linux-kernel - Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace)

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Message-ID: <1b232a8e-af99-4f7b-05c5-584b82853ac5@citrix.com>
Date:   Tue, 28 Apr 2020 17:34:36 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Joerg Roedel <jroedel@...e.de>, Andy Lutomirski <luto@...nel.org>
CC:     Joerg Roedel <joro@...tes.org>,
        Dave Hansen <dave.hansen@...el.com>,
        "Tom Lendacky" <Thomas.Lendacky@....com>,
        Mike Stunes <mstunes@...are.com>,
        "Dan Williams" <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Juergen Gross" <JGross@...e.com>, Jiri Slaby <jslaby@...e.cz>,
        Kees Cook <keescook@...omium.org>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Thomas Hellstrom" <thellstrom@...are.com>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        X86 ML <x86@...nel.org>,
        "Sean Christopherson" <sean.j.christopherson@...el.com>
Subject: Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP
 from userspace)

On 28/04/2020 08:55, Joerg Roedel wrote:
> On Mon, Apr 27, 2020 at 10:37:41AM -0700, Andy Lutomirski wrote:
>> I have a somewhat serious question: should we use IST for #VC at all?
>> As I understand it, Rome and Naples make it mandatory for hypervisors
>> to intercept #DB, which means that, due to the MOV SS mess, it's sort
>> of mandatory to use IST for #VC.  But Milan fixes the #DB issue, so,
>> if we're running under a sufficiently sensible hypervisor, we don't
>> need IST for #VC.
> The reason for #VC being IST is not only #DB, but also SEV-SNP. SNP adds
> page ownership tracking between guest and host, so that the hypervisor
> can't remap guest pages without the guest noticing.
>
> If there is a violation of ownership, which can happen at any memory
> access, there will be a #VC exception to notify the guest. And as this
> can happen anywhere, for example on a carefully crafted stack page set
> by userspace before doing SYSCALL, the only robust choice for #VC is to
> use IST.

The kernel won't ever touch the guest stack before restoring %rsp in the
syscall path, but the (minimum 2) memory accesses required to save the
user %rsp and load the kernel stack may be subject to #VC exceptions, as
are instruction fetches at the head of the SYSCALL path.

So yes - #VC needs IST.

Sorry for the noise.  (That said, it is unfortunate that the hypervisor
messing with the memory backing the guest #VC handler results in an
infinite loop, rather than an ability to cleanly terminate.)

~Andrew