[PATCH v1 0/9] Bounce buffer for untrusted devices

[Lu Baolu]

An external PCI device is a PCI peripheral device connected
to the system through an external bus, such as Thunderbolt.
What makes it different is that it can't be trusted to the
same degree as the devices build into the system. Generally,
a trusted PCIe device will DMA into the designated buffers
and not overrun or otherwise write outside the specified
bounds. But it's different for an external device. The minimum
IOMMU mapping granularity is one page (4k), so for DMA transfers
smaller than that a malicious PCIe device can access the whole
page of memory even if it does not belong to the driver in
question. This opens a possibility for DMA attack. For more
information about DMA attacks imposed by an untrusted PCI/PCIe
device, please refer to [2].

This implements bounce buffer for the untrusted external
devices. The transfers should be limited in isolated pages
so the IOMMU window does not cover memory outside of what
the driver expects. Full pages within a buffer could be
directly mapped in IOMMU page table, but for partial pages
we use bounce page instead.

In addition, the IOMMU mappings cached in the IOTLB for
untrusted devices should be invalidated immediately after
the unmap operation. Otherwise, the IOMMU window is left
open to attacks.

The implementation of bounce buffers for untrusted devices
will cause a little performance overhead, but we didn't see
any user experience problems. The users could use the kernel
parameter of "intel_iommu=nobounce" to remove the performance
overhead if they trust their devices enough.

The Thunderbolt vulnerabiltiies is public and has a nice
name as Thunderclap nowadays. Please refer to [1] [3] for
more information. This patch series aims to mitigate the
concerns.

The bounce buffer idea:

Based-on-idea-by: Mika Westerberg <mika.westerberg@xxxxxxxxx>
Based-on-idea-by: Ashok Raj <ashok.raj@xxxxxxxxx>
Based-on-idea-by: Alan Cox <alan.cox@xxxxxxxxx>

The patch series has been tested by:

Tested-by: Xu Pengfei <pengfei.xu@xxxxxxxxx>
Tested-by: Mika Westerberg <mika.westerberg@xxxxxxxxx>

[1] https://thunderclap.io/
[2] https://thunderclap.io/thunderclap-paper-ndss2019.pdf
[3] https://christian.kellner.me/2019/02/27/thunderclap-and-linux/

Lu Baolu (9):
  iommu/vt-d: Add trace events for domain map/unmap
  iommu/vt-d: Add helpers for domain mapping/unmapping
  iommu/vt-d: Add address walk helper
  iommu/vt-d: Add bounce buffer API for domain map/unmap
  iommu/vt-d: Add bounce buffer API for dma sync
  iommu/vt-d: Check whether device requires bounce buffer
  iommu/vt-d: Add dma sync ops for untrusted devices
  iommu/vt-d: Flush IOTLB for untrusted device in time
  iommu/vt-d: Use bounce buffer for untrusted devices

 .../admin-guide/kernel-parameters.txt         |   5 +
 drivers/iommu/Makefile                        |   1 +
 drivers/iommu/intel-iommu.c                   | 360 ++++++++++--
 drivers/iommu/intel-pgtable.c                 | 518 ++++++++++++++++++
 drivers/iommu/intel-trace.c                   |  14 +
 include/linux/intel-iommu.h                   |  24 +
 include/trace/events/intel_iommu.h            | 132 +++++
 7 files changed, 1010 insertions(+), 44 deletions(-)
 create mode 100644 drivers/iommu/intel-pgtable.c
 create mode 100644 drivers/iommu/intel-trace.c
 create mode 100644 include/trace/events/intel_iommu.h

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2.17.1