Re: lack of permission checking with intel-iommu?

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

On 2024/7/2 16:02, Christoph Hellwig wrote:
> On Tue, Jul 02, 2024 at 12:46:59AM -0700, Christoph Hellwig wrote:
>> Hi all,
Hi Christoph,

>>
>> 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.

Best regards,
baolu