RE: [PATCH v2 5/5] Documentation: hyperv: Describe validation of the ring buffer indices

[Michael Kelley]

From: Kameron Carr <kameroncarr@xxxxxxxxxxxxxxxxxxx> Sent: Monday, October 19, 2026 4:10 PM
> 
> Document that the guest must snapshot and validate the read and write
> indices before using them to index into the ring buffer or to compute
> the available space.
> 
> Signed-off-by: Kameron Carr <kameroncarr@xxxxxxxxxxxxxxxxxxx>

Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>

> ---
>  Documentation/virt/hyperv/coco.rst | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/virt/hyperv/coco.rst b/Documentation/virt/hyperv/coco.rst
> index 3231e51..3e20249 100644
> --- a/Documentation/virt/hyperv/coco.rst
> +++ b/Documentation/virt/hyperv/coco.rst
> @@ -200,7 +200,10 @@ validation is performed. For messages passed from the host
> to the guest in a
>  VMBus ring buffer, the length of the message is validated, and the message is
>  copied into a temporary (encrypted) buffer for further validation and
>  processing. The copying adds a small amount of overhead, but is the only way
> -to protect against a malicious host. See hv_pkt_iter_first().
> +to protect against a malicious host. See hv_pkt_iter_first(). Similarly, the
> +ring buffer header is shared with the host, so the guest must snapshot and
> +validate the read and write indices before using them to index into the ring
> +buffer or to compute available space.
> 
>  Many drivers for VMBus devices have been "hardened" by adding code to fully
>  validate messages received over VMBus, instead of assuming that Hyper-V is
> 
> --
> 2.45.4