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

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Message-ID: <4113e368-6bc8-51b7-9412-3b4df2633ad6@citrix.com>
Date:   Tue, 23 Jun 2020 14:57:20 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Joerg Roedel <jroedel@...e.de>, Andy Lutomirski <luto@...nel.org>,
        "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>,
        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 23/06/2020 13:47, Peter Zijlstra wrote:
> On Tue, Jun 23, 2020 at 12:51:03PM +0100, Andrew Cooper wrote:
>
>> There are cases which are definitely non-recoverable.
>>
>> For both ES and SNP, a malicious hypervisor can mess with the guest
>> physmap to make the the NMI, #VC and #DF stacks all alias.
>>
>> For ES, this had better result in the #DF handler deciding that crashing
>> is the way out, whereas for SNP, this had better escalate to Shutdown.
>> Crashing out hard if the hypervisor is misbehaving is acceptable.
> Then I'm thinking the only sensible option is to crash hard for any SNP
> #VC from kernel mode.
>
> Sadly that doesn't help with #VC needing to be IST :-( IST is such a
> frigging nightmare.

I presume you mean any #VC caused by RMP faults (i.e. something went
wrong with the memory owner/etc metadata) ?

If so, then yes.  Any failure here is a bug in the kernel or hypervisor
(and needs fixing) or a malicious hypervisor and the guest should
terminate for its own safety.

~Andrew