Re: Is exposing shared_info to user-land secure?

[Jeremy Fitzhardinge <jeremy@goop.org>]

Dan Magenheimer wrote:
> Is it "safe" in a paravirtualized guest to expose shared_info
> (at least read-only) to user-land?  That is, is there data
> in shared_info that could be used by a malicious program to
> compromise a guest OS (ignoring very complex side-channel
> attacks anyway)?
>
> We have apps that constantly do various time syscalls (e.g.
> to gettimeofday()) and I'm thinking if vcpu_info(cpu)->time_info
> was directly readable by an enterprise app, it could do
> the time calculations itself and save the syscall overhead.
>   

You can use the HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_info, ) 
hypercall to move the vcpu structure out of the shared info structure; 
that could be placed somewhere mappable.  Though currently I use it to 
put it into the percpu data area for quick kernel access; it definitely 
wouldn't be mappable by userspace there (well, not without padding it 
out to its own page, at least).

    J