linux-kernel - Re: [PATCH] swiotlb: fix a typo

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Date:   Wed, 31 Aug 2022 09:02:35 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Chao Gao <chao.gao@...el.com>
Cc:     linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
        hch@...radead.org, m.szyprowski@...sung.com
Subject: Re: [PATCH] swiotlb: fix a typo

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@...el.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.