swiotlb/virtio: unchecked device dma address and length

From: Felicitas Hetzelt <file@sect.tu-berlin.de>
To: <virtualization@lists.linux-foundation.org>,
	<iommu@lists.linux-foundation.org>
Cc: "Radev, Martin" <martin.radev@aisec.fraunhofer.de>,
	david.kaplan@amd.com, "Michael S. Tsirkin" <mst@redhat.com>,
	Jason Wang <jasowang@redhat.com>,
	konrad.wilk@oracle.com, Robert Buhren <robert@sect.tu-berlin.de>,
	"Morbitzer, Mathias" <mathias.morbitzer@aisec.fraunhofer.de>
Subject: swiotlb/virtio: unchecked device dma address and length
Date: Fri, 11 Dec 2020 18:31:21 +0100	[thread overview]
Message-ID: <d2ae0b1d-332b-42a1-87bf-7da2b749cac2@sect.tu-berlin.de> (raw)

Hello,
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

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