Re: [PATCH] xen: detect uninitialized xenbus in xenbus_init

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.11.2021 03:11, Stefano Stabellini wrote:
> --- a/drivers/xen/xenbus/xenbus_probe.c
> +++ b/drivers/xen/xenbus/xenbus_probe.c
> @@ -951,6 +951,18 @@ static int __init xenbus_init(void)
>               err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
>               if (err)
>                       goto out_error;
> +             /*
> +              * Uninitialized hvm_params are zero and return no error.
> +              * Although it is theoretically possible to have
> +              * HVM_PARAM_STORE_PFN set to zero on purpose, in reality it is
> +              * not zero when valid. If zero, it means that Xenstore hasn't
> +              * been properly initialized. Instead of attempting to map a
> +              * wrong guest physical address return error.
> +              */
> +             if (v == 0) {
> +                     err = -ENOENT;
> +                     goto out_error;
> +             }

If such a check gets added, then I think known-invalid frame numbers
should be covered at even higher a priority than zero. This would,
for example, also mean to ...

>               xen_store_gfn = (unsigned long)v;

... stop silently truncating a value here.

By covering them we would then have the option to pre-fill PFN params
with, say, ~0 in the hypervisor (to clearly identify them as invalid,
rather than having to guess at the validity of 0). I haven't really
checked yet whether such a change would be compatible with existing
software ...

Jan