Linux Virtualization - Jul 2019 - [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 at redhat.com>
> Suggested-by: Christoph Hellwig <hch at lst.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;
>

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 at redhat.com>
> > Suggested-by: Christoph Hellwig <hch at lst.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;
> >

On Mon, Jul 22, 2019 at 04:36:09PM +0100, Robin Murphy
wrote:
>> 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.
Makes sense to me.

On Tue, Jul 23, 2019 at 05:38:51PM +0200, Christoph Hellwig
wrote:
> On Mon, Jul 22, 2019 at 04:36:09PM +0100, Robin Murphy wrote:
> >> 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.
> 
> Makes sense to me.
Christoph - would a documented API wrapping dma_mask make sense?
With the documentation explaining how users must
desist from using DMA APIs if that returns false ...


-- 
MST

Hi,

On 7/23/19 5:38 PM, Christoph Hellwig wrote:
> On Mon, Jul 22, 2019 at 04:36:09PM +0100, Robin Murphy wrote:
>>> 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.
> 
> Makes sense to me.
> 
I am confused: if vring_use_dma_api() returns false if the dma_mask is
unset (ie. vring_use_dma_api() returns false), the virtio-blk-pci device
will not be able to get translated addresses and won't work properly.

The patch above allows the dma api to be used and only influences the
max_segment_size and it works properly.

So is it normal the dma_mask is unset in my case?

Thanks

Eric