Re: [PATCH] xen/arm: XEN_DOMCTL_get_address_size hypercall support

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/10/2025 9:01 pm, Milan Djokic wrote:
> Signed-off-by: Milan Djokic <milan_djokic@xxxxxxxx>
>
> ---
> XEN_DOMCTL_get_address_size hypercall is not implemented for arm (only for 
> x86)
> It would be useful to have this hypercall supported for arm64, in order to get
> current guest addressing mode and also to verify that 
> XEN_DOMCTL_set_address_size
> performs switch to target addressing mode (instead of relying on its returned 
> error code only).

Please don't copy this misfeature of x86 PV guests into ARM.

Letting domains be of variable bitness after domain create leads to a
whole lot of bugs, many security relevant.

32bit vs 64bit should be an input to domain_create(), not something that
is edited after the domain has been constructed.

> diff --git a/xen/arch/arm/arm64/domctl.c b/xen/arch/arm/arm64/domctl.c
> index 8720d126c9..f227309e06 100644
> --- a/xen/arch/arm/arm64/domctl.c
> +++ b/xen/arch/arm/arm64/domctl.c
> @@ -33,6 +34,37 @@ static long switch_mode(struct domain *d, enum domain_type 
> type)
>      return 0;
>  }
>  
> +static long get_address_size(struct domain *d, uint32_t *address_size)
> +{
> +    long rc = 0;
> +    struct vcpu *v;
> +    /* Check invalid arguments */
> +    if ( d == NULL || address_size == NULL) {
> +        rc = -EINVAL;
> +    }
> +    /* Domain structure type field and actual vcpu mode must be aligned */
> +    if(rc == 0) {
> +        for_each_vcpu(d, v) {
> +            if(vcpu_get_mode(v) != d->arch.type) {
> +                rc = -EFAULT;
> +            }
> +        }

This is deeply suspicious.

Under what circumstances can the vCPU setting be different from the
domain setting?

~Andrew