rtlwifi: rtl8723ae: avoid accessing the data mapped to streaming DMA

In rtl8723e_tx_fill_cmddesc(), skb->data is mapped to streaming DMA on
line 531:
  dma_addr_t mapping = dma_map_single(..., skb->data, ...);

On line 534, skb->data is assigned to hdr after cast:
  struct ieee80211_hdr *hdr = (struct ieee80211_hdr *)(skb->data);

Then hdr->frame_control is accessed on line 535:
  __le16 fc = hdr->frame_control;

This DMA access may cause data inconsistency between CPU and hardwre.

To fix this bug, hdr->frame_control is accessed before the DMA mapping.
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201119015218.12220-1-baijiaju1990@gmail.com

drivers/net/wireless/realtek/rtlwifi/rtl8723ae/trx.c

@@ -528,12 +528,12 @@ void rtl8723e_tx_fill_cmddesc(struct ieee80211_hw *hw,
 	u8 fw_queue = QSLT_BEACON;
 	__le32 *pdesc = (__le32 *)pdesc8;
 
-	dma_addr_t mapping = dma_map_single(&rtlpci->pdev->dev, skb->data,
-					    skb->len, DMA_TO_DEVICE);
-
 	struct ieee80211_hdr *hdr = (struct ieee80211_hdr *)(skb->data);
 	__le16 fc = hdr->frame_control;
 
+	dma_addr_t mapping = dma_map_single(&rtlpci->pdev->dev, skb->data,
+					    skb->len, DMA_TO_DEVICE);
+
 	if (dma_mapping_error(&rtlpci->pdev->dev, mapping)) {
 		rtl_dbg(rtlpriv, COMP_SEND, DBG_TRACE,
 			"DMA mapping error\n");