Messages in this thread | | | Subject | Re: [PATCH v2 0/6] Add support for privileged mappings | From | Robin Murphy <> | Date | Mon, 11 Jul 2016 15:02:24 +0100 |
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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@arm.com> > 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(-) >
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