linux-kernel - Re: [PATCH 2/2] virtio/virtio_ring: Fix the dma_max_mapping

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Message-ID: <e4a288f2-a93a-5ce4-32da-f5434302551f@arm.com>
Date:   Mon, 22 Jul 2019 16:36:09 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Eric Auger <eric.auger@...hat.com>, eric.auger.pro@...il.com,
        hch@....de, m.szyprowski@...sung.com, mst@...hat.com,
        jasowang@...hat.com, virtualization@...ts.linux-foundation.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] virtio/virtio_ring: Fix the dma_max_mapping_size call

On 22/07/2019 15:55, Eric Auger wrote:
> Do not call dma_max_mapping_size for devices that have no DMA
> mask set, otherwise we can hit a NULL pointer dereference.
> 
> This occurs when a virtio-blk-pci device is protected with
> a virtual IOMMU.
> 
> Fixes: e6d6dd6c875e ("virtio: Introduce virtio_max_dma_size()")
> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> Suggested-by: Christoph Hellwig <hch@....de>
> ---
>   drivers/virtio/virtio_ring.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index c8be1c4f5b55..37c143971211 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -262,7 +262,7 @@ size_t virtio_max_dma_size(struct virtio_device *vdev)
>   {
>   	size_t max_segment_size = SIZE_MAX;
>   
> -	if (vring_use_dma_api(vdev))
> +	if (vring_use_dma_api(vdev) && vdev->dev.dma_mask)

Hmm, might it make sense to roll that check up into vring_use_dma_api() 
itself? After all, if the device has no mask then it's likely that other 
DMA API ops wouldn't really work as expected either.

Robin.

>   		max_segment_size = dma_max_mapping_size(&vdev->dev);
>   
>   	return max_segment_size;
>