Re: lack of permission checking with intel-iommu?

[Baolu Lu <baolu.lu@linux.intel.com>]

On 2024/7/2 16:47, Yi Liu wrote:
>>>>
>>>> we've recently had an interesting case where a device unexpectedly
>>>> modified a dma buffer passed to it.  On AMD systems this caused iommu
>>>> faults because the buffer was mapped read only (DMA_FROM_DEVICE), but
>>>> on an Intel system everything worked fine as the IOMMU did not trip up
>>>> on the device writes to that buffer.  Any idea why that could sneak
>>>> through?
>>> Sorry if this was confusing, the permission checking is of course about
>>> the device reading from a buffer mapped read-only for the device
>>> (as that is what DMA_FROM_DEVICE is), which on intel still allows it
>>> to read.
>>
>> Is it about IOMMU blocking "device reading from buffer" or "device
>> writing to buffer"?
>>
>> The intel iommu driver might use first-stage translation for kernel DMA.
>> It seems not to block the "device reading from buffer" if I remember it
>> correctly.
> 
> does the current kernel use the first-stage page table by default?

Use first-stage page table for kernel DMA by default.

Best regards,
baolu