[PATCH v3] dmaengine: tegra186-gpc-dma: Read channel state under the lock

[Ginger Li <ginger.jzllee@gmail.com>]

My static analyzer detected a potential issue in the dmag engine.
tegra_dma_tx_status() reads tdc->status before taking tdc->vc.lock, and
tegra_dma_isr() snapshots tdc->dma_desc before taking the same lock.  All
writers of these fields (tegra_dma_start(), tegra_dma_xfer_complete(),
tegra_dma_pause(), tegra_dma_resume() and tegra_dma_terminate_all()) update
them under tdc->vc.lock.

The unlocked read in tegra_dma_tx_status() can therefore report DMA_PAUSED
for a transfer that has already completed, and the unlocked read in
tegra_dma_isr() can pick up a tdc->dma_desc which is being cleared
concurrently, in which case the handler keeps using a stale descriptor
pointer.

Read both fields while holding tdc->vc.lock.

Fixes: ee17028009d4 ("dmaengine: tegra: Add tegra gpcdma driver")
Signed-off-by: Ginger Li <ginger.jzllee@gmail.com>
---
v3:
 - Acknowledge the usage of static analysis in detecting this problem.
---
 drivers/dma/tegra186-gpc-dma.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/dma/tegra186-gpc-dma.c b/drivers/dma/tegra186-gpc-dma.c
--- a/drivers/dma/tegra186-gpc-dma.c
+++ b/drivers/dma/tegra186-gpc-dma.c
@@ -606,7 +606,7 @@ static irqreturn_t tegra_dma_isr(int irq, void *dev_id
 static irqreturn_t tegra_dma_isr(int irq, void *dev_id)
 {
 	struct tegra_dma_channel *tdc = dev_id;
-	struct tegra_dma_desc *dma_desc = tdc->dma_desc;
+	struct tegra_dma_desc *dma_desc;
 	struct tegra_dma_sg_req *sg_req;
 	u32 status;
 
@@ -619,6 +619,7 @@ static irqreturn_t tegra_dma_isr(int irq, void *dev_id
 	}
 
 	spin_lock(&tdc->vc.lock);
+	dma_desc = tdc->dma_desc;
 	status = tdc_read(tdc, tdc->regs->status);
 	if (!(status & TEGRA_GPCDMA_STATUS_ISE_EOC))
 		goto irq_done;
@@ -786,10 +787,10 @@ static enum dma_status tegra_dma_tx_status(struct dma_
 	if (ret == DMA_COMPLETE)
 		return ret;
 
+	spin_lock_irqsave(&tdc->vc.lock, flags);
 	if (tdc->status == DMA_PAUSED)
 		ret = DMA_PAUSED;
 
-	spin_lock_irqsave(&tdc->vc.lock, flags);
 	vd = vchan_find_desc(&tdc->vc, cookie);
 	if (vd) {
 		dma_desc = vd_to_tegra_dma_desc(vd);
-- 
2.43.0