Re: [Xen-devel] HVMOP_guest_request_vm_event only works from guest in ring0

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 08/03/16 23:44, Andrew Cooper wrote:
> On 03/08/2016 21:37, Bread Cutter wrote:
>> Hello all,
>>
>> I'm writing an executable that runs inside of a guest, and I planned
>> to use vmcall to talk to a tool running in Dom0, using the vm_event
>> API. It didn't work, and looking through the code, the first thing
>> hvm_do_hypercall() does is check if the guest is in ring0. If not, it
>> returns EPERM and exits.
>>
>> In the case of HVMOP_guest_request_vm_event, I'd rather it be up to my
>> code if a call can be made from CPL>0. Is this done intentionally?
> 
> In general, allowing hypercalls from user context is unsafe, and the
> subject of several arguments in the past.
> 
> However, in this specific case there are plenty of ways for userspace to
> get the attention of an introspection agent, although in inefficient
> ways.  As such, blocking access is pointless.  In XenServer, we have
> whitelisted that specific hypercall.
> 
> You want something like:
> 
> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> index c1b8392..c7a2cdf 100644
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -5194,8 +5194,14 @@ int hvm_do_hypercall(struct cpu_user_regs *regs)
>      switch ( mode )
>      {
>      case 8:       
> +        if ( eax == __HYPERVISOR_hvm_op &&
> +             regs->rdi == HVMOP_guest_request_vm_event )
> +            break;
>      case 4:
>      case 2:
> +        if ( eax == __HYPERVISOR_hvm_op &&
> +             regs->ebx == HVMOP_guest_request_vm_event )
> +            break;
>          hvm_get_segment_register(curr, x86_seg_ss, &sreg);
>          if ( unlikely(sreg.attr.fields.dpl) )
> {

Indeed, if everyone agrees that the patch is acceptable I'm happy to
send it to xen-devel. It'd obviously be great if this ends up upstream.


Thanks,
Razvan

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