Documentation: hyperv: Describe validation of the ring buffer indices

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@linux.microsoft.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>

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