p54: avoid accessing the data mapped to streaming DMA

In p54p_tx(), skb->data is mapped to streaming DMA on line 337:
  mapping = pci_map_single(..., skb->data, ...);

Then skb->data is accessed on line 349:
  desc->device_addr = ((struct p54_hdr *)skb->data)->req_id;

This access may cause data inconsistency between CPU cache and hardware.

To fix this problem, ((struct p54_hdr *)skb->data)->req_id is stored in
a local variable before DMA mapping, and then the driver accesses this
local variable instead of skb->data.

Cc: <stable@vger.kernel.org>
Signed-off-by: Jia-Ju Bai <baijiaju@tsinghua.edu.cn>
Acked-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200802132949.26788-1-baijiaju@tsinghua.edu.cn

drivers/net/wireless/intersil/p54/p54pci.c

@@ -333,10 +333,12 @@ static void p54p_tx(struct ieee80211_hw *dev, struct sk_buff *skb)
 	struct p54p_desc *desc;
 	dma_addr_t mapping;
 	u32 idx, i;
+	__le32 device_addr;
 
 	spin_lock_irqsave(&priv->lock, flags);
 	idx = le32_to_cpu(ring_control->host_idx[1]);
 	i = idx % ARRAY_SIZE(ring_control->tx_data);
+	device_addr = ((struct p54_hdr *)skb->data)->req_id;
 
 	mapping = dma_map_single(&priv->pdev->dev, skb->data, skb->len,
 				 DMA_TO_DEVICE);
@@ -350,7 +352,7 @@ static void p54p_tx(struct ieee80211_hw *dev, struct sk_buff *skb)
 
 	desc = &ring_control->tx_data[i];
 	desc->host_addr = cpu_to_le32(mapping);
-	desc->device_addr = ((struct p54_hdr *)skb->data)->req_id;
+	desc->device_addr = device_addr;
 	desc->len = cpu_to_le16(skb->len);
 	desc->flags = 0;