linux-kernel - Re: TDX #VE in SYSCALL gap (was: [RFD] x86: Curing the exception and syscall trainwreck in hardware)

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Message-ID: <CALCETrWw4Jr2iqpPdH-YoAT7oUUNbyWsm2P2+ghYsQ=R8bc9Ew@mail.gmail.com>
Date:   Tue, 25 Aug 2020 10:41:46 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Pu Wen <puwen@...on.cn>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <alexander.levin@...rosoft.com>,
        Dirk Hohndel <dirkhh@...are.com>,
        Jan Kiszka <jan.kiszka@...mens.com>,
        Tony W Wang-oc <TonyWWang-oc@...oxin.com>,
        "H. Peter Anvin" <hpa@...ux.intel.com>,
        "Mallick, Asit K" <asit.k.mallick@...el.com>,
        Gordon Tetlow <gordon@...lows.org>,
        David Kaplan <David.Kaplan@....com>
Subject: Re: TDX #VE in SYSCALL gap (was: [RFD] x86: Curing the exception and
 syscall trainwreck in hardware)

On Tue, Aug 25, 2020 at 10:36 AM Luck, Tony <tony.luck@...el.com> wrote:
>
> > > Or malicious hypervisor action, and that's a problem.
> > >
> > > Suppose the hypervisor remaps a GPA used in the SYSCALL gap (e.g. the
> > > actual SYSCALL text or the first memory it accesses -- I don't have a
> > > TDX spec so I don't know the details).
>
> Is it feasible to defend against a malicious (or buggy) hypervisor?
>
> Obviously, we can't leave holes that guests can exploit. But the hypervisor
> can crash the system no matter how clever TDX is.

Crashing the system is one thing.  Corrupting the system in a way that
could allow code execution is another thing entirely.  And the whole
point of TDX is to defend the guest against the hypervisor.