Re: [PATCH] virtio-iommu: Default to bypass during boot

[Jean-Philippe Brucker]

On Sun, Feb 21, 2021 at 06:45:18AM -0500, Michael S. Tsirkin wrote:
> On Thu, Feb 18, 2021 at 11:59:30AM +0100, Jean-Philippe Brucker wrote:
> > Currently the virtio-iommu device must be programmed before it allows
> > DMA from any PCI device. This can make the VM entirely unusable when a
> > virtio-iommu driver isn't present, for example in a bootloader that
> > loads the OS from storage.
> > 
> > Similarly to the other vIOMMU implementations, default to DMA bypassing
> > the IOMMU during boot. Add a "boot-bypass" option that lets users change
> > this behavior.
> > 
> > Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> 
> wouldn't this be a spec violation?
> 
> 
> When the device is reset, endpoints are not attached to any domain.
> 
> If the VIRTIO_IOMMU_F_BYPASS feature is negotiated, all accesses from
> unattached endpoints are allowed and translated by the IOMMU using the
> identity function. If the feature is not negotiated, any memory access
> from an unattached endpoint fails. Upon attaching an endpoint in
> bypass mode to a new domain, any memory access from the endpoint fails,
> since the domain does not contain any mapping.

Right, but the device behavior regarding BYPASS is specified as:

  If the driver does not accept the VIRTIO_IOMMU_F_BYPASS feature, the
  device SHOULD NOT let endpoints access the guest-physical address space.

So I figured that the spec could be read as "before feature negotiation
it's undefined whether the IOMMU is bypassed or not" and so a device
implementation can choose either (previously discussed at [1]). But it's a
stretch, we should clarify this.

Also, the OS might want to know whether DMA was bypassing the IOMMU before
the device is initialized. If there are strong security requirements, then
boot-bypass means the system was vulnerable to DMA attacks during boot.

So I'd like to add a new feature bit for this, VIRTIO_IOMMU_F_BOOT_BYPASS,
that tells whether DMA bypasses the IOMMU before feature negotiation. It's
only an indicator, accepting the feature doesn't change anything. I'll
send the spec change shortly.

Thanks,
Jean

[1] https://lore.kernel.org/qemu-devel/20200109104032.GA937123@myrica/

> Maybe it's not too late to change the spec here - want to try sending
> a spec patch?