wcn36xx: add proper DMA memory barriers in rx path

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>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211023001528.3077822-1-benl@squareup.com

diff --git a/drivers/net/wireless/ath/wcn36xx/dxe.c b/drivers/net/wireless/ath/wcn36xx/dxe.c
index 0e0bbcd..aff04ef 100644 (file)
--- a/drivers/net/wireless/ath/wcn36xx/dxe.c
+++ b/drivers/net/wireless/ath/wcn36xx/dxe.c
@@ -606,6 +606,10 @@ 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);
@@ -616,9 +620,15 @@ 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;
        }