Patch "virtio_pci: Limit DMA mask to 44 bits for legacy virtio devices" has been added to the 4.8-stable tree

This is a note to let you know that I've just added the patch titled

virtio_pci: Limit DMA mask to 44 bits for legacy virtio devices

to the 4.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
virtio_pci-limit-dma-mask-to-44-bits-for-legacy-virtio-devices.patch
and it can be found in the queue-4.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Will Deacon <[email protected]>
Date: Wed, 14 Sep 2016 17:33:26 +0100
Subject: virtio_pci: Limit DMA mask to 44 bits for legacy virtio devices

From: Will Deacon <[email protected]>

commit a0be1db4304f51c99af6b6e515549794182a94df upstream.

Legacy virtio defines the virtqueue base using a 32-bit PFN field, with a read-only register indicating a fixed page size of 4k.

This can cause problems for DMA allocators that allocate top down from the DMA mask, which is set to 64 bits. In this case, the addresses are silently truncated to 44-bit, leading to IOMMU faults, failure to read from the queue or data corruption.

This patch restricts the coherent DMA mask for legacy PCI virtio devices to 44 bits, which matches the specification.

---
 drivers/virtio/virtio_pci_legacy.c |   16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

@@ -212,10 +212,18 @@ int virtio_pci_legacy_probe(struct virti
 		return -ENODEV;
 	}
 
-	rc = dma_set_mask_and_coherent(&pci_dev->dev, DMA_BIT_MASK(64));
-	if (rc)
-		rc = dma_set_mask_and_coherent(&pci_dev->dev,
-						DMA_BIT_MASK(32));
+	rc = dma_set_mask(&pci_dev->dev, DMA_BIT_MASK(64));
+	if (rc) {
+		rc = dma_set_mask_and_coherent(&pci_dev->dev, DMA_BIT_MASK(32));
+	} else {
+		/*
+		 * The virtio ring base address is expressed as a 32-bit PFN,
+		 * with a page size of 1 << VIRTIO_PCI_QUEUE_ADDR_SHIFT.
+		 */
+		dma_set_coherent_mask(&pci_dev->dev,
+				DMA_BIT_MASK(32 + VIRTIO_PCI_QUEUE_ADDR_SHIFT));
+	}
+
 	if (rc)
 		dev_warn(&pci_dev->dev, "Failed to enable 64-bit or 32-bit DMA.  Trying to continue, but this might not work.\n");
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.8/virtio_pci-limit-dma-mask-to-44-bits-for-legacy-virtio-devices.patch