Re: [Xen-devel] Hypercall by DomU Application

[Tim Deegan <tim@xxxxxxx>]

At 20:16 -0500 on 25 Oct (1319573807), Srujan Kotikela wrote:
> Hi,
> 
> I am working on a security architecture. In this architecture, the
> application in DomU has to communicate directly with the hypervisor. But as
> I can see, the xen architecture allows only DomU kernel to raise a
> hypercall. I am planning to enable application to communicate with xen
> directly. I am assuming, setting up a trap gate with Ring-3 access should do
> the trick. I have few questions regarding this.
> 
> Is my idea feasible? ==> (  _set_gate(idt_table+HYPERCALL_VECTOR, 15, 3,
> &hypercall); )

Seems like it would be easy to find out. :)

> Are there any security/performance/functional implications with this
> approach?

Well, it totally undermines the security of the kernel if the
application can get the hypervisor to alter memory (since the
hypervisor doesn't know about the kernel's datastructures or policies)
but if you're very restrictive about what hypercalls can be called frum
user-mode, it should be OK. 

One thing to look out for is making sure that the hypercall arguments
are actually mapped properly when the call happens (since the kernel
controls paging).

Tim.

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