Re: [PATCH v2 0/6] Add support for privileged mappings

From: Robin Murphy
Date: Mon Jul 11 2016 - 10:02:34 EST


Hey Mitch,

Thanks for having the necessary go at the DMA API - I think the series
looks broadly workable now.

On 09/07/16 03:09, Mitchel Humpherys wrote:
> The following patch to the ARM SMMU driver:
>
> commit d346180e70b91b3d5a1ae7e5603e65593d4622bc
> Author: Robin Murphy <robin.murphy@xxxxxxx>
> Date: Tue Jan 26 18:06:34 2016 +0000
>
> iommu/arm-smmu: Treat all device transactions as unprivileged
>
> started forcing all SMMU transactions to come through as "unprivileged".
> The rationale given was that:
>
> (1) There is no way in the IOMMU API to even request privileged mappings.
>
> (2) It's difficult to implement a DMA mapper that correctly models the
> ARM VMSAv8 behavior of unprivileged-writeable =>
> privileged-execute-never.
>
> This series rectifies (1) by introducing an IOMMU API for privileged
> mappings and implements it in io-pgtable-arm.
>
> This series rectifies (2) by introducing a new dma attribute
> (DMA_ATTR_PRIVILEGED_EXECUTABLE) for users of the DMA API that need
> privileged, executable mappings, and implements it in the arm64 IOMMU DMA
> mapper. The one known user (pl330.c) is converted over to the new
> attribute.
>
> Jordan and Jeremy can provide more info on the use case if needed, but the
> high level is that it's a security feature to prevent attacks such as [1].

My understanding of that hack is that it involves switching the GPU into
privileged mode to get at the mapping of the IOMMU registers in the
first place, so I don't see at a glance how having privileged mappings
defends against that. What it clearly does do, however, is get us _to_
the point where it's necessary to do such a privilege switch in the
first place, as opposed to everything being trivially wide-open, which
is no bad thing.

Robin.

>
> [1] https://github.com/robclark/kilroy
>
> Changelog:
>
> v1..v2
>
> - Added a new DMA attribute to make executable privileged mappings
> work, and use that in the pl330 driver (suggested by Will).
>
>
> Jeremy Gebben (1):
> iommu/io-pgtable-arm: add support for the IOMMU_PRIV flag
>
> Mitchel Humpherys (5):
> iommu: add IOMMU_PRIV attribute
> Revert "iommu/arm-smmu: Treat all device transactions as unprivileged"
> common: DMA-mapping: add DMA_ATTR_PRIVILEGED_EXECUTABLE attribute
> arm64/dma-mapping: Implement DMA_ATTR_PRIVILEGED_EXECUTABLE
> dmaengine: pl330: Make sure microcode is privileged-executable
>
> Documentation/DMA-attributes.txt | 9 +++++++++
> arch/arm64/mm/dma-mapping.c | 6 +++---
> drivers/dma/pl330.c | 7 +++++--
> drivers/iommu/arm-smmu.c | 5 +----
> drivers/iommu/dma-iommu.c | 15 +++++++++++----
> drivers/iommu/io-pgtable-arm.c | 16 +++++++++++-----
> include/linux/dma-attrs.h | 1 +
> include/linux/dma-iommu.h | 3 ++-
> include/linux/iommu.h | 1 +
> 9 files changed, 44 insertions(+), 19 deletions(-)
>