Re: [PATCH v5 16/16] x86/tdx: Add cmdline option to force use of ioremap_host_shared

Michael S. Tsirkin
2021-10-12T21:30:51Z

On Tue, Oct 12, 2021 at 02:18:01PM -0700, Andi Kleen wrote:
>
> > Interesting. VT-d tradeoffs ... what are they?
>
> The connection to the device is not encrypted and also not authenticated.
>
> This is different that even talking to the (untrusted) host through shared
> memory where you at least still have a common key.

Well it's different sure enough but how is talking to host less secure?
Cold boot attacks and such?

> > Allowing hypervisor to write into BIOS looks like it will
> > trivially lead to code execution, won't it?
>
> This is not about BIOS code executing. While the guest firmware runs it is
> protected of course. This is for BIOS structures like ACPI tables that are
> mapped by Linux. While AML can run byte code it can normally not write to
> arbitrary memory.

I thought you basically create an OperationRegion of SystemMemory type,
and off you go. Maybe the OSPM in Linux is clever and protects
some memory, I wouldn't know.

> The risk is more that all the Linux code dealing with this hasn't been
> hardened to deal with malicious input.
>
> -Andi

--
MST