Virtualization - Dec 2020 - swiotlb/virtio: unchecked device dma address and length

----- Original Message -----
> .snip.
> > > > This raises two issues:
> > > > 1) swiotlb_tlb_unmap_single fails to check whether the index
generated
> > > > from the dma_addr is in range of the io_tlb_orig_addr array.
> > > That is fairly simple to implement I would think. That is it can
check
> > > that the dma_addr is from the PA in the io_tlb pool when
SWIOTLB=force
> > > is used.
> > 
> > 
> > I'm not sure this can fix all the cases. It looks to me we should
map
> > descriptor coherent but readonly (which is not supported by current
DMA
> > API).
> 
> I think I am missing something obvious here. The attacker is the
hypervisor,
> aka
> the owner of the VirtIO device (ring0). The attacker is the one that
> provides the addr/len - having that readonly from a guest perspective
> does not change the fact that the hypervisor can modify the memory range
> by mapping it via a different virtual address in the hypervisor? (aka
> aliasing it).
Right, but if we allow hypervisor to provide arbitrary addr/len, does
it mean hypervisor can read uncrypted content of encrypted memory of
guest through swiotlb?

Thanks
> > 
> > Otherwise, device can modify the desc[i].addr/desc[i].len at any time
to
> > pretend a valid mapping.
> 
> With the swiotlb=force as long as addr/len are within the PA boundaries
> within the SWIOTLB pool this should be OK?
> 
> After all that whole area is in cleartext and visible to the attacker.
> 
>

----- Original Message -----
> 
> 
> ----- Original Message -----
> > .snip.
> > > > > This raises two issues:
> > > > > 1) swiotlb_tlb_unmap_single fails to check whether the
index
> > > > > generated
> > > > > from the dma_addr is in range of the io_tlb_orig_addr
array.
> > > > That is fairly simple to implement I would think. That is it
can check
> > > > that the dma_addr is from the PA in the io_tlb pool when
SWIOTLB=force
> > > > is used.
> > > 
> > > 
> > > I'm not sure this can fix all the cases. It looks to me we
should map
> > > descriptor coherent but readonly (which is not supported by
current DMA
> > > API).
> > 
> > I think I am missing something obvious here. The attacker is the
> > hypervisor,
> > aka
> > the owner of the VirtIO device (ring0). The attacker is the one that
> > provides the addr/len - having that readonly from a guest perspective
> > does not change the fact that the hypervisor can modify the memory
range
> > by mapping it via a different virtual address in the hypervisor? (aka
> > aliasing it).
> 
> Right, but if we allow hypervisor to provide arbitrary addr/len, does
> it mean hypervisor can read encrypted content of encrypted memory of
> guest through swiotlb?
> 
> Thanks
Actually not. I think you're right.

Thanks
> 
> > > 
> > > Otherwise, device can modify the desc[i].addr/desc[i].len at any
time to
> > > pretend a valid mapping.
> > 
> > With the swiotlb=force as long as addr/len are within the PA
boundaries
> > within the SWIOTLB pool this should be OK?
> > 
> > After all that whole area is in cleartext and visible to the attacker.
> > 
> > 
>