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

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Date:   Mon, 22 Jul 2019 11:45:22 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Eric Auger <eric.auger@...hat.com>, eric.auger.pro@...il.com,
        hch@....de, m.szyprowski@...sung.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 Mon, Jul 22, 2019 at 04:36:09PM +0100, Robin Murphy wrote:
> 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.

Nope, Eric pointed out it's just dma_addressing_limited that is broken.

Other APIs call dma_get_mask which handles the NULL mask case fine.


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