[PATCH] dmaengine: xilinx_dma: Add missing check for empty list

[Sebastian von Ohr <vonohr@smaract.com>]

The DMA transfer might finish just after checking the state with
dma_cookie_status, but before the lock is acquired. Not checking
for an empty list in xilinx_dma_tx_status may result in reading
random data or data corruption when desc is written to. This can
be reliably triggered by using dma_sync_wait to wait for DMA
completion.

Signed-off-by: Sebastian von Ohr <vonohr@smaract.com>
---
 drivers/dma/xilinx/xilinx_dma.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/drivers/dma/xilinx/xilinx_dma.c b/drivers/dma/xilinx/xilinx_dma.c
index a9c5d5cc9f2b..5d5f1d0ce16c 100644
--- a/drivers/dma/xilinx/xilinx_dma.c
+++ b/drivers/dma/xilinx/xilinx_dma.c
@@ -1229,16 +1229,16 @@ static enum dma_status xilinx_dma_tx_status(struct dma_chan *dchan,
 		return ret;
 
 	spin_lock_irqsave(&chan->lock, flags);
-
-	desc = list_last_entry(&chan->active_list,
-			       struct xilinx_dma_tx_descriptor, node);
-	/*
-	 * VDMA and simple mode do not support residue reporting, so the
-	 * residue field will always be 0.
-	 */
-	if (chan->has_sg && chan->xdev->dma_config->dmatype != XDMA_TYPE_VDMA)
-		residue = xilinx_dma_get_residue(chan, desc);
-
+	if (!list_empty(&chan->active_list)) {
+		desc = list_last_entry(&chan->active_list,
+				       struct xilinx_dma_tx_descriptor, node);
+		/*
+		 * VDMA and simple mode do not support residue reporting, so the
+		 * residue field will always be 0.
+		 */
+		if (chan->has_sg && chan->xdev->dma_config->dmatype != XDMA_TYPE_VDMA)
+			residue = xilinx_dma_get_residue(chan, desc);
+	}
 	spin_unlock_irqrestore(&chan->lock, flags);
 
 	dma_set_residue(txstate, residue);
-- 
2.17.1