linux-kernel - Re: [PATCH] Allow RDTSC and RDTSCP from userspace

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Message-ID: <45000aa0-a6d1-03c3-069b-0e9a07c0284d@intel.com>
Date:   Mon, 27 Apr 2020 11:47:07 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        Mike Stunes <mstunes@...are.com>, joro@...tes.org,
        dan.j.williams@...el.com, dave.hansen@...ux.intel.com,
        hpa@...or.com, jgross@...e.com, jslaby@...e.cz,
        keescook@...omium.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, luto@...nel.org,
        peterz@...radead.org, thellstrom@...are.com,
        virtualization@...ts.linux-foundation.org, x86@...nel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH] Allow RDTSC and RDTSCP from userspace

On 4/25/20 5:49 AM, Joerg Roedel wrote:
>> That's a fun point because it means that the (untrusted) hypervisor can
>> cause endless faults.  I *guess* we have mitigation for this with our
>> stack guard pages, but it's still a bit nasty that the hypervisor can
>> arbitrarily land a guest in the double-fault handler.
>>
>> It just all seems a bit weak for the hypervisor to be considered
>> untrusted.  But, it's _certainly_ a steep in the right direction from SEV.
> Yeah, a malicious hypervisor can do bad things to an SEV-ES VM, but it
> can't easily steal its secrets from memory or registers. The #VC handler
> does its best to just crash the VM if unexpected hypervisor behavior is
> detected.

This is the kind of design information that would be very useful to
reviewers.  Will some of this information make it into the cover letter
eventually?  Or, Documentation/?

Also, for the security purists, an SEV-ES host is still trusted (in the
same TCB as the guest).  Truly guest-untrusted VMMs won't be available
until SEV-SNP, right?