Messages in this thread | | | Date | Wed, 31 Aug 2022 09:02:35 +0100 | Subject | Re: [PATCH] swiotlb: fix a typo | From | Robin Murphy <> |
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On 2022-08-31 05:22, Chao Gao wrote: > On Tue, Aug 30, 2022 at 10:23:51AM +0100, Robin Murphy wrote: >> On 2022-08-26 10:50, Chao Gao wrote: >>> "overwirte" isn't a word. It should be "overwrite". >>> >>> Signed-off-by: Chao Gao <chao.gao@intel.com> >>> --- >>> BTW, I am wondering if copying the original buffer to the tlb buffer >>> unconditionally will leak the original buffer to the VMM, especially >>> when VMM isn't trusted e.g., by confidential VMs. Would it be better >>> to zero the tlb buffer for dir == DMA_FROM_DEVICE? >> >> No, at the point of dma_map(), the buffer contents are owned by the caller, >> so if parts of that buffer are sensitive and shouldn't be exposed to DMA, >> then don't map the whole buffer for DMA. There are more DMA API >> implementations than SWIOTLB. >> > > I am not sure if all existing drivers ensure that all buffers allocated > for DMA_FROM_DEVICE are zeroed/poisoned so that they don't have sensitive > data before dma_map(). If that isn't the case, bouncing the original contents > (left by the previous user of the buffer) effectively makes the contents > visible to host/VMM. I am afraid it may be a concern for confidential VMs.
Sure, and in a scheme where pages can be dynamically shared in-place instead of using SWIOTLB to bounce through a pre-shared buffer, then those same drivers will still expose the same stale data. Similarly, a driver could massively over-map with DMA_TO_DEVICE or DMA_BIDIRECTIONAL and expose all manner of potential secrets that way. It's a concern that cannot be addressed at the DMA API level. Anyone who wants to completely trust drivers not to do anything stupid in a confidential compute scenario is going to have to audit and possibly fix those drivers.
Robin.
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