Re: [Xen-devel] [PATCH v2] x86/hvm: Allow the guest to permit the use of userspace hypercalls

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.01.16 at 17:51, <andrew.cooper3@xxxxxxxxxx> wrote:
> Currently, hypercalls issued from HVM userspace will unconditionally fail 
> with
> -EPERM.
> 
> This is inflexible, and a guest may wish to allow userspace to make
> hypercalls.
> 
> Introduce HVMOP_set_hypercall_dpl which allows the guest to alter the
> permissions check for hypercalls.  It behaves exactly like the dpl field for
> GDT/LDT/IDT entries.
> 
> As the dpl is initialised to 0, hypercalls are restricted to cpl0 code until
> the OS explicitly chooses an alternative.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> --
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Ian Campbell <ian.campbell@xxxxxxxxxx>
> CC: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
> 
> v2:
>  * Fix rcu lock and dpl check.

That's a bold statement considering ...

> @@ -6839,6 +6840,31 @@ long do_hvm_op(unsigned long op, 
> XEN_GUEST_HANDLE_PARAM(void) arg)
>          rc = do_altp2m_op(arg);
>          break;
>  
> +    case HVMOP_set_hypercall_dpl:
> +    {
> +        xen_hvm_hypercall_dpl_t a;
> +        struct domain *d;
> +
> +        if ( copy_from_guest(&a, arg, 1 ) )
> +            return -EFAULT;
> +
> +        d = rcu_lock_domain_by_any_id(a.domid);
> +        if ( d == NULL )
> +            return -ESRCH;
> +
> +        if ( current->domain != d )
> +            return -EPERM;
> +
> +        if ( !is_hvm_domain(d) )
> +            return -EINVAL;
> +
> +        if ( a.dpl > 3 )
> +            return -EDOM;
> +
> +        d->arch.hvm_domain.hypercall_dpl = a.dpl;
> +        break;
> +    }

... there's no unlock anywhere here.

Jan


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