[PATCH 2/2] dmaengine: dma-axi-dmac: fix use-after-free on unbind

["Nuno Sá via B4 Relay" <devnull+nuno.sa.analog.com@kernel.org>]

From: Nuno Sá <nuno.sa@analog.com>

The DMA device lifetime can extend beyond the platform driver unbind if
DMA channels are still referenced by client drivers. This leads to
use-after-free when the devm-managed memory is freed on unbind but the
DMA device callbacks still access it.

Fix this by:
 - Allocating axi_dmac with kzalloc_obj() instead of devm_kzalloc() so
its lifetime is not tied to the platform device.
 - Implementing the device_release callback that so that we can free
the object when reference count gets to 0 (no users).
 - Adding an 'unbound' flag protected by the vchan lock that is set
during driver removal, preventing MMIO accesses after the device has been
unbound.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
---
 drivers/dma/dma-axi-dmac.c | 47 ++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 43 insertions(+), 4 deletions(-)

diff --git a/drivers/dma/dma-axi-dmac.c b/drivers/dma/dma-axi-dmac.c
index df2668064ea2..99454e096588 100644
--- a/drivers/dma/dma-axi-dmac.c
+++ b/drivers/dma/dma-axi-dmac.c
@@ -176,6 +176,8 @@ struct axi_dmac {
 
 	struct dma_device dma_dev;
 	struct axi_dmac_chan chan;
+
+	bool unbound;
 };
 
 static struct axi_dmac *chan_to_axi_dmac(struct axi_dmac_chan *chan)
@@ -184,6 +186,11 @@ static struct axi_dmac *chan_to_axi_dmac(struct axi_dmac_chan *chan)
 		dma_dev);
 }
 
+static struct axi_dmac *dev_to_axi_dmac(struct dma_device *dev)
+{
+	return container_of(dev, struct axi_dmac, dma_dev);
+}
+
 static struct axi_dmac_chan *to_axi_dmac_chan(struct dma_chan *c)
 {
 	return container_of(c, struct axi_dmac_chan, vchan.chan);
@@ -616,6 +623,11 @@ static int axi_dmac_terminate_all(struct dma_chan *c)
 	LIST_HEAD(head);
 
 	spin_lock_irqsave(&chan->vchan.lock, flags);
+	if (dmac->unbound) {
+		/* We're gone */
+		spin_unlock_irqrestore(&chan->vchan.lock, flags);
+		return -ENODEV;
+	}
 	axi_dmac_write(dmac, AXI_DMAC_REG_CTRL, 0);
 	chan->next_desc = NULL;
 	vchan_get_all_descriptors(&chan->vchan, &head);
@@ -644,9 +656,12 @@ static void axi_dmac_issue_pending(struct dma_chan *c)
 	if (chan->hw_sg)
 		ctrl |= AXI_DMAC_CTRL_ENABLE_SG;
 
-	axi_dmac_write(dmac, AXI_DMAC_REG_CTRL, ctrl);
-
 	spin_lock_irqsave(&chan->vchan.lock, flags);
+	if (dmac->unbound) {
+		spin_unlock_irqrestore(&chan->vchan.lock, flags);
+		return;
+	}
+	axi_dmac_write(dmac, AXI_DMAC_REG_CTRL, ctrl);
 	if (vchan_issue_pending(&chan->vchan))
 		axi_dmac_start_transfer(chan);
 	spin_unlock_irqrestore(&chan->vchan.lock, flags);
@@ -1206,6 +1221,14 @@ static int axi_dmac_detect_caps(struct axi_dmac *dmac, unsigned int version)
 	return 0;
 }
 
+static void axi_dmac_release(struct dma_device *dma_dev)
+{
+	struct axi_dmac *dmac = dev_to_axi_dmac(dma_dev);
+
+	put_device(dma_dev->dev);
+	kfree(dmac);
+}
+
 static void axi_dmac_tasklet_kill(void *task)
 {
 	tasklet_kill(task);
@@ -1216,6 +1239,16 @@ static void axi_dmac_free_dma_controller(void *of_node)
 	of_dma_controller_free(of_node);
 }
 
+static void axi_dmac_disable(void *__dmac)
+{
+	struct axi_dmac *dmac = __dmac;
+
+	spin_lock(&dmac->chan.vchan.lock);
+	dmac->unbound = true;
+	spin_unlock(&dmac->chan.vchan.lock);
+	axi_dmac_write(dmac, AXI_DMAC_REG_CTRL, 0);
+}
+
 static int axi_dmac_probe(struct platform_device *pdev)
 {
 	struct dma_device *dma_dev;
@@ -1225,7 +1258,7 @@ static int axi_dmac_probe(struct platform_device *pdev)
 	u32 irq_mask = 0;
 	int ret;
 
-	dmac = devm_kzalloc(&pdev->dev, sizeof(*dmac), GFP_KERNEL);
+	dmac = kzalloc_obj(struct axi_dmac);
 	if (!dmac)
 		return -ENOMEM;
 
@@ -1270,9 +1303,10 @@ static int axi_dmac_probe(struct platform_device *pdev)
 	dma_dev->device_prep_interleaved_dma = axi_dmac_prep_interleaved;
 	dma_dev->device_terminate_all = axi_dmac_terminate_all;
 	dma_dev->device_synchronize = axi_dmac_synchronize;
-	dma_dev->dev = &pdev->dev;
+	dma_dev->dev = get_device(&pdev->dev);
 	dma_dev->src_addr_widths = BIT(dmac->chan.src_width);
 	dma_dev->dst_addr_widths = BIT(dmac->chan.dest_width);
+	dma_dev->device_release = axi_dmac_release;
 	dma_dev->directions = BIT(dmac->chan.direction);
 	dma_dev->residue_granularity = DMA_RESIDUE_GRANULARITY_DESCRIPTOR;
 	dma_dev->max_sg_burst = 31; /* 31 SGs maximum in one burst */
@@ -1326,6 +1360,11 @@ static int axi_dmac_probe(struct platform_device *pdev)
 	if (ret)
 		return ret;
 
+	/* So that we can mark the device as unbound and disable it */
+	ret = devm_add_action_or_reset(&pdev->dev, axi_dmac_disable, dmac);
+	if (ret)
+		return ret;
+
 	ret = devm_request_irq(&pdev->dev, dmac->irq, axi_dmac_interrupt_handler,
 			       IRQF_SHARED, dev_name(&pdev->dev), dmac);
 	if (ret)

-- 
2.53.0