Re: swiotlb/virtio: unchecked device dma address and length

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Dec 15, 2020 at 11:20:48AM +0800, Jason Wang wrote:
> 
> On 2020/12/15 上午5:49, Konrad Rzeszutek Wilk wrote:
> > On Fri, Dec 11, 2020 at 06:31:21PM +0100, Felicitas Hetzelt wrote:
> > > Hello,
> > Hi! Please see below my responses.
> > 
> > > we have been analyzing the Hypervisor-OS interface of Linux
> > > and discovered bugs in the swiotlb/virtio implementation that can be
> > > triggered from a malicious Hypervisor / virtual device.
> > > With SEV, the SWIOTLB implementation is forcefully enabled and would
> > > always be used. Thus, all virtio devices and others would use it under
> > > the hood.
> > > 
> > > The reason for analyzing this interface is that, technologies such as
> > > Intel's Trusted Domain Extensions [1] and AMD's Secure Nested Paging [2]
> > > change the threat model assumed by various Linux kernel subsystems.
> > > These technologies take the presence of a fully malicious hypervisor
> > > into account and aim to provide protection for virtual machines in such
> > > an environment. Therefore, all input received from the hypervisor or an
> > > external device should be carefully validated. Note that these issues
> > > are of little (or no) relevance in a "normal" virtualization setup,
> > > nevertheless we believe that it is required to fix them if TDX or SNP is
> > > used.
> > > 
> > > We are happy to provide more information if needed!
> > > 
> > > [1]
> > > https://software.intel.com/content/www/us/en/develop/articles/intel-trust-domain-extensions.html
> > > 
> > > [2]https://www.amd.com/en/processors/amd-secure-encrypted-virtualization
> > > 
> > > Bug:
> > > OOB memory write.
> > > dma_unmap_single -> swiotlb_tbl_unmap_single is invoked with dma_addr
> > > and length parameters that are under control of the device.
> > > This happens e.g. in virtio_ring:
> > > https://elixir.bootlin.com/linux/v5.10-rc7/source/drivers/virtio/virtio_ring.c#L378
> > Heya!
> > 
> > Thank you for pointing this out! I've a couple of questions and hope you can
> > help me out with them.
> > 
> > Also CC-ing AMD / TDX folks.
> > > 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).

Neither is this supported but encrypted memory technologies.

> Otherwise, device can modify the desc[i].addr/desc[i].len at any time to
> pretend a valid mapping.
> 
> Thanks
> 
> 
> > 

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