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

From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To: Felicitas Hetzelt <file@sect.tu-berlin.de>,
	ashish.kalra@amd.com, jun.nakajima@intel.com, hch@lst.de
Cc: "Radev, Martin" <martin.radev@aisec.fraunhofer.de>,
	david.kaplan@amd.com, "Michael S. Tsirkin" <mst@redhat.com>,
	virtualization@lists.linux-foundation.org,
	Robert Buhren <robert@sect.tu-berlin.de>,
	iommu@lists.linux-foundation.org, "Morbitzer,
	Mathias" <mathias.morbitzer@aisec.fraunhofer.de>
Subject: Re: swiotlb/virtio: unchecked device dma address and length
Date: Mon, 14 Dec 2020 16:49:50 -0500	[thread overview]
Message-ID: <20201214214950.GC18103@char.us.oracle.com> (raw)
In-Reply-To: <d2ae0b1d-332b-42a1-87bf-7da2b749cac2@sect.tu-berlin.de>

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.

> 2) when swiotlb_bounce is called the device controls the length of the
> memory copied to the cpu address.

So.. this sounds very similar to the Intel Thunder.. something issue
where this exact issue was fixed by handing the DMA off to the SWIOTLB
bounce code.

But if that is broken, then that CVE is still not fixed?

So the issue here is that swiotlb_tbl_unmap_single(..,mapping_size,) is
under the attacker control. Ugh.

One way could be to have a io_tlb_orig_addr-ish array with the length
of mappings to double check?

Couple more questions:
 - Did you have already some PoC fixes for this? 
 - Is there a CVE associated with this?
 - Is there a paper on this you all are working on?

Thank you!
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