Re: [Discussion] Xen grants and access permissions

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 16/02/2023 11:13 am, Viresh Kumar wrote:
> Hi Oleksandr,
>
> As you already know, I am looking at how we can integrate the Xen
> grants work in our implementation of Rust based Xen vhost frontend [1].
>
> The hypervisor independent vhost-user backends [2] talk to
> xen-vhost-frontend using the standard vhost-user protocol [3]. Every
> memory region that the backends get access to are sent to it by the
> frontend as memory region descriptors, which contain only address and
> size information and lack any permission flags.
>
> I noticed that with Xen grants, there are strict memory access
> restrictions, where a memory region may be marked READ only and we
> can't map it as RW anymore, trying that just fails. Because the
> standard vhost-user protocol doesn't have any permission flags, the
> vhost libraries (in Rust) can't do anything else but try to map
> everything as RW.
>
> I am wondering how do I proceed on this as I am very much stuck here.
>

(unhelpful comment) This is what happens when people try to reinvent the
wheel a little more square than it was before.

If the guest grants the page read-only, then you can only map it read
only.  Anything else is a violation of the security model.

So either you need to adjust the guest to always grant read/write, or
you need to teach virtio that read only is actually a real concept.

~Andrew