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Re: [Xen-devel] Is exposing shared_info to user-land secure?

To: "dan.magenheimer@xxxxxxxxxx" <dan.magenheimer@xxxxxxxxxx>
Subject: Re: [Xen-devel] Is exposing shared_info to user-land secure?
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Fri, 01 Aug 2008 13:31:23 -0700
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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).


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