggregate()
->ieee80211_amsdu_realloc_pad()->pskb_expand_head(), the address of
skb_shinfo(head)->frag_list will be changed. However, the
ieee80211_amsdu_aggregate() not update frag_tail after call
pskb_expand_head(). That will cause the second skb can't bind to the
head skb appropriately.So we update the address of frag_tail to fix it.

Fixes: 6e0456b545 ("mac80211: add A-MSDU tx support")
Signed-off-by: Chih-Kang Chang <gary.chang@realtek.com>
Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://lore.kernel.org/r/20210830073240.12736-1-pkshih@realtek.com
[reword comment]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/tx.c

@@ -3380,6 +3380,14 @@ static bool ieee80211_amsdu_aggregate(struct ieee80211_sub_if_data *sdata,
 	if (!ieee80211_amsdu_prepare_head(sdata, fast_tx, head))
 		goto out;
 
+	/* If n == 2, the "while (*frag_tail)" loop above didn't execute
+	 * and  frag_tail should be &skb_shinfo(head)->frag_list.
+	 * However, ieee80211_amsdu_prepare_head() can reallocate it.
+	 * Reload frag_tail to have it pointing to the correct place.
+	 */
+	if (n == 2)
+		frag_tail = &skb_shinfo(head)->frag_list;
+
 	/*
 	 * Pad out the previous subframe to a multiple of 4 by adding the
 	 * padding to the next one, that's being added. Note that head->len