Add support for privileged mappings
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Cover letter [PATCH 0/3] Add support for privileged mappings
Quoted 23 lines · Mitchel Humpherys
The following patch to the ARM SMMU driver:
commit
d346180e70b91b3d5a1ae7e5603e65593d4622bc
Author: Robin Murphy <[email protected]>
Date: Tue Jan 26 18:06:34 2016 +0000iommu/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 attempts to rectify (1) by introducing an IOMMU API for privileged mappings (and implementing it in io-pgtable-arm). It seems like
(2) can be safely ignored for now under the assumption that any users of theIOMMU_PRIVflag will be using the low-level IOMMU APIs directly, rather than going through the DMA APIs.
Quoted 3 lines
Robin, Will, what do you think? 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].
So I think the problem that the offending patch tried to fix is that the PL330 DMA controller (drivers/dma/pl330.c) uses dma_alloc_coherent to allocate its microcode buffer, but the so-called "manager" thread that fetches the microcode does so with privileged accesses and consequently fails.
Whilst this series is a step in the right direction for fixing that, I don't think you can claim that only low-level users need this, given that we have in-tree code which would break without it. Perhaps you just need to extend things slightly more to expose this to the DMA API as well (or, alternatively, hack the PL330 driver some how).
Will