[PATCH] wcn36xx: add proper DMA memory barriers in rx path

[Benjamin Li <benl@squareup.com>]

This is essentially exactly following the dma_wmb()/dma_rmb() usage
instructions in Documentation/memory-barriers.txt.

The theoretical races here are:

1. DXE (the DMA Transfer Engine in the Wi-Fi subsystem) seeing the
dxe->ctrl & WCN36xx_DXE_CTRL_VLD write before the dxe->dst_addr_l
write, thus performing DMA into the wrong address.

2. CPU reading dxe->dst_addr_l before DXE unsets dxe->ctrl &
WCN36xx_DXE_CTRL_VLD. This should generally be harmless since DXE
doesn't write dxe->dst_addr_l (no risk of freeing the wrong skb).

Fixes: 8e84c2582169 ("wcn36xx: mac80211 driver for Qualcomm WCN3660/WCN3680 hardware")
Signed-off-by: Benjamin Li <benl@squareup.com>
---
 drivers/net/wireless/ath/wcn36xx/dxe.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wireless/ath/wcn36xx/dxe.c b/drivers/net/wireless/ath/wcn36xx/dxe.c
index 8e1dbfda6538..93994b2e8e03 100644
--- a/drivers/net/wireless/ath/wcn36xx/dxe.c
+++ b/drivers/net/wireless/ath/wcn36xx/dxe.c
@@ -613,6 +613,10 @@ static int wcn36xx_rx_handle_packets(struct wcn36xx *wcn,
 	dxe = ctl->desc;
 
 	while (!(READ_ONCE(dxe->ctrl) & WCN36xx_DXE_CTRL_VLD)) {
+		/* do not read until we own DMA descriptor */
+		dma_rmb();
+
+		/* read/modify DMA descriptor */
 		skb = ctl->skb;
 		dma_addr = dxe->dst_addr_l;
 		ret = wcn36xx_dxe_fill_skb(wcn->dev, ctl, GFP_ATOMIC);
@@ -623,9 +627,15 @@ static int wcn36xx_rx_handle_packets(struct wcn36xx *wcn,
 			dma_unmap_single(wcn->dev, dma_addr, WCN36XX_PKT_SIZE,
 					DMA_FROM_DEVICE);
 			wcn36xx_rx_skb(wcn, skb);
-		} /* else keep old skb not submitted and use it for rx DMA */
+		}
+		/* else keep old skb not submitted and reuse it for rx DMA
+		 * (dropping the packet that it contained)
+		 */
 
+		/* flush descriptor changes before re-marking as valid */
+		dma_wmb();
 		dxe->ctrl = ctrl;
+
 		ctl = ctl->next;
 		dxe = ctl->desc;
 	}
-- 
2.25.1