Re: [PATCH 4/4] thunderbolt: Export IOMMU based DMA protection support to userspace

[Yehezkel Bernat]

On Mon, Nov 12, 2018 at 6:06 PM Mika Westerberg
<mika.westerberg@xxxxxxxxxxxxxxx> wrote:
>
> Recent systems shipping with Windows 10 version 1803 or later may
> support a feature called Kernel DMA protection [1]. In practice this
> means that Thunderbolt connected devices are placed behind an IOMMU
> during the whole time it is connected (including during boot) making
> Thunderbolt security levels redundant. Some of these systems still have
> Thunderbolt security level set to "user" in order to support OS
> downgrade (the older version of the OS might not support IOMMU based DMA
> protection so connecting a device still relies on user approval then).
>
> Export this information to userspace by introducing a new sysfs
> attribute (iommu_dma_protection). Based on it userspace tools can make
> more accurate decision whether or not authorize the connected device.
>
> In addition update Thunderbolt documentation regarding IOMMU based DMA
> protection.
>
> [1] https://docs.microsoft.com/en-us/windows/security/information-protection/kernel-dma-protection-for-thunderbolt
>
> Signed-off-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
> ---

I can't comment on the IOMMU side, but the Thunderbolt side looks good to me.

Just one point:
Have you considered the option to add this property per (TBT?) device?
If the kernel may decide to enable/disable the IOMMU or AST per device, maybe
it should be on this level.
Or maybe the IOMMU decision isn't going to change (it's system-wide) and the AST
decision will be communicated per device by a new sysfs attribute anyway, if
needed?